Recent news stories routinely feature “hot market” U.S. cities with astronomical housing prices that end up displacing residents with moderate or low incomes. San Francisco’s epic housing battles pit longtime residents against tech workers. In Portland, Oregon, city council extended the state of emergency it declared in 2015 to address the local affordable housing crisis. In Denver, Mayor Michael Hancock pledged $150 million for affordable housing in the next decade. Boston Mayor Martin J. Walsh plans to build 53,000 units by 2030, while neighboring Cambridge adds density in infill areas and near transit. And in Boulder, Colorado, public officials seek to add a host of housing options through an approach they call “gentle infill.” 

“Hot markets exist for many reasons, but in Portland, San Francisco, Boulder, and other cities, housing issues are clearly a result of strong economic development,” says Peter Pollock, FAICP, manager of Western programs for the Lincoln Institute of Land Policy. In these places, a jobs-housing imbalance leads to inadequate shelter options. The “gentle” or “sensitive” infill approach is about “trying to find ways to make infill compatible with surroundings to achieve urban design goals and enable production of more housing,” he says. The term also “puts a positive spin on something that may not be universally accepted”—namely, density—“and suggests that we can do a better job.”

While half of all households nationwide are spending more than 30 percent of their income on housing, many residents in hot market cities are spending more than 50 percent and being forced to leave. Housing activists, such as those at the annual U.S. YIMBY (“Yes in my backyard”) gathering, are challenging city planners and elected officials to create more diverse infill options to house people, stem displacement, make better transit connections, and create more environmentally sustainable communities.

How Did We Get Here?

Desirable cities are growing rapidly because they’re attracting millennials and cultural creatives for job opportunities and lifestyle amenities, and the newcomers have gravitated in numbers that far exceed places to live. The tech industry, with its influxes of well-paid workers, is often blamed for driving up housing costs and causing displacement. But other factors are also in play. Many cities built little if any housing during the Great Recession. Mortgage credit is tighter. Construction costs are escalating. New housing is priced at market rates that drive up the cost for existing homes. Zoning that favors single-family detached houses or luxury apartments has led to expensive housing monocultures. What’s being viewed as a crisis in many cities is the loss of housing not just for lower-income residents but also for workforce and middle-income residents—teachers, nurses, firefighters, small business owners, young professionals, young families, and others who typically provide a foundation for communities.

Restoring the “Missing Middle”

The good news is that cities across the United States are already working on solutions. Communities are overturning policies that prohibit housing or place tight restrictions on where and how it can be built, to allow for more diverse and affordable places to live. Many urban planners and public officials are focused on developing housing types that restore the “missing middle,” to shelter moderate and middle-income households. 

The missing middle, a concept that grew out of new urbanism, includes row houses, duplexes, apartment courts, and other small to midsize housing designed at a scale and density compatible with single-family residential neighborhoods. Since the 1940s, this type of development has been limited by regulatory constraints, the shift to car-dependent development, and incentives for single-family home ownership. Three- or four-story buildings at densities of 16 to 35 dwelling units per acre used to be a standard part of the mix in urban neighborhoods. Many urban planners say this scale and density of housing is needed again to offer diversity, affordability, and walkable access to services and transit. Cities are using a variety of additional approaches to inject more moderately priced housing into residential neighborhoods, from shrinking or subdividing lots to adding accessory dwelling units (ADUs) to expanding legal occupancy in homes. Some of these gentle infill approaches are showing great potential or in fact adding needed units on a faster track. 

How does gentle infill work? It depends on the city, as demonstrated by the following examples from Portland, Oregon; Boulder, Colorado; and Cambridge, Massachusetts.

Portland, Oregon: More Housing is Better

Portland typically ranks atop lists of “best places” to live but has recently slipped a few notches because of its housing prices, which ballooned 13 percent in 2015. According to a recent study released by Metro, the regional government organization, Portland area rents increased 63 percent since 2006, while the average income of renters rose only 39 percent. The population grew by 12,000 in 2015, to more than 632,000 residents in 250,000-plus households. 

Since 1973, Portland has been living with statewide urban planning that mandates an urban growth boundary to protect farmland and forests from urban sprawl and to ensure efficient use of land, public facilities, and services within the urban boundary. This city has an ambitious agenda to meet its growth projections with several big planning efforts: a new zoning map and the 2035 Comprehensive Plan, its first update in 30 years, adopted by city council in June 2016; a new land use code with regulations that affect a range of growth from multifamily and mixed-use development to transportation corridors and parking; and Central City 2035, a long-range development plan for the city center and its districts. 

The city is relying on policy changes in view of the 142,000 additional jobs, 135,000 extra households, and 260,000 more people that it will need to accommodate by 2035, according to Metro. About 30 percent of new housing will be built in the city center, 50 percent in mixed-use centers and corridors, and 20 percent in Portland’s single-family residential zones, which comprise about 45 percent of the city’s 133 square miles of land. The city has about 12,000 buildable lots, assuming that some current lots can be subdivided to provide more sites.

Since 2010, an estimated 20,000 new residential units have been built or are in the pipeline, and tax increment financing in designated urban renewal areas has invested $107 million in new and preserved affordable housing. In 2016, the state legislature lifted a 17-year ban on inclusionary zoning, which will allow the city to require builders to set aside units for new workforce housing. The city is focused on funding strategies to provide more affordable homes for households below 80 percent of the area median income (AMI). To increase the number of middle-income units for people earning more than 80 percent of AMI, the city is relying on policy changes, rather than funding strategies.

By the end of 2016, a stakeholder advisory committee for the Residential Infill Project (RIPSAC) will provide advice regarding the size and scale of houses, small-lot development, and alternative housing types. One proposal under consideration is to allow more internal conversions of large historic houses into multiple units, an approach that would provide more housing while avoiding teardowns and preserving the historic fabric of neighborhoods. Building on the legacy of small homes that exist from a century ago, Portland is looking to add little houses on undersized, pre-platted lots. And the city is considering whether to allow the development of more tall “skinny” homes of up to 1,750 square feet on 2,500 square-foot lots, half the square footage of land required under R-5 single-family zoning.

“Five or ten years ago, people would ask, ‘Why is this house being built on a narrow lot?’” says RIP project manager Morgan Tracy. “Now it’s not so surprising. They’re really becoming popular because they’re at a lower price point for buyers.”

Policy changes regarding accessory dwelling units have helped generate new moderately priced housing and have drawn the attention of public officials from other cities in search of solutions to their own housing crises. ADU construction has exploded since 2010, when the city waived development fees covering sewer, water, and other infrastructure connections, reducing construction costs by $8,000 to $11,000 per unit. The waiver inspired a surge in construction: almost 200 ADUs were permitted in 2013—six times the yearly average from 2000 to 2009. In 2015, the city granted 350 new ADU permits, for a current total of more than 1,500 units. Tracy says ADUs “are a well-accepted means of producing more housing because they’re better integrated into a site and don’t necessitate a home being demolished.”

Any single-family house in the main zoning districts can have an ADU, and a proposal would allow up to two units—an interior apartment plus a separate carriage house or granny flat. The city does not limit the number of ADUs within a neighborhood or require off-street parking. It has also streamlined some ADU standards to allow for improved designs with slightly greater height and setbacks. RIPSAC is considering proposals to allow any house to have two ADUs, both interior and detached, triplexes on corner lots where duplexes are now allowed, and duplexes on interior lots, with a detached ADU. Allowing duplexes on interior lots and triplexes on corners “doesn’t mean everyone will take advantage” of the policy changes, says Tracy, noting that only 3 percent of corners now have duplexes. But “if every property owner took advantage of additional unit potential, we would double the number of housing units in each neighborhood.”

The next phase of infill housing policy considerations will address how medium-density housing types might fit into small infill and multi-dwelling sites. The city has already been moving in that direction: Portland’s Infill Design Toolkit guide focuses on integrating rowhouses, triplexes and fourplexes, courtyard housing, and low-rise multifamily buildings into neighborhoods.

“What may be shocking and alarming for some people becomes more acceptable as you see it more,” says Tracy. “We’re seeing that with duplexes and triplexes in single-family neighborhoods. The last time we built them was in the 1930s and ’40s. We’re trying to promote a wider diversity of housing forms, and some folks are supportive because they understand the need to be able to house more people on available land.”

Boulder: More Housing Is Better, But There Are Down Sides

Boulder is studying what other cities are doing to encourage gentle infill, and a recent trip to Portland by city officials, staff, and business leaders offered perspective on what could work at home. Like Portland, Boulder has determined to halve carbon emissions by 2030, provide more infill housing in the developed city core, protect open space, and encourage public transportation use. But with one-sixth of Portland’s population and different challenges and opportunities, Boulder seeks its own consensus on what gentle infill means.

Located 25 miles northwest of Denver in the foothills of the Rockies, Boulder also ranks high on the lists of healthy, livable, and entrepreneurial places. The natural beauty and high quality of life in this 25.8-square-mile city of 105,000 have attracted start-ups and established tech firms such as Google and Twitter. The influx has fed a digitally paced lifestyle and “1 percent” housing market in which the median single-family detached house costs over $1 million.

In the past two years, housing prices overall have risen 31 percent. Factors beyond the tech industry have limited affordability for many years (disclosure: for nearly 25 years, I’ve lived, worked, and raised two kids in a formerly modest Boulder neighborhood that has been largely rebuilt with higher-end homes). The University of Colorado-Boulder, a key economic driver with 38,000 faculty, staff, and students, generates significant housing demand. A jobs-housing imbalance translates to an estimated 60,000 cars arriving and departing daily, despite regional and local bus service.

State law prohibits rent control, and the state’s “condominium construction defects legislation” has squelched that type of construction for middle-income housing. Boulder is also home to many independently wealthy “trustafarians” and speculative buyers who purchase homes with cash from selling property in other high-end markets. Some are second or third residences; others are reserved for short-term rentals like airbnb. In June 2015, city council voted to restrict short-term vacation rentals, saying they impacted affordability and reduced the number of long-term housing opportunities.

Development limitations include few residential lots, a 45,000-acre ring of protected open space around the city, and a height limit, to preserve mountain views, capped at between 35 and 55 vertical feet, depending on planned development intensity and location near transit. The city is within sight of a theoretical build-out; a forecast of 6,760 additional units by 2040 is being considered for the current update of the Boulder Valley Comprehensive Plan. A 2015 housing survey conducted for the plan indicated that most residents were willing to increase density and building height to allow for more housing, at least in some parts of the city.

Since 1989, while the percentage of lower-income households has held steady, middle-income households have declined from 43 percent to 37 percent of the populace. The segment disappearing at the fastest rate is households earning between $65,000 and $150,000 as well as families with children. City council, the planning board, and local newspaper op-ed pages field lively debates over the “Aspenization” of Boulder and infill housing options that could slow or reverse the city’s momentum toward greater exclusivity and less diversity.

Boulder has been working on affordability and inclusivity for some time. Its inclusionary zoning ordinance produced 3,300 affordable housing units between 2000 and 2016. Developers of projects with five or more units are required to construct 20 percent as permanently affordable, build off-site, donate land, or make a cash-in-lieu payment to the city’s affordable housing fund. The city’s goal is 10 percent permanently affordable housing; some 7.3 percent of the city’s housing stock now qualifies.

Part of the affordable program is aimed at middle-income housing: the city has a goal of creating 450 permanently affordable units for households earning 80 to 120 percent of AMI. Between 2000 and 2016, 107 units for middle-income households were built in new mixed-income neighborhoods on land annexed in north Boulder. Many are in the Holiday neighborhood, a mixed-use model of 42 percent affordable units integrated within a total of 333 townhomes, row houses, flats, live-work studios, and cohousing. Recently built middle-income units are located in the Northfield Commons neighborhood, where half of the 43 percent of affordable units in duplexes, fourplexes, sixplexes, and townhomes are reserved for middle-income households.

“It’s very expensive to subsidize people making $70,000 to $130,000 per year,” says Aaron Brockett, a city council member and former planning board member, referencing a middle-income housing study prepared for the city that defined Boulder’s middle market as 80 to 150 percent of AMI. He advocates for “market solutions like smaller units as a trade-off in those areas that have amenities and services such as mixed-use areas where people can walk to transit and redeveloping areas.”

In preparing a comprehensive housing strategy, Boulder is exploring ideas for middle-income infill housing in transit corridors, commercial strips, business parks, and industrial areas that could be rezoned and redeveloped, and in walkable mixed-use neighborhood centers in residential areas. “The 15-minute neighborhood is the Holy Grail for a lot of communities, but it takes a lot of work,” says Jay Sugnet, project manager for Housing Boulder. “Are they in single-family neighborhoods or at the edge of service-industrial areas? Where are you willing to locate those, and what’s appropriate? You also need a concentration of people to support retail. Boulder has lots of commercial corridors, but they need a sufficient number of people to support all of them.”

The city also plans to adjust the ADU ordinance to achieve more middle-income affordability in neighborhoods of mostly single-family detached houses, which comprise about 41 percent of the city’s 46,000-unit housing stock. An ADU ordinance in effect since 1981 has permitted only 186 ADUs and 42 OAUs (owner’s accessory units) because of requirements regarding off-street parking, minimum lot size, and limits on ADU density. “We’d like ADUs for diversity of housing in neighborhoods,” says David Driskell, executive director of planning, housing, and sustainability. “Physically we could put in quite a few here, but, politically, there will be quite a lot of discussion about parking and traffic impacts.”

City council is considering “creative adjustments” to existing housing that could have less impact on the footprint and “character” of residential areas, such as loosening code restrictions on the number of unrelated people who can share a home. In most residential zones, no more than three unrelated people can share a house, even if it has six bedrooms and multiple bathrooms. A ballot measure petition launched recently by University of Colorado graduate students asks Boulder voters to overturn the occupancy limit and adopt a “one person = one bedroom” policy. Allowing higher occupancy is controversial. Although it would provide more places for students and others to live legally, it could further drive up housing costs for families, as monthly rent in group houses, particularly close to the university, often costs as much as $1,000 per bedroom.

The city is also discussing a revision of its 20-year-old cooperative housing ordinance. No co-op projects have been permitted because the ordinance was “essentially a path to No,” says Driskell. Three affordable rental co-ops were established under other measures. City council is considering a more welcoming ordinance that supporters say would benefit the city by offering a sustainable and community-oriented lifestyle for single residents, young families, seniors, and people who work lower-wage jobs.

“We tend to be a regulatory city, and we have really embraced deliberative planning,” says Susan Richstone, deputy director of planning, housing, and sustainability. “It hasn’t always been easy, but we’re having the discussions and making changes in planning and zoning levels within a regulatory framework. It’s in our DNA.”

“Density is a bogeyman here, and people are up in arms,” says Bryan Bowen, an architect and planner who is a member of the Boulder Planning Board and the city’s Middle Income Working Group.  Residents are anxious about both modest homes being scrapped and replaced with 5,000 square-foot $1.5 million new homes and the possibility of greater density with more large edgy-looking multifamily apartment buildings. “That’s probably why gentle infill feels good, though it has an interpretive quality. It’s a question of what people find to be compatible and palatable.” There’s no consensus yet about which infill approach will work best, Bowen says. “But frankly, in moderation, some application of all of them might be needed.”

Accessory Dwelling Units (ADUs): A Preferred Infill Housing Approach

Demographic changes such as aging populations, shrinking household size, college-loan-strapped millennials, and cultural preferences are leading many cities to allow home owners to build ADUs, also known as in-law apartments, granny flats, and carriage houses. Advocates say ADUs—built in the interior of a home, rebuilt from a garage, or newly built as a separate cottage—offer affordable options for elderly parents, adult kids, and caregivers. They’re also a source of rental income that can help residents stay in their homes. As older home owners wish to downsize and age in place, some are choosing to live in the ADU and rent out their main house.

Typically ranging from 200 square feet to more than 1,000 square feet, ADUs are part of a long tradition of modest apartments and multigenerational houses that were common before the era of single-family suburban homes. Many housing advocates are keen on ADUs as a way to add units quickly, with home owners financing the infill of existing neighborhoods, compared to the lengthy and costly process of land acquisition and development of larger-scale multifamily projects by municipalities, nonprofit affordable housing organizations, and private developers. At Denver’s Bridging the Gap housing summit in May, a session on small-scale affordability posed a potential scenario for the city: 70 neighborhoods multiplied by 300 ADUs per neighborhood would equal 21,000 moderately priced housing units.

At the 2015 YIMBY conference in Boulder, Susan Somers of AURA (formerly Austinites for Urban Rail Action) in Austin, Texas, described a coalition effort to become “an ADU city” and achieve much greater housing density in the mostly single-family detached city. They accomplished their mission; in November 2015, the Austin City Council passed a resolution relaxing ADU regulations and allowing them on smaller lots. AURA hopes to help home owners entitle 500 new ADUs annually. The units provide “affordable housing and a source of income to allow folks to stay in their homes,” says Somers. In gentrifying East Austin, “this is how families stay together.”

Cambridge: Bridging the Income Gap

Cambridge, located across the Charles River and three miles west of Boston, has the most expensive housing in Massachusetts and bears keen pressure to produce more missing-middle options. The population has increased more than 10 percent since 2000, to 110,000 residents within a compact 6.5 square miles, and is projected to grow by 6,200 homes before 2030, according to the Metropolitan Area Planning Council (MAPC), the regional planning agency for Metro Boston. The city has 117,000 jobs and more than 52,000 housing units, about half of them located in mixed-use commercial areas. The average listed single-family home price in 2015 exceeded $1.2 million. Median monthly rent for a one-bedroom apartment was $2,300.

“Cambridge has become a bifurcated place of very high income and very low income,” says Andre Leroux, executive director of the Massachusetts Smart Growth Alliance. “It’s hard for middle-class people to live there.” Cambridge has the infrastructure to support much greater density and to add significantly more residential development and huge residential towers, “but it doesn’t want to be downtown Boston.”

The city is in the first year of a three-year comprehensive plan process, its first since 2000 (the state does not require municipalities to develop comprehensive plans). Affordable housing for low, moderate, and middle incomes—a resounding theme through the public process—is the number-one priority, says Iram Farooq, assistant city manager for community development.

“For a lot of working people, there are fewer affordable options in the city,” says Farooq. The greatest population decline has occurred among residents earning between 50 and 80 percent of AMI, she says. Middle-income households earning between 80 and 120 percent of the area’s AMI are also leaving the city for housing options elsewhere in the urban region. She notes that a city program that offered low-interest financing to home buyers earning up to 120 percent of AMI experienced little demand.

“Just creating the program doesn’t mean people are going to use it. With the same financial commitment, they are able to go three miles down the road and find a nicer or bigger house for the same money. Being able to hold onto the middle is more challenging than at other income levels.”

The city is using regulatory strategies to fund more affordable housing. An incentive zoning ordinance enacted in 1988 required linkage payments to offset the effects of commercial development on the housing market. In 2015, the city updated the ordinance, increasing the rate for developers from $4.58 to $12 per square foot and broadening the requirement to include any nonresidential development, including healthcare and university facilities, labs, and office space. The city is also considering new zoning for infill sites and an expansion of its inclusionary housing ordinance, which now requires 11.5 percent affordability in new projects, to 20 percent affordable units for moderate, middle-income, and low-income households.

Cambridge has been building infill housing, mostly in projects ranging from 50 to 300 units, on larger sites. East Cambridge, for example, has seen the development of thousands of housing units in the past decade, along with millions of square feet of office space and restaurants, on land that was formerly industrial. The city is requiring residential units with all new development; 40 percent of a new commercial project in East Cambridge’s Kendall Square will be dedicated to housing. Some of this new development is subsidized for the middle class. But few parcels exist in residential areas, land costs are high, and residents are pushing back.

For years, housing advocates have been urging the city to add more infill housing and increase density in Central Square, the historic municipal center of the city. Located on Massachusetts Avenue, Central Square has a subway station and a bus-transfer station where eight bus routes converge. The area has some three- and four-story buildings as well as one- and two-story buildings that could be redeveloped for dense mixed-use housing next to transit. The square historically had taller, denser buildings before some third and fourth stories were removed to reduce taxes during the Depression. In 2012, however, some neighbors tried to persuade the city to downzone Central Square.

“Downzoning is not appropriate in a crisis in which we’re so restricted in our ability to build housing,” says Jesse Kanshoun-Benanav, an urban planner and affordable housing developer who started the civic group A Better Cambridge in response to the downzoning effort, to promote increased density for infill housing opportunities. The city council tabled the downzoning effort and since then has been allowing zoning changes in Central Square and providing incentives such as additional height and density in exchange for the development of more affordable housing.

At the eastern end of Central Square, Twining Properties is developing Mass + Main, a multiparcel mixed-use project with a 195-foot tower and 270 apartments, 20 percent of which will be affordable for low, moderate, and middle-income residents. The project required a zoning variance, notes Farooq. “We’re now hearing political desire to rezone the rest of Central Square. People don’t seem to be as opposed to density as height, so we’ll have to explore what that means in terms of urban form.”

Townhouses, duplexes, and triple deckers are the norm in Cambridge, and only 7.5 percent are single-family detached homes. New rules passed in May that allow the conversion of basements into accessory dwelling units in single- and two-family homes throughout the city could enable 1,000 legal ADUs. The ADUs don’t need a zoning variance, and off-street parking is not required. The square footage of the new units won’t count as gross floor area (ADUs previously were prohibited in most cases due to the existing floor-area ratio and requirements for lot area per dwelling unit). Supporters say the rules won favor because they allow for more efficient use of large homes and won’t alter the look of the neighborhood.

“It’s important that there are people in the city who are willing to accept trade-offs,” says Farooq, noting that the YIMBY movement has “great political capital” to counter NIMBY pushback against infill housing. “There is a community desire to see more housing, and many young people, including a lot of renters, recognize that it’s important to increase the supply and not have steep increases in rent, to make housing more manageable and accessible.”

Regional Approaches

Leroux from the Massachusetts Smart Growth Alliance and others across the nation say that housing needs should be addressed as a regional issue, and cities and towns should work together to allow urban infill housing and approaches like ADUs under state zoning laws. In June, the Massachusetts Senate passed a bill that would reform 1970s-era zoning laws to permit ADUs and multifamily housing districts in every community. A coalition including the Alliance; the Senate President; mayors; and advocates for the environment, public health, affordable housing, and transportation supported the bill, which is poised to become state law next legislative session. A legal and policy strategy, it includes a fair-housing clause that prohibits communities from making discriminatory land-use decisions, which Leroux and others say increase segregation in many metropolitan areas, as low-income residents, including people of color, get pushed out of redeveloping urban neighborhoods.

Suburban communities also need to do their fair share, he says. Many suburbs are still zoning and building for the auto-oriented market, with “a lot of modest homes being torn down and replaced with McMansions,” he says. “We think there’s a grand bargain to be made between cities and towns and the real estate development community to unshackle development near walkable places, infrastructure, and transportation while curbing sprawl and protecting natural areas.” To allow for more diverse housing growth, he says, the Alliance and others are promoting “as-of-right,” or permitted zoning uses, in walkable areas, commercial centers, villages, town centers, and urban squares, because “that’s where the market is and where we need to let the market do its job.”

This article originally appeared in July 2016 Land Lines.

Kathleen McCormick, principal of Fountainhead Communications, LLC, lives and works in Boulder, Colorado, and writes frequently about sustainable, healthy, and resilient communities.

Photograph: Fred King

People who work with me are often surprised by the extent to which my philosophical canon derives from low-budget offbeat films, typically from the 1980s. When in need of wisdom, I frequently turn to the teachings of Repo Man or, for this essay, Terry Gilliam’s allegorical masterpiece Time Bandits. In the movie, a group of public workers are employed by the Supreme Being to fill holes in the time-space continuum left from the haste of creating the universe in seven days: “It was a bit of a botched job, you see.”

Like the Time Bandits, policy makers are often tasked to fill holes—actual potholes in roadways, or more theoretical holes that are the artifacts of dysfunctional private markets, such as the inadequate supply of affordable housing. For example, housing economists in the United States have become quite adept at tracking the size of the hole, which has only become harder to fill since the federal government committed to address it as a national policy priority beginning with the Housing Act of 1949, part of President Harry S. Truman’s Fair Deal.

In his 1949 State of the Union address, President Truman noted that to fill the needs of millions of families with inadequate housing, “Most of the houses we need will have to be built by private enterprise, without public subsidy.” Nearly 70 years later, our collective failure to solve the affordable housing deficit may stem from wrongheaded analysis of the problem, and the conclusion that market-based solutions can be designed to solve the mismatch between the supply of affordable housing and demand for it.

To support this claim, permit me a short departure into market theory. From the now-preferred mathematical approach to economic analysis, a market is simply a system of partial differential equations that is solved by a single price. The equations capture the complex decisions made by consumers and producers of goods—reconciling consumers’ preferences and budgets with producers’ production techniques, capital, and transaction costs—to arrive at a price that clears the market by settling the transactions of all suppliers and consumers willing to trade at that price.

Acclaimed economists Arrow, Debreu, and McKenzie proved the theoretical existence of a single set of prices that can simultaneously solve for the “general equilibrium” of all markets in a national or global economy. One important aspect of this Nobel Prize–winning contribution was the observation that a unique price cleared each market—one market, one price. There was no expectation that a single price could maintain equilibrium in two markets. And this is the fundamental flaw of the housing market—it is actually two markets, not one. Housing markets supply both shelter for local consumption and a globally tradable investment good made possible by broad capital markets that serve global investors. This dual-market status used to pertain to owner-occupied housing, but, with the proliferation of real estate investment trusts, rental markets are now in the same boat.

Markets for consumption goods behave very differently than investment markets, responding to different “fundamentals.” On the supply side, prices for consumption goods are dictated by production costs, while prices in investment markets are dictated by expected returns. On the demand side, such things as tastes and preferences, household incomes, and demographics determine the price of housing as shelter. Investment demand for housing is dictated by factors like liquidity and liquidity preferences of investors, expected returns on alternative investments, or interest rates.

In developed countries, global capital markets and the market for shelter collide locally with little chance of reconciliation. Local households compete with global investors to decide the character and quantity of housing that is produced. In markets that attract global investment, plenty of housing is produced, but shortages of affordable units are acute, and worsen over time. This is because a huge share of new housing is produced to maximize investment return, not to meet the needs of the local population for shelter. For example, there is no shortage of global investment willing to participate in developing $100 million apartments in New York City. But affordable housing, being much harder to finance, is in short supply. And in markets that have been abandoned by global capital, house prices fall below production costs, and surplus housing accumulates and decays. In extreme cases such as Detroit, market order can only be restored by demolishing thousands of abandoned homes and buildings.

Perhaps it is time that we question the conclusion that market-based solutions can address the challenge of sheltering a country’s population. Truman concluded that “By producing too few rental units and too large a proportion of high-priced houses, the building industry is rapidly pricing itself out of the market.” But Truman was thinking about the market for shelter, not investment. Remarkably, the number of housing units in developed countries significantly exceeds the number of households. In 2016, the U.S. Census estimated that there were 135 million units of housing in the country and 118 million households. One in seven housing units was vacant. This over-supply of housing characterizes every metropolitan market in the United States—even markets with extreme shortages of affordable housing. In 2016, 10.3 percent of housing units were vacant in New York, 6.0 percent in the San Francisco Bay area, 8.2 percent in Washington, DC, and a stunning 13.7 percent in Honolulu. The problem is that many households have insufficient incomes to afford the housing that is available.

In the end, rather than fill the holes in the fabric of time and space, the Time Bandits decided to take advantage of them to “get bloody stinking rich.” The bandits sought to capitalize on celestial imperfections, the way global investors seek returns from short-term market dislocations. To illustrate the dangers of such naked speculation in unregulated markets, consider an apocalyptic tale from a very different market. In 1974, heavy rains during planting season in Bangladesh suggested that rice might be in short supply at harvest time, and rice prices started to rise. Savvy commodity speculators realized that there would be a good return on any rice that was held off the market. The actual harvest produced a bumper crop, but the interaction between market expectations and market manipulations by commodity investors produced one of the worst famines of the 20th century—with an estimated 1.5 million famine-related fatalities. The famine did not result from real food shortages. The collision of the market for goods and the market for speculative investment priced rice out of the reach of the local populations, with landless families suffering mortality at three times the rate of families with land.

Perhaps shelter and food are too important to be left to unregulated markets to allocate. Perhaps public policy should focus on protecting a share of the market—and the public—from the ravages of speculation. In this special anthology issue of Land Lines, Loren Berlin describes efforts to preserve affordable housing in the form of manufactured homes and to promote permanent affordability of that stock through the conversion of manufactured housing communities to limited equity cooperatives. Community land trusts and inclusionary housing policies are also effective ways to insulate shelter from speculation, as demonstrated by Lincoln Institute research. After almost seven decades of failed efforts to get private markets to meet populations’ needs for affordable shelter, it might be time to develop, and to export, these other approaches based on a more realistic understanding of the complexity of housing and capital markets.

This article originally appeared in July 2015 Land Lines.

Los drones revolucionan la recolección de datos y el mapeo, y dan lugar a grandes cambios y nuevas oportunidades en los campos de la gestión de suelo, políticas y defensa. 

Los vehículos aéreos no tripulados (UAV) empezaron a usarse de forma generalizada en todo el mundo hace una década aproximadamente, cuando su costo cayó rápidamente en el mercado de consumo. En los países en vías de desarrollo y en las zonas que se urbanizan a toda velocidad, los drones se están convirtiendo en una herramienta esencial para garantizar los derechos territoriales, actualizar los mapas en línea en tiempo real y comprender los patrones de los asentamientos no planificados. Desde América Latina hasta Asia Meridional se están lanzando drones en los sectores de la información geoespacial y la gestión territorial. Esta actividad está a cargo de agrimensores, para definir parcelas urbanas específicas; tasadores, para determinar el valor de un terreno en un territorio periurbano; y empleados privados y públicos, para actualizar información territorial.

Los drones pueden poseer cámaras aéreas multiespectrales de formato pequeño y producir imágenes tanto del entorno visible como del espectro infrarrojo; esta capacidad técnica ofrece un complemento importante a la fotografía aérea tradicional e incluso a las imágenes satelitales de alta resolución. Dado que los UAV pueden volar muy bajo y seguir patrones estrechos y repetitivos, pueden crear imágenes detalladas con resolución de un centímetro o, mejor, permitir crear imágenes tridimensionales. 

Además, su potencial democrático está generando entusiasmo, dado que dan más poder a los ciudadanos, a las organizaciones no gubernamentales y a otras redes informales más pequeñas. “Los drones marcarán la diferencia en los procesos de políticas y de toma de decisiones, dado que los ciudadanos participan en la creación de datos en momentos críticos”, destaca Diego Alfonso Erba, ingeniero tasador y experto en sistemas de gestión de suelo en América Latina. “Los ciudadanos pueden controlarlos, tomar fotos de una situación y compartir los resultados con las autoridades. En situaciones que evolucionan a toda velocidad, en las que se observa la generación de asentamientos informales, extracciones de recursos no autorizadas o conflictos, los drones pueden ofrecer pruebas a los sistemas legales”. 

El uso pionero de los drones en América Latina para enriquecer y mejorar las políticas y la gestión de suelo está haciendo eco en todo el mundo. “En China estamos haciendo lo mismo”, dice Zhi Lui, director del programa en China del Instituto Lincoln y director del Centro de desarrollo urbano y políticas de suelo de la Universidad de Pekín y el Instituto Lincoln (PLC), de Beijing. En Asia Oriental, los drones están ayudando en las investigaciones y experimentos nuevos de alta tecnología para modernizar los registros de usos contemporáneos y para ayudar a abordar otros desafíos a gran escala, como la implementación potencial de un impuesto inmobiliario.

Catastros: registros territoriales públicos en América Latina 

En América Latina y Asia, los drones demuestran ser particularmente útiles en la evolución de los “catastros” territoriales: registros públicos que gestionan la información relacionada con parcelas y que tienen una función clave en la toma de decisiones sobre el uso territorial en toda América Latina.

En gran parte de la región los sistemas territoriales de catastro existentes provienen de un modelo “ortodoxo” importado hace siglos desde la Europa colonial, según relata Erba, coautor de Making Land Legible: Cadastres for Urban Planning and Development in Latin America (Para leer el suelo urbano​: catastros multifinalitarios para la planificación y el desarrollo de las ciudades de América Latina), publicado en 2016 por el Instituto Lincoln. Erba encabeza un trabajo que pretende actualizar estos sistemas de registro territorial a los que se conoce como “catastros multipropósito (MPC, por su sigla en inglés)”; y los drones tienen un papel crítico en esta evolución. 

Los catastros tradicionales u “ortodoxos” se mantienen como registros públicos gracias a organismos gubernamentales. No son válidos para la creación de políticas urbanas actuales porque solo cubren parcelas privadas y dan pocos detalles sobre los atributos físicos, legales y económicos. En cambio, los catastros multipropósito se mantienen gracias a las partes interesadas voluntarias de una jurisdicción que se comprometen a enviar información más completa e inclusiva sobre una ciudad. Los MPC pueden incluir datos alfanuméricos y catastros temáticos o específicos de un dominio, relacionados con el entorno, los sistemas de transporte o las redes de servicios, y se pueden organizar por organismos gubernamentales o privados. Algunos beneficios pueden ser un mejor planeamiento urbano, impuestos más equitativos que aumentarán la renta y una base impositiva más amplia. 

“La integración de datos que ofrece el modelo de MPC es la forma más directa de identificar y controlar las características económicas, físicas, legales, ambientales y sociales de las parcelas y sus ocupantes”, observan Erba y su coautor, Mario Piumetto, un tasador territorial especialista en sistemas de información geográfica. “Los planificadores necesitan esta información para gestionar el crecimiento de las ciudades, definir estrategias de financiación urbana, reducir la informalidad y analizar el impacto de las intervenciones del gobierno” (Erba y Piumetto, 2016). Con la democratización de las herramientas de monitoreo geoespacial, la tecnología de los drones ayuda a facilitar el camino hacia los MPC con múltiples interesados. 

Las ciudades latinoamericanas consolidadas con catastros existentes utilizan drones para abordar desafíos asociados con la construcción informal. Por ejemplo, Erba y Piumetto destacan la villa 31, una de las zonas más valiosas de Buenos Aires, donde unas 40.000 personas erigieron construcciones informales de hasta cinco pisos en una superficie de 100 manzanas. En 2016, el gobierno lanzó un sondeo por dron junto con un escáner láser a nivel del suelo que creó un modelo 3D y generó estadísticas sobre la ocupación de viviendas, calles y espacios públicos. Con esta imagen más precisa del desarrollo residencial, las agencias y las partes interesadas están mejor posicionadas para que los habitantes informales hagan una transición para poseer las propiedades formalmente y participen en los procesos de planificación. 

Ecuador también es una prueba de cómo los catastros mejorados por drones pueden promover la capacidad de recuperación. La ciudad de Portoviejo utiliza drones para hacer cumplir las reglamentaciones que prohíben ocupar espacios públicos. Las autoridades compararon registros de 2010 con imágenes de control tomadas por drones y se determinó que más de 7.000 construcciones nuevas violaban la normativa. En abril de 2016 este registro de asentamiento en tiempo real, que es más preciso, demostró ser imprescindible cuando un terremoto de 7.8 puntos causó estragos en las estructuras de todo Portoviejo y mató a más de 200 personas. Las fotos que se tomaron luego del terremoto se compararon con grabaciones recientes de drones; así, se facilitaron los trabajos de rescate y reconstrucción.

Valuación masiva con ayuda de SIG en China

En China, los drones podrían resultar de gran utilidad en los trabajos actuales para calcular el valor de las propiedades (ver pág. 8). Desde 2003, China espera que se introduzca un impuesto inmobiliario municipal sobre la tenencia privada de propiedades residenciales, algo que durante muchas décadas, los gobiernos municipales no pudieron hacer. Sin embargo, la mayoría de las ciudades se enfrentan a una barrera técnica inmensa: no hay un sistema de tasación de propiedades ni una base de datos. Los investigadores esperan que los drones puedan ayudar a formar la base subyacente para hacer las tasaciones. 

“El asunto es cómo podemos ayudar a tantas ciudades de China a desarrollar con rapidez un sistema de catastro, que es la base de los sistemas de impuestos inmobiliarios”, confiesa Liu, y destaca que el PLC financia un proyecto de investigación en China orientado a ofrecer innovaciones en esta materia. En la etapa siguiente, los investigadores de China deberán combinar los datos de derechos de propiedad con representaciones 3D de parcelas creadas por tecnologías de drones. Es indispensable que el gobierno entregue a los investigadores todos los datos relativos a derechos de propiedad, como información formal de propiedad y el tamaño de las unidades, parcelas y edificios, para poder emparejar las imágenes 3D de forma precisa. Liu destaca que, en muchas ciudades, no se sabe si estos registros están totalmente digitalizados. Si bien los datos generados por drones no pueden ofrecer la documentación de propiedad que falta, eventualmente, los datos mejorados sobre las parcelas acelerarán el proceso de generar un sistema de catastro preciso. 

Chun Zhang, profesor de planificación de ciudades en la Universidad Beijing Jiaotong y líder del proyecto financiado por el PLC, dice que en este momento los drones están usando fotografía con efecto diorama (mediante el cual los rasgos fotografiados pueden parecer una representación en miniatura) y crean modelos 3D con las imágenes capturadas. Luego, el proyecto ofrecerá información espacial básica. Hoy, las técnicas de drones se aplican en pueblos pequeños como Jimingyi, Shexian y Gubeikou. Pero a medida que los investigadores avanzan con los experimentos con drones, se están encontrando con límites técnicos y normativos. “La zona de sondeo no puede ser demasiado grande”, destaca Zhang, debido a los límites de la batería del dron. “La dificultad más importante para los investigadores es el control aéreo en ciertas zonas [limitadas]; por ejemplo, en el 6.º anillo de Beijing”. Pero esto no debería ser un problema si los gobiernos municipales deciden usar drones para desarrollar una base de datos de propiedades en 3D. 

La tasación de las propiedades escapa al alcance del proyecto de investigación actual de Zhang, pero será un desafío a gran escala en China. En última instancia, el proceso de trabajo intensivo se podría resolver mediante métodos informáticos con la ayuda de datos generados por drones. Liu indica que en Estados Unidos los gobiernos locales usan técnicas de valuación masiva asistida por computadora (CAMA) desde hace mucho tiempo para tasar todas las propiedades de una zona determinada. “En China, trabajamos con unas pocas ciudades que están perfeccionando el modelo de valuación masiva asistida por computadora para incorporar gran cantidad de datos y poder tasar el valor de una propiedad con mayor precisión”, dice Liu. Ese tipo de trabajo podría ser la siguiente etapa de la investigación. Pero la etapa actual aún se concentra en descifrar qué nivel de unificación se puede lograr entre los registros de propiedad existentes y los datos de los drones. 

En el contexto de los registros territoriales, resulta imprescindible usar drones para la identificación inicial y provisoria de límites físicos de propiedad, en las ciudades y jurisdicciones en las que todavía no hay un sistema formal de administración territorial y la estructura del territorio se desconoce.

Caminos hacia la revolución de los drones

Hoy, los drones funcionan con una capacidad crucial en un abanico de casos de uso en políticas de suelo y atienden necesidades culturales y legales, pero su historia de desarrollo y uso es evidentemente más profunda. La evolución hacia un uso comercial y recreativo más amplio (que incluye una definición más específica de políticas de suelo) es un poco la típica historia de los efectos secundarios de la innovación tecnológica. Su desarrollo original y el prototipo de las tecnologías de vuelo se dieron en gran parte en el contexto de la investigación militar. Pero algunos de los logros técnicos más importantes necesarios para que la instrumentación requerida para volar estuviera disponible a precios razonables fueron el resultado de las “guerras de los smartphones”, en las que varias empresas de tecnología de comunicación competían en el perfeccionamiento de hardware y software eficiente para brújulas, giroscopios, altímetros y otros instrumentos (Anderson, 2017).

Aun así, aunque la tecnología estuviera lista y la economía fuera ideal para que el público la usara de forma generalizada, el entorno de políticas para el uso de los drones debía madurar. Por ejemplo, en Estados Unidos, la Administración Federal de Aviación intentó luchar con la demanda comercial y de consumo, y al mismo tiempo equilibrar las preocupaciones sobre los conflictos con las rutas de vuelo de las aeronaves tripuladas y la invasión potencial de la privacidad y los derechos territoriales. Como se indicó anteriormente, estos tipos de debates sobre políticas se están llevando a cabo en todo el mundo.

Sin embargo, muchas de las tecnologías en desarrollo se concentran en los territorios agrícolas, donde la competencia de intereses y los conflictos son mínimos. Se espera que la agricultura sea el sector principal para el uso comercial de las tecnologías de drones. Dado que se puede utilizar instrumentación de un dron para medir rastros de radiación y el espectro infrarrojo, estos tienen un potencial masivo para hacer mejoras en el rendimiento de las cosechas y en la agricultura en general (Wihbey, 2015). Pero los beneficios no fueron distribuidos de forma igualitaria en la última década, dado que los países como Japón y Canadá abrieron el espacio aéreo agrícola, mientras que Estados Unidos aún debate dónde abrir las políticas de espacio aéreo para la agricultura (Lewis, 2017). Para llevar la tecnología a una escala agrícola, se necesitará que los drones vuelen a una latitud muy superior, fuera de la vista de los operadores terrestres. En cualquier caso, la idea de “agricultura de precisión” se puso de moda en todo el mundo con potenciales beneficios medioambientales, como reducción y mejor concentración en el uso de pesticidas y otros químicos. Y seguramente, los avances alcanzados en los ambientes agrícolas rurales también se podrán usar, por ejemplo, para controlar reservas forestales y poblaciones de vida silvestre, y para los trabajos mundiales para limitar la expansión de asentamientos no planificados y garantizar la sustentabilidad ecológica (Paneque-Gálvez et al., 2014). 

Las políticas relacionadas con la capacitación, licencias y certificaciones que necesitan los operadores de drones siguen evolucionando en muchos países y, por supuesto, el sondeo territorial formal en sí posee sus propios estándares profesionales que integran estas nuevas tecnologías. El uso recreativo o de ciudadanos, y el control informal del suelo y los espacios urbanos están destinados a complicarse cada vez más, dado que surgen nuevos desafíos y posibilidades de observación con el uso de técnicas de “enjambre” y múltiples drones de forma simultánea. También surge un potencial de mayor autonomía, porque el software los hace más inteligentes y los independiza de los operadores humanos (The Economist, 2017).

Desafíos

Los drones podrían resultar ser una herramienta indispensable para afrontar los problemas extensos de uso de suelo que se espera que surjan en las siguientes décadas a medida que el mundo se urbaniza velozmente, como viviendas inasequibles o escasez de suelo disponible para espacios abiertos (Wihbey, 2016). De hecho, pueden ofrecer una especie de “salto en rango” tecnológico parecido a la conectividad a Internet desde los celulares, que permitió a muchas personas y sociedades de los países en vías de desarrollo conectarse con la web sin la necesidad de tener líneas de banda ancha en el hogar. 

Zhi Liu, director del PLC, considera que los catastros multipropósito podrían ofrecer soluciones, pero muchas ciudades de Asia necesitarán avances tecnológicos, además de voluntad política y apoyo público, para mejorar y actualizar sus catastros, dado su crecimiento veloz. Los experimentos en las ciudades pequeñas y pueblos de China pueden resultar útiles para otras ciudades más grandes de la región, o incluso para países de todo el mundo. 

Las normativas en toda América del Sur y Central evolucionan para mantenerse al día con el uso extendido de los drones como herramientas para actualizar las políticas de suelo en la región. Los funcionarios estiman que en 2015 había 20.000 drones en funcionamiento solo en Brasil, con funciones principales en la agricultura, minería, inspecciones de infraestructuras, seguridad y control de fronteras y la diagramación de zonas ambientales y ciudades, según Erba y Piumetto. En mayo de 2017, gracias a este crecimiento, la Agência Nacional de Aviação Civil o ANAC de Brasil emitió nuevas normas de seguridad y operación, que citan y siguen específicamente definiciones de otras autoridades de aviación civil, como las que se encuentran en Estados Unidos y la Unión Europea (ANAC, 2017). 

En México, la Dirección General de Aeronáutica Civil emitió una serie de normas similares que apuntan a evitar accidentes y proteger a terceros y propiedades en tierra y en vuelo. En Argentina, los vuelos que superan los 122 metros precisan autorización, y también hay limitaciones que dependen del peso del equipo, las zonas sobre las que vuela y la información que se recopila.

Nuevas fronteras para los drones

Muchas instituciones de todo el mundo se han interesado en aprovechar las tecnologías de los drones para resolver problemas administrativos antiguos, en particular en zonas que sufrieron las condiciones adversas de conflictos o dificultades económicas. Por ejemplo, el Banco Mundial destacó los trabajos en los Balcanes luego del conflicto, donde en algunas zonas de Kosovo todavía hay problemas, ya que los dueños de propiedades, en su mayoría hombres, fueron asesinados en la guerra regional de la década de 1990. Las mujeres que quedaron en la zona han luchado por restablecer el orden en lo que respecta a las propiedades y las políticas de suelo, dada la falta de registros formales. El Banco Mundial comentó: “El tiempo, el costo y la complejidad de los sondeos y registros convencionales de suelo. . . son un obstáculo para estas mujeres. Suelen llevar años y son demasiado costosos, por lo que estas mujeres no tienen información ni protección legal de sus derechos” (Banco Mundial, 2016). Por lo tanto, se están usando drones en conjunto con la Autoridad Mapeadora de Kosovo para realizar actividades de mapeo catastral. 

Además, los expertos del Banco Mundial destacan que los drones resultan ser armas efectivas en la lucha por los derechos de suelo en zonas subdesarrolladas del continente africano (Totaro, 2017). Mientras que cerca del 90 por ciento de Europa está mapeada a nivel local, apenas el 3 por ciento del continente africano posee mapas con la misma resolución. Dado que las zonas costeras se desarrollan a toda velocidad para hoteles y para uso comercial o residencial, los drones podrían ayudar a las comunidades a mantenerse al día con el desarrollo y adquirir una recaudación tributaria acorde. 

En resumen, la fuerza de los drones proviene de la información cargada de detalles que pueden recopilar a un costo relativamente bajo; incluso pueden crear modelos 3D de buena calidad de calles y propiedades, y acelerar la recopilación de datos. Pero se deben tener en cuenta ciertas debilidades. Los UAV poseen limitaciones en el territorio de cobertura, la velocidad y la autonomía de vuelo. Las condiciones meteorológicas adversas también son un problema importante. 

Hasta ahora, los drones demostraron ser más efectivos en las operaciones urbanas, que suelen requerir muchos detalles y gran cantidad de datos. En toda decisión de lanzar drones para cualquier tarea se deben considerar costos y beneficios. Podría ser suficiente con las imágenes satelitales en alta resolución (que hoy llegan a los 30 cm de resolución); si la zona a sondear se extiende por más de 25 km, los archivos de imágenes satelitales pueden ser más apropiados y eficientes. 

Aun así, los drones ofrecen posibilidades que ninguna otra tecnología aérea de sondeo ofrece, dado su lanzamiento masivo en el mercado. Erba dice: “Los drones democratizarán la recolección y el análisis de la información geoespacial. Pronto, todos tendrán acceso a las herramientas que hace algunos años solo poseían los dueños de satélites. Se podrían enviar fotos a la nube todo el tiempo”. Y destaca que esta nueva habilidad podría fortalecer muchos tipos de transparencia y responsabilidad, y además ofrecer eficiencia al gobierno: “Las fotos aéreas de zonas invadidas o deforestadas tomadas en tiempo real se podrían enviar directamente al funcionario responsable del control urbano. Esta información de suma importancia se puede poner a disponibilidad sin costos para el estado y se puede utilizar de inmediato para entrar en acción”. 

Ya sea que dicha acción sea una aplicación más uniforme de normativas, mejor recaudación de impuestos o datos más abundantes y dinámicos para los registros territoriales, estas tecnologías nuevas se preparan para traer cambios inmensos en varios aspectos de las políticas de suelo de todo el mundo.

John Wihbey es profesor asistente en periodismo y medios nuevos de la Universidad Northeastern. Sus obras y su investigación se centran en asuntos tecnológicos, cambio climático y sustentabilidad.

Fotografía: iStock.com/dabidy

Referencias 

ANAC (Agência Nacional de Aviação Civil). 2017. “Orientações Para Usuários de Drones.” Brasilia, Brasil: ANAC.

Anderson, Chris. 2017. “Drones Go to Work.” Harvard Business Review, 7 de junio. https://hbr.org/cover-story/2017/05/drones-go-to-work.

The Economist. 2017. “Drone Technology Has Made Huge Strides.” 10 de junio. www.economist.com/news/technology-quarterly/21723001-originally-military-technology-drones-are-now-benefiting-rapid-advances.

Erba, Diego Alfonso, y Mario Andrés Piumetto. 2016. Making Land Legible: Cadastres for Urban Planning and Development in Latin America. Enfoque en Políticas de Suelo. Cambridge, MA: Instituto Lincoln de Políticas de Suelo.

Lewis, Jason. 2017. “Striking a Balance on Drone Regulation.” The Hill, 10 de julio. http://origin-nyi.thehill.com/blogs/congress-blog/technology/341300-striking-a-balance-on-drone-regulation. 

Man, Joyce Yanyun. 2012. “China’s Property Tax Reform: Progress and Challenges.” Land Lines 24 (abril): 15-19.

Paneque-Gálvez, Jaime, Michael K. McCall, Brian M. Napoletano, Serge A. Wich y Lian Pin Koh. 2014. “Small Drones for Community-Based Forest Monitoring: An Assessment of Their Feasibility and Potential in Tropical Areas.” Forests 5 (6): 1481-1507.

Totaro, Paola. 2017. “Newest Technologies Becoming Weapons in Fight for Land Rights.” Reuters, 20 de marzo. www.reuters.com/article/us-global-landrights-technology/newest-technologies-becoming-weapons-in-fight-for-land-rights-idUSKBN16R2IE.

Wihbey, John. 2016. “Boundary Issues: The 2016 Atlas of Urban Expansion Indicates Global De-Densification.” Land Lines 28 (octubre): 18–25.

Wihbey, John. 2015. “Agricultural Drones May Change the Way We Farm.” The Boston Globe, 22 de agosto. www.bostonglobe.com/ideas/2015/08/22/agricultural-drones-change-way-farm/WTpOWMV9j4C7kchvbmPr4J/story.html.

Banco Mundial. 2016. “Drones Offer Innovative Solution for Local Mapping” (“Los drones constituyen una solución innovadora para hacer mapas locales”). Washington, DC: Banco Mundial, 7 de enero. www.worldbank.org/en/news/feature/2016/01/07/drones-offer-innovative-solution-for-local-mapping.