Planning Practice Feature

Excising Airports and Railyards for Better Urban Parkland Comparisons

by Peter Harnik and Tim O'Grady

If you want to compare the adequacy of parkland of cities, what are the measures that can be used, and which is best?

Traditionally, the most common measurement used has been acres of parkland per 1,000 persons. The implication of this measure is how well parkland serves the population, specifically how "crowded" each park would be if everyone went out and used the space simultaneously. A second measure is the percentage of a city's land area devoted to parks; it helps to measure how well parkland serves as an environmental resource for the city, capturing stormwater and mitigating the heat island effect. A third method measures the percentage of the population within a half-mile of parkland; that metric is an indicator of proximity of parkland, i.e., within a reasonable walking distance.

If every city had equal population densities, we wouldn't need three measures of parkland-level of service. Population densities of cities are not equal, however, and each of the three measures described above has its own advantages, disadvantages, and implications. To illustrate, consider that New York City is one of the "greenest" cities in the United States when parkland is measured as a percentage of total city land area, at 19.7 percent. But with its high population density, all the parkland (38,201 acres) in New York City provides just 4.7 acres per 1,000 persons. In contrast, Indianapolis appears to be generous in its supply of parkland, providing 13.4 acres of parks for every 1,000 residents, yet only 5 percent of Indianapolis is covered by its 11,203 acres of parks. Considering the third measure, a smaller proportion of Indianapolis residents (31.7 percent) can get to a park within a half-mile (about a 10-minute walk) than residents of New York City (96.4 percent).

Since 2000, The Trust for Public Land's Center for City Park Excellence (CCPE) ("the center") has been calculating and tracking park acreage and many other park attributes in the largest U.S. cities. It took us several years of trial and error to establish accurate figures, and over time the database has become more accurate and reliable. See www.tpl.org/center-city-park-excellence.

In 2010, the center received concerns from park advocates in Denver who contended that our statistics weren't accurately portraying parkland in their city. Considering the first measure, Denver had a respectable statistic of 9.8 acres of park land per 1,000 residents. But as a percentage of city size, the Mile High City measured an unimpressive 6 percent. Moreover, the city itself was characterized by the Trust for Public Land as a "medium-low-population-density" city, comparable to San Diego and St. Petersburg. After study, we determined that a key difference in the two measures could be attributed to Denver International Airport, which was built on land annexed from Adams County in 1989. By annexing 53 square miles of land (22,721 acres for the airport, plus a large buffer), the city instantly mushroomed in total land area by more than 50 percent without adding any new residents (see Figure 1). Statistically, the airport acquisition reduced Denver's citywide density from 8.43 persons per acre (equivalent to Pittsburgh's density) to 6.43 (more like Garland, Texas), yet the residents of Denver didn't experience any real reduction in population density. Statistically, the annexation of the gigantic airport (the nation's largest) would imply that Denver's 5,900-acre park system has become less effective: with the airport annexation, Denver's percentage of total city area devoted to parkland was only 6 percent compared to 8 percent before the airport was annexed.


Denver Park and Airport Annexation
Figure 1 Denver Park and Airport Annexation

In considering the concerns raised about our statistics for Denver, the center staff agreed it didn't make sense for our measures of parkland to penalize cities for airport acreage that would never need any internal parkland. We also decided the same logic holds for urban railyards, or large properties dedicated to freight transport, train storage, and maintenance. Such areas take up large amounts of space but don't contain residents and thus don't require parkland. Using a geographic information system (GIS), we calculated the acreage devoted to air and rail facilities in each of the 100 most populous U.S. cities. Airports were identified using the "USA Airport Hub Size" GIS layer from ESRI, a well-known GIS software company. The results are shown in Table 1. Note that we excluded military bases with airports if there are full-time residents on the premises.  We identified railyards (multiple railroad tracks converging into contiguous sites of train activity) from satellite imagery and then constructed polygons to represent land area for use in the GIS. The results for railyards are shown in Table 2. 

Table 1. U.S. Cities with the Most Airport Acreage

CityNo. of AirportsAirport Acreage
Denver122,721
Houston312,200
Orlando211,008
Jacksonville49,759
Oklahoma City49,197
Chicago26,628
Colorado Springs16,418
Irving, Texas15,801
Kansas City35,176
New York25,151
Indianapolis24,869
Columbus44,868
Anchorage Borough34,787
Los Angeles54,485
Memphis24,260

Source: The Trust for Public Land, Center for City Park Excellence

Table 2. U.S. Cities with the Most Railyard Acreage

CityRailyard Acreage
Chicago2,262
Memphis1,277
Houston1,266
Kansas City1,147
Jacksonville1,025
Fort Worth962
Philadelphia957
Columbus811
Louisville/Jefferson792
Atlanta762
Lincoln735
Portland716
Cleveland673
Detroit645
New York595

Source: The Trust for Public Land, Center for City Park Excellence

From Table 1, it is clear that Denver leads the nation with the most land dedicated to airports. Incidentally, Denver also led the nation with the largest acreage (23,123 acres) dedicated to airports and railyards, combined. For the 100 most populous cities, center staff calculated the percent of total city land area devoted to parkland after excluding airports and railyards from the total acreage of the city. We then compared those results to the data on parkland as a percentage of total city land area. We then sorted the cities according to the greatest positive effect on the parkland measure as a result of excluding air and rail facilities from the city's area calculation.

The results are shown in Table 3. Except for the unusual case of the urbanized portion of Honolulu, Denver led all cities in the United States based on this modified measure. Denver's percentage of total city land area devoted to parkland is 6.03 percent; however, when airports and railyards are excluded, Denver's percentage is 7.89 percent.

Table 3. Parkland as Percentage of City Total Land Area after Excluding Airports and Railyards

CityParkland as a Percentage of Total City AreaParkland as a Percentage of Total City Area, Excluding Airports and RailyardsNet Difference between Measures (Percentage of Area Computed that is Parkland)
Honolulu (urban)31.01%33.05%2.04%
Denver6.03%7.89%1.86%
Oakland16.98%18.27%1.29%
Boston15.91%16.85%0.94%
Orlando4.54%5.45%0.91%
Portland16.63%17.40%0.77%
Lincoln11.05%11.75%0.70%
Irving (TX)4.36%5.04%0.68%
Arlington (VA)12.28%12.86%0.58%
Newark5.47%6.03%0.56%
Chicago8.57%9.13%0.56%
Colorado Springs8.96%9.45%0.49%
Cleveland6.17%6.54%0.37%
Boise5.46%5.74%0.28%
Reno4.54%4.75%0.21%

Source: The Trust for Public Land, Center for City Park Excellence

We contend that removing airport and railyard acreage from cities' total land area gives park planners a more appropriate measure of the proportion of a city that is parkland. The center has therefore modified its most recent analyses contained in 2014 City Park Facts to include an "adjusted city acreage" measure that represents cities' total land area, minus acreage for airports and railyards. We hope our revelation will spark more debate about the ways parkland in cities is measured in the future.

Peter Harnik is director of The Trust for Public Land's Center for City Park Excellence and author of Urban Green: Innovative Parks for Resurgent Cities. Tim O'Grady was a 2013 research intern for the center.