Analyzing the land cover of an urban environment using high-resolution orthophotos

To estimate the impact of light-colored surfaces (roofs and pavements) and urban vegetation (trees, grass, shrubs) on meteorology and air quality of a city, it is essential to accurately characterize various urban surfaces. Of particular importance is the characterization of the area fraction of various surface-types as well as the vegetative fraction. In this paper, a method is discussed for developing data on surface-type distribution and city-fabric (land cover) makeup (percentage of various surface-types) using high-resolution orthophtos. We devised a semi-automatic Monte Carlo method to sample the data and visually identify the surface-type for each pixel. The color aerial photographs for Sacramento covered a total of about 65 km(2), at 0.3-m resolution. Five major land-use types were examined: (1) downtown and city center, (2) industrial, (3) offices, (4) commercial, and (5) residential. In downtown Sacramento, the top view (above-the-canopy) shows that vegetation covers 30% of the area, whereas roofs cover 23% and paved surfaces (roads, parking areas, and sidewalks) 41%. In the industrial areas, vegetation covers 8-14% of the area, whereas roofs cover 19-23%, and paved surfaces 29-44%. The surface-type percentages in the office area were 21% trees, 16% roofs, and 49% paved areas. In commercial areas, vegetation covers 5-20%, roofs 19-20%, paved surfaces 44-68%. Residential areas exhibit a wide range of percentages among their various surface-types. On average, vegetation covers about 36% of the area, roofs about 20%, and paved surfaces about 28%. Trees mostly shade streets, parking lots, grass, and sidewalks. In most non-residential areas; paved surfaces cover 50-70% of the under-the-canopy area. In residential areas, on average, paved surfaces cover about 35% of the area. Land-use/land cover (LULC) data from the United States Geological Survey (USGS) was used to extrapolate these results from neighborhood scales to metropolitan Sacramento. Of an area of roughly 800 km(2), defining most urban areas of the metropolitan Sacramento, about half is residential. The total roof area comprises about 150 km(2) and the total paved surfaces (roads, parking areas, sidewalks) are about 310 km(2). The total vegetated area covers about 230 km(2). The remaining 110 km(2) consist of barren land and miscellaneous surfaces. (C) 2002 Elsevier Science B.V All rights reserved.