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 km², 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 km², defining most urban areas of the metropolitan Sacramento, about half is residential. The total roof area comprises about 150 km² and the total paved surfaces (roads, parking areas, sidewalks) are about 310 km². The total vegetated area covers about 230 km². The remaining 110 km² consist of barren land and miscellaneous surfaces.