Topography

Elevation data sets and their derived products (Table 2) are used extensively in ecological modeling. Digital elevation data are available as digitized points or contour

Table 2 Products commonly derived from digital elevation models (DEM)

Slope steepness Slope aspect

Hillshade and perspective views Viewshed/line of sight

Topographic features Ridges Peaks Channels Pits Passes Plateaus

Hydrologic parameters Flow direction Flow accumulation Predicted watercourses Watershed boundaries lines, triangulated irregular networks (TIN), and as gridded surfaces or images.

When using elevation data it is important to know what the elevation values represent. These values can represent the surface of the bare Earth, the surface of the features on the Earth (i.e., top of the canopy), or somewhere in between.

A relatively new topographic data set is the Shuttle Radar Topographic Mission (SRTM) 'digital elevation model' (DEM). The resolution of this data set is 30 m and it covers land areas between 56° south latitude and 60° north latitude. It has been degraded to 90 m for most areas of the world outside of the United States. The data for the SRTM DEM were collected using an interferometric radar instrument mounted on the Space Shuttle. Over forested areas the elevation value provided by the SRTM data set represents a point somewhere roughly half-way between the ground surface and the top of the canopy. The exact point depends on the structural characteristics of the forest stand.

Lidar is increasing being used to collect elevation data with vertical accuracy on the order of centimeters. Lidar instruments are flown on aircraft and are routinely used for monitoring coastal areas. The accuracy and speed of lidar elevation data collection is unmatched by other methods.

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