Title: LiDAR Elevation Datasets - Bare Earth Derived Slope, Aspect and Hillshade
Originator: USDA - NRCS
Abstract:
These layers are derived from the one and two meter Light Detection and Ranging (LiDAR) bare earth elevation layers.
Slope is the calculated maximum rate of change in value from a cell to its neighbors. Basically, the maximum change in
elevation over the distance between the cell and its eight neighbors identifies the steepest downhill descent from the cell.
The following two slope products are served:
-Slope Degree is expressed in angular degree of slope. The steeper the slope the higher the number, with zero being flat
and 90 being vertical.
-Slope Percent is percent rise over run expressed as a percentage. For a 45 degree slope, the rise is equal to the run, and
the percent rise is 100.The range of values is negative to essentially infinity. The steeper the slope the higher
the percentage.
Aspect identifies the downslope direction of the maximum rate of change in value from each cell to its neighbors. Aspect can be thought of as the slope direction. The values of the each cell will be the compass direction of the aspect 0-360. Aspect is measured clockwise in degrees from 0 (due north) to 360 (again due north), coming full circle. Flat areas having no downslope direction are given a value of -1.
Hillshade is a grayscale 3D model of the surface, with the sun's relative position taken into account for shading the
image. This function uses the altitude and azimuth properties to specify the sun's position. The properties altitude
and azimuth together indicate the sun's relative position that will be used for creating the 3D hillshade model. These
maps are generated with an azimuth of 315º (North-West) and an altitude of 45º. Cells that are in a shadow of another cell
are coded as zero. All other cell values fall in the range from 1-255. A more complete definition of altitude and
azimuth is the following:
-Altitude is the sun's angle of elevation above the horizon and ranges from 0 to 90 degrees. A value of 0 degrees
indicates that the sun is on the horizon—that is, on the same horizontal plane as the frame of reference. A value of
90 degrees indicates that the sun is directly overhead.
-Azimuth is the sun's relative position along the horizon (in degrees). This position is indicated by the angle of the
sun measured clockwise from due north. An azimuth of 0 degrees indicates north, east is 90 degrees, south is 180 degrees,
and west is 270 degrees.
Format: GeoTIFF 7.5’ quads
Spatial Reference Information: Universal Transverse Mercator, North American Datum 1983
