GEOG 480
Exploring Imagery and Elevation Data in GIS Applications

Slope, Aspect, and Hillshade


Slope is the steepness or the degree of incline of a surface. Slope cannot be computed from the lidar points directly; one must first create either a raster or TIN surface. Then, the slope for a particular location is computed as the maximum rate of change of elevation between that location and its surroundings. Slope can be expressed either in degrees or as a percentage. Aspect is the orientation of slope, measured clockwise in degrees from 0 to 360, where 0 is north-facing, 90 is east-facing, 180 is south-facing, and 270 is west-facing.

Diagram of Slope Calculations. See surrounding text and see text description in link below
Slope Calculations
Click here to see a text description of this image.
There is an image of a long, but short triangle depicting how to calculate values related to slope. Along the bottom, it says Run change in x is 125 feet. Along the height of the triangle it says rise change in z is 15 feet. Percent slope is rise over run times 100. In this case, 15 divided by 125 times 100 is 12 percent. Degree of slope is arctan of rise over run, which is 6.85 degrees.
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Hillshading is a technique used to visualize terrain as shaded relief, illuminating it with a hypothetical light source. The illumination value for each raster cell is determined by its orientation to the light source, which is based on slope and aspect. In the lab activity, you will experiment with placement of the light source, but, for now, it will suffice to say that positioning the light source in the northwest works best to simulate a natural landscape to the human eye. Depending on your application, you might also want to simulate the true position of the sun at a particular date and time of year.