GIS software, calculating slope from a
Digital Elevation Model, will use a
matrix of 5 points to determine slope at each point. For example, for the points
___________________
| | | |
| x | 500 | |
|_____|_____|_____|
| | | |
| 500 | 700 | 900 |
|_____|_____|_____|
| | | |
| y | 900 | |
|_____|_____|_____|
the GIS software would see that the average difference in
elevation between the center point and those surrounding it
(I made each difference 200 meters here for simplicity) was 200 meters. It would then divide that by the distance between the points—the
raster cell size—Imagine the cell size is 600 meters, and you get a (200m/600m=)33% slope, quite
steep. Important to note is that the slope is attributed to the
center point. To calculate, for example, the slope of the bolded 500-meter point on the left, one would compare it to a different 5-cell matrix centered on it, along with cells
x and
y, the 700-meter high cell, and a cell to the left not pictured on the matrix shown here.
cf. aspect