In
geography, "aspect" refers basically to the
direction that a
slope is facing. For example, the southern side of a
mountain would have a southerly aspect, while the southern side of a
gorge running east-to-west would have a northerly aspect; that is, the slope would face north. Aspect is important for a number of reasons: it can determine
weather and
climate for a given location. South-facing slopes are likely to be warmer than north-facing slopes; west-facing slopes (in the
temperate zone) will be wetter than east-facing slopes; winds will hit different slopes differently; &c.
Aspect can usually be one of 9 categories: the four cardinal directions, the four in-between (southeast and the like), and flat.
GIS software will calculate aspect from a Digital Elevation Model file using a matrix of five points. For example, given these different sets of raster cells:
___________________ ___________________ ___________________
| | | | | | | | | | | |
| | 500 | | | | 500 | | | | 500 | |
|_____|_____|_____| |_____|_____|_____| |_____|_____|_____|
| | | | | | | | | | | |
| 500 | 700 | 900 | | 500 | 700 | 500 | | 500 | 900 | 500 |
|_____|_____|_____| |_____|_____|_____| |_____|_____|_____|
| | | | | | | | | | | |
| | 900 | | | | 900 | | | | 500 | |
|_____|_____|_____| |_____|_____|_____| |_____|_____|_____|
one would see that in the first set, the center point (which aspect is being calculated for) is facing north-west. To go through the calculation, we see that the western point is 200 feet lower than the center point, which is 200 feet lower than the eastern point. Thus, the center point "faces" west. We also see that the northern point is lower than the center point, which is lower than the southern point; thus, the center point also faces north. Together, it faces to the northwest.
For the second set of points, the northern point is lower than the center point, and the center point is lower than the southern point. Thus, the center point faces north. But both the eastern and western points are lower than the center point, and both by the same amount. While the center point is certainly not flat in the east-west axis, nay, it is a peak in that axis, it is deemed "flat" for purposes of aspect; that is, it faces neither east nor west. Thus, the aspect for the second center point is northerly.
For the third set of points, we see that the difference between the center point and all those surrounding it is the same. Again, while it is not exactly "flat" but a peak, the third point faces not north, east, south, or west, but straight up. Thus, the center point of the third matrix is deemed to have a "flat" aspect.
cf. slope