Geography Reference
In-Depth Information
32
64
128
16
1
8
4
2
FIGURE 8-22 Elevation values in a tiny
raster
FIGURE 8-21 Direction
code for the flow of water
FIGURE 8-24 Flow allowed directly off
edge cells of the raster
FIGURE 8-23 Flow direction following
steepest slope
The range in altitude is from 100 to 91, sloping gradually from east to west and a bit from north to south.
When the Flow Direction calculation is applied to this raster, the resulting raster looks like Figure 8-23.
Note that water flows from each cell to the nearest neighbor cell so that the water flows down the
steepest slope, except from the cell with lowest elevation in the southwest, where it flows off the raster.
Normally, cells along the edge of the raster are treated as any other cells in the raster, except that if none
of the five adjacent edge cells have lower elevation than the edge cell under consideration, the flow
will be directly off the side of the raster. But there is an option (force all edge cells to flow outward),
which shoves water off the raster, even if edge cells have lower elevation neighbors. Thus, the raster in
Figure 8-24 would be generated instead.
The Ultimate Destination of Water Is off the Raster Area
The lowest point on the raster must be on an edge. This requirement is not as stringent as it sounds.
Basically, you want to deal with land that has no ponds or lakes. You want a network of valleys that will
hold only linear bodies of water, at least one of which will flow off the edge. As already indicated, the
ArcGIS hydrologic tools presented here do not work with lakes. They are strictly for stream networks.
Lakes, which would constitute sinks, are not allowed.
It is worth remarking on the rather strange choice of numbers used to indicate flow direction. You've
learned that water flows from any given cell to one of the eight adjacent cells. In the previous exercise on
proximity, the directions were indicated simply by the integers 1 through 8. Why, then, are we dealing
with numbers such as 32 and 64?
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