Geography Reference
In-Depth Information
1 0 0
7 6 4
7 0 0
0 1 0
6 5 4
0 5 0
0 1 1
5 4 3
0 4 3
Figure 10.1
Retaining grid cells indicated by 1 in a binary grid.
IDW-OK pred. (mm)
Value
High : 35.02
N
Low : -23.59
Inland water
0
20
40 km
Figure 10.2
IDW minus OK predictions of precipitation in July 2006
(both using 16 nearest neighbours).
by any other layer. h e result is that only features in areas of interest are transferred to
the new raster layer as all cells of interest are multiplied by 1, whereas cells outside the
area of interest are multiplied by 0. Figure 10.1 gives a simple example of this
operation.
Figure 10.2 shows the map obtained when IDW predictions of precipitation
(Figure 9.7) are subtracted from OK predictions (Figure 9.13). In this case, the map
algebra command corresponds to OUTPUT = IDWMAP - OKMAP. Burrough and
McDonnell (1998) provide more examples of map algebra.
Positive values indicate that the IDW predictions are larger while negative values
indicate that OK predictions are larger. In other words, where values are positive,
IDW predicts larger precipitation amounts than OK does and, where they are nega-
tive, OK predicts larger precipitation amounts than IDW does. h is kind of approach
of ers a simple means to compare grids. One obvious potential applications area is the
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