Analysis of grids and surfaces - Spatial Data Analysis: An Introduction for GIS Users - page 158

1

45 44 44 43 42

43 42 39 36 41

2

38 32 34 35 38

39 41 37 38 39

3

32 41 35 31 39

Values Zones

43.286

36.667

37.083

Zonal means

Figure 10.3 Zonal mean.

Spatial fi lters

10.4

h e idea of moving windows for processing images was introduced in Section 4.6.
Such operations are called spatial i lters. h e application of a mean i lter, as illustrated
in Section 4.6, results in a smoothed version of the original image. In contrast, applica-
tion of a standard deviation i lter will result in a new image that highlights the edges of
features in the original image. Figure 10.4 shows an input grid (the same one as used
in Section 4.6) and the output of a mean i lter and a standard deviation i lter.
Comparison of the values of the standard deviation output in Figure 10.4 with the
input grid shows how areas with more variable values have larger standard deviations.
As with the example in Section 4.6, the edge pixels are not included in the outputs.
Figure 10.5 shows elevations in Northern Ireland. Figure 10.6 shows the mean of
elevation for a 3 ¥ 3 pixel moving window. Comparison with the original elevation
map shows that the range of values in the mean map is smaller than in the original
map as the ef ect of outliers (values that are large or small locally) is reduced through