Geography Reference
In-Depth Information
FIGURE 8-5 Attribute table of the type IV regions of the
sample raster
If RegionGroup is applied to Basic_Raster, and Type VIII regions are asked for, the result looks like
Figure 8-6. Note that the 13 cells of Zone 23 are divided into only two groups; the region numbers are 4
and 6. (Where there were 17 Type IV region groups before there are now only 15 Type VIII region groups.
Can you see which other region group was affected by including corner neighbors?)
When rasters are composed of integers, those integers generally represent categories. They are quite
frequently just nominal data. It is useful if the categories are present in the table as well, so they can
be used in selection. For example, suppose the Basic_Raster numbers referred to zoning. Perhaps the
categories might be as follows:
21 Heavy Industrial
22 Light Industrial
23 Commercial
24 Parkland
25 Agricultural
26 Multi-Family Housing
27 Single Family Detached
29 Single Family Attached
Then a table might be created that looks like Figure 8-7.
As mentioned, cells may contain NoData. How NoData cells are handled during analysis operations
depends on the particular analysis tool and/or decisions made by the user. Sometimes NoData is
ignored. Sometimes NoData prevents the operation from occurring. There is not such a thing as a
“NoData zone.” NoData is not represented in the attribute table, but when the raster is displayed you can
indicate how you want NoData symbolized.
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