Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 502

Converting Zones to Regions to Find Individual Sites
You noticed from the attribute table that only four numbers appear in the Value column. This means
that all the cells of the areas we are interested in (indicated by Value = 4) reside together in a single zone.
A zone consists of cells that all have the same value. So, the areas that are candidates for the Wildcat
Boat facility are all connected in a way—they are part of a single zone. You may recall that a raster
region consists of one or more sets of disconnected cells that are in the same zone. That is, for a given
zone, regions are clumps of cells that are completely disconnected from other cells in the zone. So it is
the regions of zone 4 that we want as distinct entities that will form the individual sites we need. We can
form these using the Region Group tool in ArcToolbox.
37. Locate the Region Group tool in ArcToolbox. Where is it?
____________ ______________ ________________ ______________
38. While you are in the Spatial Analyst Tools toolbox, count the number of sub-toolboxes in the
Spatial Analyst toolbox. ________. Expand a few of the sub-toolboxes and note the number of
tools. This is to say that this is a big chunk of software, all by itself.
Recall that the term “connected cells” can have two meanings. Cells can be connected by their
sides, in which case each cell has up to four neighbors. Or cells can be considered connected if
either their sides or corners touch, in which case each cell has up to eight neighbors.
The tool lets you add a LINK field to the attribute table of the new raster. The link value is the
old zone value. You will need that, since the candidate areas are all of zone 4 and you need to
be able to distinguish them in the resulting raster attribute table.
40. For number of neighbors to use, select EIGHT. Leave Add Link Field To Output checked. Click OK.
41. RG_WC_Boat will be added to the map. It consists of dozens of distinct zones, formed by all the
regions of all the zones in SUM_LuSoStSw. Observe the map. Open the attribute table of RG_WC_
BOAT. Notice, by looking at Count, that some of these zones are quite small; remember that each
cell contains only 4 square meters. Notice that the Link value in each record is 1, 2, 3, or 4.Sort
the table by LINK value, descending. Select the records with LINK value 4. Observe the map.
As a last step, to isolate the sites, you will use the ArcToolbox command Extract by Attributes.
You will want the Link value to be 4. You will also want the areas to be at least 2000 square
meters. How many cells would 2000 square meters be? ________
42. Clear any selections you may have made and close the attribute table. Locate the ArcToolbox
command Extract by Attributes. Where is it?
________________________.________________________.________________________
43. Start the tool. The Input raster should be RG_WC_Boat. Browse to make the Output raster
FinalSites_SA in Wildcat_Boat_SA.mdb . For the Where clause, press the SQL (Structured Query
Language) button and construct the following expression:
“LINK” = 4 AND “Count” >= 500
and click OK in Query Builder window.