Spatial Analysis Based on Raster Data Processing - Introducing Geographic Information Systems with ArcGIS - page 510

has “been invented” by at least three different people (and was used informally by Descartes), giving
rise to Thiessen polygons, Dirichlet domains, and Voronoi cells. The polygons tessellate a portion of the
Cartesian plane. Unlike the procedures used above, the value at the point has no bearing on the size,
shape, or extent of the area around it. Only the position of the point is important.
Making Thiessen polygons is a little more involved than making the other rasters we have used, because
they expand the extent of the raster area. There are ways to correct this, but we will use a “workaround”
to accomplish the same thing. This will (a) make the polygons, and (b) acquaint you with the idea of
workarounds which is a handy frame of mind to be in when dealing with today's very complex and not
particularly robust software.
We will first use “Only Points” to make a shapefile. Then we will convert that shapefile into a raster.
20. In ArcMap start a new blank map. Add as data Only_Points.shp. Label the points layer with
the FID field. Open the attribute table. (You will find a field there named Population. If you
try to delete it you will be told NO and given the reason that comes down to Tables must
have at least three fields. Whatever.) With Search, find the Create Thiessen Polygons tool.
Where is it? _______________________________
Start the tool. For Input Features click the drop-down menu arrow, and you should see the
shapefile Only_Points. Click it. For the Output feature type browse to the Experiment_with_
Interpolation folder in the Look in box and, in the Name box type T_D_V.shp - noting that you are
making a shapefile, not a raster, and that it is in the folder that is one level up from the Surfaces
file geodatabase. Accept ONLY_FID as the output field. OK. Observe the lower right of screen for
the progress and completion of the execution of the tool. The shapefile will be added to the map.
21. Make sure that Only_Points is at the top of the T/C. Label the T_D_V layer with its Input_FID
field with text size 10 points, color red. Click the Full Extent button and examine the shapefile
display.
22. Right-click the display and, in Data Frame Properties, make the Map and Display units both
kilometers. Use the Measure tool to convince yourself that the lines separating the polygons lie
halfway between pairs of adjacent, generating points, by measuring from the first point to the
second point, clicking, and then measuring back to the line. The segment distance should be
about one-third the total distance.
23. Use Polygon to Raster to convert the shapefile to a raster. Where is the tool? _______________
Start the tool. Convert the shapefile T_D_V to the raster Thiessen_Dirichlet_Voronoi with
the following parameters: Value field should be Input_FID. Cell assignment type should be
MAXIMUM AREA. Set the Cellsize to 5.0. Make sure the Thiessen_Dirichlet_Voronoi raster goes
into the Surfaces.gdb file geodatabase.
24. Make sure the point feature class is at the top of the T/C. Turn off the T_D_V shapefile,
leaving the Thiessen_Dirichlet_Voronoi raster on. Go to full extent. Use the Identify tool on
the raster to verify that the raster zone number (its Input_FID) is the same as the point value.
Bearing in mind that the cell size is 5 kilometers on a side, how large is the area designated
as zone 9? ___________ square kilometers.
25. Close ArcMap.