Chapter 7

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For data that falls in the middle of the Continuous--Discrete range and in the middle of the Abrupt--Smooth range (see Figure 1.1), a grid cell representation should be used. This is accomplished by converting a polygon coverage into a grid data layer; if necessary, smoothing the resulting grid; then displaying the grid. POLYGRID is an ARC command that converts a polygon coverage into the GRID cell format.

Conversion of Polygons to Grids

POLYGRID is an ARC command that converts polygon coverages into GRID cell format. This is best for nominal data, or other polygon data that needs to retain the shape of the original polygon. For ratio data, the GRID operators:IDW, SPLINE and TREND allow conversion of polygons to grids. These operators take a point file (polygon centers can be used) which also have a `Z' value and calculate intermediate values, in order to populate the resulting grid. IDW is the inverse distance weighted routine; this is quick, but it may smooth the data by eliminating extreme values. SPLINE retains extremes, but requires more processing time. TREND calculates an overall trend for the input data. POLYGRID.aml links the ARC command to ARCPLOT. An ARC version of the ARCPLOT AML is called POLYGRID.

POLYGRID <in_cover> <data_item> <out_grid> {resolution} {p|l|i|s|t}

<in_cover>
the input polygon coverage
<data_item>
a numeric data field in the attribute table of <in_cover>
<out_grid>
the resulting grid
{resolution}
cell resolution--defaults to one hundredth of the smaller axis
{p|l|i|s|t}
the method used to generate the grid--defaults to p: Polygon, poInt, Idw, Spline, Trend

Lookup Tables for Ratio Grids

Because of the impracticality of calculating optimal breakpoints for ratio grid layers, this AML (GRIDLUT.aml) only generates equal interval, or mean and standard deviation classifications. These are based on the grid statistics table, rather than calculated directly from the data set. Lookup tables for nominal grids can be generated with SETMAN.aml.

GRIDLUT <grid> <lookup_name> <classes> {interval|standard_deviation}

<grid>
the name of the grid layer
<lookup_name>
a name for the resulting lookup table
<classes>
the number of classes in the resulting lookup table
{interval|standard_deviation}
the classification method: equal Interval (the default) or Standard deviation