Monovariate Maps
Monovariate symbolization for graduated symbols is based on the manipulation of size to specify data. This makes graduated symbols appropriate for ordinal to ratio data.
Ordinal to Ratio Data--Graduated Circles
Figure 6.1a: GC.aml Discontinuous and abruptly changing, ratio area data displayed with graduated circles.
This AML (GC.aml) uses the commands SPOTSIZE and POLYGONSPOT to generate a graduated circle map. Because SPOTSIZE requires the minimum and maximum data values, the AML searches the data set for the extremes, and then displays the data. See Figure 6.1a.
GC <cover> <size_item> {minimum_size}
{maximum_size}
<cover>- the polygon coverage to be displayed
<size_item>- a numeric data field in the attribute table of <cover>
{minimum_size}- the circle representing the smallest data value--defaults to 0.05 inches
{maximum_size}- the circle representing the largest data value--defaults to 0.5 inches.
Ordinal to Ratio Data--Cartograms
Figure 6.1b: GG.aml Ratio area data displayed with cartograms. This shows the AML's defaults--note the extremely small sizes and the misplaced polygons.
There are several varieties of cartograms; this AML simply scales a polygon be a data value, and places the scaled polygon in the center of the original polygon's extents. Because this does not handle unusually shaped polygons well, the AML allows the scaled polygons to be repositioned. Since the representation of data in cartograms is done with the polygon outline, cartograms are best used to display density data (the AML calculates density from the data item and the polygon size). See Figure 6.1b and GG.aml.
GG <cover> <size_item>
{scale_factor}
<cover>- the polygon coverage to be displayed
<size_item>- a numeric data field in the attribute table of <cover>
{scale_factor}- specifies the exponent used to calculate cartogram size--defaults to 1.