Chapter 8 Part 4

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Multivariate Dot Density Symbolization

More than two variables can be shown in a dot-density map by making use of two or three of the dimensions used to specify color. Hue and intensity allow three variables to be shown and RGB symbolization allow four variables to be shown.

Ratio Data, Nominal Data and Ordinal Data--Texture, Hue and Intensity

Figure 8.2b DDHI.aml Ratio data displayed with dot density texture, nominal data displayed with hue and ordinal data (ratio meta-data) displayed with intensity.

The use of color intensity can be used to display an ordinal data set, such as meta-data. This can be accomplished by adding intensity to the dot density and hue AML. See DDHI.aml and Figure 8.2b.

DDHI <dot_lookup> <hue_lookup> <intensity_lookup> {dot_size}

<dot_lookup>
specifies shade separation in hundredths of PAGEUNITS
<hue_lookup>
specifies HLS hue (from 0 to 300)
<intensity_lookup>
specifies HLS saturation (from 20 to 100)
{dot_size}
specifies the size of a dot--defaults to 0.03 inches.


Three Ordinal to Ratio Data Sets--Red, Green and Blue Symbolization

Figure 8.2c DDRGB.aml Four ratio data sets, one displayed with dot density texture, one with red, one with green and one with blue. Color data values are linearyly stretched.
Figure 8.2d DDL.aml and RGBL.aml A legend for figure 8.2c. These two AMLs display all of the data shown, without creating a four dimensional display.

Three numeric data sets can be displayed by assigning each to one of the color primaries (red, green and blue). In order to ensure that all data values are represented, DDRGB.aml linearly stretches each of the color data sets, so that each value ranges from 0 to 250, or 0 to 100 for the CMY color model (this may show changes at the lower ranges of the data better than RGB). See Figures 8.2c and d.

DDRGB <dot_lookup> <red_item> <green_item> <blue_item> {r|c} {dot_size}

<dot_lookup>
specifies shade separation in hundredths of PAGEUNITS
<red_item> <green_item> <blue_item>
numeric data fields in the coverage referred to by
<dot_lookup>
{r|c}
selects the Rgb (the default) or the Cmy color model
{dot_size}
specifies the size of a dot--defaults to 0.03 inches.



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