Chapter 3 Part 4

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Multivariate Symbolization

Multivariate point symbolization can be accomplished by several means, each of which is suited to various combinations of data levels. Because of the increased complexity involved in multivariate mapping, care must be taken to insure legends are well designed and convey the methods that should be used in interpreting map symbols.

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

Figure 3.6a PRGB.aml Three ratio data sets displayed by linearly stretching the data sets from 0 to 255 and using data set one for red, two for green and three for blue.

The 'false color' images that are often generated with satellite derived data use red, green and blue to symbolize data values from three spectral bands. This technique is applied here to allow display of three data values. See Figures 3.6a and RGBL.aml for a legend. Note that the AML (PRGB.aml) performs a linear-stretch on the items in the Point Attribute Table. This AML also allows color specification as cyan, magenta, and yellow. This can be helpful because this color scheme tends to allow numbers in the low end of the data range to be distinguished more readily than the RGB scheme.

PRGB <cover> <red_item> <green_item> <blue_item> {markerset} {markersymbol} {markersize} {point|node} {r|c}

<cover>
the coverage to be displayed
<red_item> <green_item> <blue_item>
numeric items in the attribute table of <cover>
{markerset} {markersymbol}
specify which symbol will be drawn with--defaults to current
{markersize}
specifies the size of the marker--defaults to 0.15 inches
{point|node}
symbolize point (the default) or node features
{r|c}
display with the RGB (default) or CMY color system.


Nominal Data, Ratio Data and Ordinal Data--Shape, Size and Value

Figure 3.6b PSZV.aml A nominal data set displayed with shape, a ratio data set displayed with size, and an ordinal data set (size meta-data) displayed with color value.

Shape can be used to display nominal data, and size can be used to display ratio data. This AML (PSZV.aml) adds to this bivariate representation by allowing color value to be used to display an ordinal data set. Color value can be used to display uncertainty on monochrome output devices, such as laser printers. The calculation of symbol size is discussed on page 39; see Figure 3.6b.

PSZV <shape_lookup> <size_item> <value_lookup> {markerset} {size_exponent} {size_factor} {minimum_size} {hue} {intensity}

<shape_lookup>
specifies marker symbol numbers
<size_item>
a numeric data item in the coverage referred to by <shape_lookup>
<value_lookup>
specifies HLS lightness data (from 0 to 100)
{markerset}
a markerset for shapes--defaults to the current markerset
{size_exponent} {size_factor} {minimum_size}
define the scaling of sizes; defaults: size_exponent = 1; size_factor = 0.15; minimum_size = 0.02 inches
{hue} {intensity}
define a color that will be value shaded.


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

Figure 3.6c PHIZ.aml A nominal data set displayed with color hue, ratio data displayed with size, and an ordinal data set (size meta-data) displayed with color intensity.

Like the previous AML, size, hue and intensity should be used to display a nominal data variable, a ratio data variable, and an ordinal data variable (meta-data for the ratio data variable would be appropriate). This AML however uses color hue and intensity, and therefore requires full color displays. See Figure 3.6c and PHIZ.aml.

PHIZ <hue_lookup> <intensity_lookup> <size_item> {markerset} {markersymbol} {size_exponent} {size_factor} {minimum_size}

<hue_lookup>
specifies HLS hue data (from 0 to 360)
<intensity_lookup>
specifies HLS saturation data (from 0 to 100)
<size_item>
a numeric data item in the coverage referred to by the lookup tables:
{markerset} {markersymbol}
specify a marker--the current marker is the default
{size_exponent} {size_factor} {minimum_size}
define the scaling of sizes; defaults: size_exponent = 1; size_factor = 0.15; minimum_size = 0.02 inches.


Ratio Data--Point Pie Graphs

Figure 3.6d PP.aml Ratio data displayed with size; each pie slice is a nominal difference within the ratio whole--pie slice values are percentage of the whole.

This AML generates pie symbols for point data by use of the POINTSPOT command. The type of data that should be used in the creation of this type of map is a size value that represents a sum of several other values; these other values should have nominal distinctions. POINTSPOT uses the sum value to calculate the size of the circle, and it calculates the pie slice size that will be drawn as a function of the ratio a sub-value to the whole. Colors for each slice are calculated by the AML. See Figures 3.6d and 6.3d for a legend, and PP.aml.

PP <cover> <point|node> <size_item> <minimum_size> <maximum_size> <number_of_subitems> <item_1...item_n>

<cover>
the coverage to be displayed
<point|node>
symbolize points or nodes
<size_item>
a numeric item in <cover> that specifies the size of the circle
<minimum_size> <maximum_size>
the smallest and largest circle sizes
<number_of_subitems>
the number of pie slice data items--the value of n, next
<item_1...item_n>
names of numeric items in <cover> that specify the size of pie slices.



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