Multivariate Symbolization
Multivariate choropleth maps can expand on some of the bivariate mapping techniques in order to allow meta-data to be presented with data, in an otherwise bivariate map. Multivariate maps can also be used to present more than two data sets to the map reader.
Two Nominal or Ordinal Data Sets, Ordinal Data Sets--Texture with Value
Figure 5.7c: CTLVV.aml Two classed ratio (ordinal) data sets displayed with texture with meta-data for each data set displayed by using color value.
For monochrome displays, intersecting lines allow control of texture to convey a data value. By adding control of line value, meta-data can be presented along with the two primary data sets. This AML requires two lookup tables with values that will be divided by 100 to set texture and two lookup tables with HLS value data (0 to 80). See Figure 5.7c and CTLVV.aml. (The legends are generated with CBL.aml and AL.aml).
CTLVV <horizontal_lookup>
<horizontal_value_lookup> <vertical_lookup>
<vertical_value_lookup> {hue} {intensity}
{line_size}
<horizontal_lookup>- specifies line separation in hundredths of PAGEUNITS
<horizontal_value_lookup>- specifies HLS lightness (from 0 to 80)
<vertical_lookup>- specifies line separation i hundredths of PAGEUNITS
<vertical_value_lookup>- specifies HLS lightness (from 0 to 80)
{hue} {intensity}- specify a hue to be value shaded
{line_size}- specifies line width--defaults to 0.03 inches.
Three Ordinal to Ratio Data Sets--Red, Green and Blue Symbolization
Figure 5.8a: CRGB.aml Three ratio data sets displayed with red, green and blue color ranges. This is the 'false color' imagery often used with satellite data.
Figure 5.8c: CRGB.aml Three ratio data sets displayed with cyan, magenta and yellow color ranges. This color model shows variability at the low end of the data better than RGB.
Three ratio data sets can be displayed by scaling the data to the range of the RGB color system (0 to 255) and then using the data values to specify color. This AML requires three numeric data items in the polygon attribute table; a switch can be specified to direct the AML to use the CMY color system, which can represent data values at the low end of each range better than the RGB system. See Figures 5.8a and 5.8c, and CRGB.aml.
CRGB <cover> <red_item>
<green_item> <blue_item> {r|c}
<cover>- the polygon coverage to be displayed
<red_item> <green_item> <blue_item>- three numeric data fields in the Polygon Attribute Table of <cover>
{r|c}- Rgb or Cmy color space.
Multivariate Legends--RGB Space
/ Figure 5.8b: RGBL.amlA legend for figure 5.8a. The AML finds the minimum and maximum data values, and displays these along with the colors at the corners of the color cube.
Figure 5.8d: RGBL.aml A legend for figure 5.8c. The AML increases the size of the front face of the cube, and the front four circles, in order to enhance the legend's '3-D' appearance.
Because the specification of color requires three dimensions, development of a legend is difficult--it must appear to be three dimensional. This AML takes the same three numeric data items used for the previous AML and displays the colors and data values associated with the corners of the color cube. The AML will also work with points, lines and grids. See Figures 5.8b and 5.8d, and RGBL.aml.
RGBL <cover> <point|line|poly|grid>
<red_item> <green_item> <blue_item>
<start_x> <start_y>
<size_x> <size_y> {r|c} {textset} {font}
{point} {floating_point_precision}
<cover>- the coverage that contains the data
<point|line|poly|grid>- the type of <cover>
<red_item> <green_item> <blue_item>- numeric fields in the attribute table of <cover>
<start_x> <start_y>- the lower left corner of the back plane of the legend
<size_x> <size_y>- the size of the back plane, in PAGEUNITS
{r|c}- Rgb or Cmy color space
{textset} {font} {point}- define the labelling text--defaults to a 10 point roman font
{floating_point_precision}- the number of decimal places to be shown--defaults to 2.