Bivariate and Multivariate Symbolization
As with single variable displays, bivariate displays are limited to the control of color. This is best accomplished by use of hue and intensity. Three variables can be represented in grid cell format by use of Red, Green and Blue encoding. This is the method used to generate color satellite images, for example.
Nominal Data, and Ordinal Data--Hue and Intensity
Figure 7.1c: RHI.aml Nominal data displayed with hue and ordinal data displayed with intensity (compare figure 5.5a).
Hue should be used for nominal data (with a lookup table generated by SETMAN.aml). Intensity should be used for ordinal data (with a lookup table generated by GRIDLUT.aml). Legends can be created with CBL.aml. See Figure 7.1c and RHI.aml.
RHI <hue_lookup> <intensity_lookup>
{nominal_vat_item} {ordinal_vat_item}
<hue_lookup>- specifies HLS hue (from 0 to 300)
<intensity_lookup>- specifies HLS saturation (from 20 to 100)
{nominal_vat_item}- specifies an item in the hue grid attribute table other than VALUE
{ordinal_vat_item}- specifies an item in the intensity grid attribute table other than VALUE.
Three Ordinal to Ratio Data Sets--Red, Green and Blue Symbolization
Figure 7.1d: RRGB.aml The ratio data sets displayed with red, green and blue.
This AML takes three grids, assigns the first to red, the second to green, and the third to blue. Each grid is linearly stretched from 0 (no color) to 255 (maximum color), and displayed. Legends can be created with RGBL.aml. See Figure 7.1d and RRGB.aml.
RRGB <red_data_grid> <green_data_grid>
<blue_data_grid>
<red_data_grid>- a grid that will be displayed in shades of red;
<green_data_grid>- a grid that will be displayed in shades of green
<blue_data_grid>- a grid that will be displayed in shades of blue.