Ternary Legend

After creating a ternary image in a map, you will most likely want to add a ternary legend. A ternary legend is a triangular graphics element that indicates the colours assigned to each variable on the ternary image. Use the Grid and Image > Display on Map > Ternary Legend menu option (TERNLEG GX) to create a ternary colour legend.

Ternary Legend dialog options

Colour model

Select the colour model: "CMY", "RGB", "CMY inverted", or "RGB inverted".

Script Parameter: TERNLEG.MODEL

Red-Cyan label

Indicate the colour label for the red or cyan corner of the legend.

Script Parameter: TERNLEG.LABEL1

Green-Magenta label

Indicate the colour label for the green or magenta corner of the legend.

Script Parameter: TERNLEG.LABEL2

Blue-Yellow label

Indicate the colour label for the blue or yellow corner of the legend.

Script Parameter: TERNLEG.LABEL3

Legend size (mm)

Specify the size of the legend triangle in mm.

Script Parameter: TERNLEG.SIZE

[OK]

Click the OK button and the Locate Legend prompt will be displayed enabling you to interactively select the location for the bottom center of the legend triangle.

Locate Legend

Click the OK button and your cursor will be changed to a cross-hair. Click on the map to define the location for the bottom center of the legend triangle (in map mm), or use the parameters below when running the GX as a script.

Script Parameter: TERNLEG.X

Script Parameter: TERNLEG.Y

Application Notes

Ternary legends attempt to display a 3-dimensional combination of colours in two dimensions. At each of the corners, the primary colours are displayed at full intensity, without blending from the other colours. At the opposite edge from each corner, that colour’s intensity drops to zero.

Our ternary legends are designed so that at the central "spot", the colour intensity is one-half for each colour, e.g. RGB = (127, 127, 127). The colour intensities increase linearly to this spot (which is 1/3 of the way to the opposite corner, and continue increasing at the same rate to reach 255 (the maximum) at a point 2/3 of the way to corner. From this point to the corner, the colour is at its full intensity with diminishing quantities of the other two colours. This convention means that the central spot "grey" is the same colour for both CMY and RGB diagrams.