Other important visual variables

Shape. Objects with different shapes are presumed to be different in kind. A star as a symbol for a capital city indicates that it is different, not necessarily larger or smaller, than a city represented by a circle or a dot. A single-dashed line indicates a border of a different kind than a solid line or one that is double-dashed.

Texture. Like shape, a texture or pattern difference, in general, implies no inherent order. Overlaying a semi-transparent textured pattern over a solid color is an effective way of visualizing multiple area variables like forest cover and ownership. Textures include, but are certainly not limited to, cross-hatching, rows of dots or other shapes, and "brick" patterns. The swamp icon on U.S.G.S. topographical maps is an example of the use of texture to represent an area feature.

Density. The density of a texture can represent ordered data to a limited extent. A semi-transparent overlay of a dot pattern that varies in density can be used to effectively represent such information as levels of an airborne toxin or relative uncertainty of the data "underneath" the pattern.

Size. Not surprisingly, size is a very important visual variable for ordered data. As in proportional symbol maps, the relative size of icons gives the viewer a qualitative estimate of the relationships of the attributes of the spatial units in the display.