Impervious surface data are fast becoming popular datasets for analysts to use in all kinds of computations from measuring growth over time to determining hot spots to identifying illegal building activities. Showing them on a map is not a difficult task, although in a highly populated area, you may find that impervious surface pixels or polygons tend to overwhelm the map if there are other details that you wish to highlight as well. Some impervious data are presented as Boolean pixels: a pixel either is or is not impervious surface. Other impervious data represent a range of imperviousness within the pixel, usually in categories such as zero to 10 percent, 10 to 50 percent , and 50 to 100 percent. Boolean data are displayed as solid color pixels, usually in a dark gray, black, or red. Range data are displayed color ramps of gray or some other color. Ancillary data will help the viewer to understand the data better if the viewer is not immediately familiar with impervious surface data. For example, having a roads layer or parcel layer underneath the impervious pixels allows the viewer to see that the impervious pixels overlap with the roads and rooftops. If the data are at a small scale, then country boundaries and city points will help the viewer see that impervious surfaces are concentrated in cities and highly developed countries. (See Figure 6.84 for color suggestions.) When displaying a series of impervious surface maps in order to illustrate change over time, my recommendation is to get rid of all superfluous details. Think very carefully about each and every element on the map element to decide whether it can be taken off. All the scales will match, of course, so the scale bar can be placed in the margins of the map page. The north arrow is another element that will be the same regardless, so it can be relegated to the margins as well. Most importantly, see if you can get rid of the map frame. Your map viewer will then focus on the message — that impervious surfaces are increasing in certain areas over time, for example — rather than the graphic elements that only serve to clutter up the page (see Figures 6.85 through 6.89 for examples how to depict this feature in certain situations).