By Charlie Frye, Esri Chief Cartographer
Not long ago we were asked how to create tint bands for boundaries like the ones on National Geographic’s and other national atlas political maps. We didn’t have an immediate answer, but promised to look into it.
Tint bands effectively look like a wide line symbol that is drawn just inside the boundary of a country or state. Some atlases use a wide pastel tint band with a narrower brighter or heavier shade tint band on top of the wide tint band. Tint bands have been used in cartography for centuries, for example this image came from an 18th century map that is available on the U.S. Library of Congress’s American Memory Web site. The tint band is light red and was actually hand-painted onto this map as part of an assembly-line production process.
The tint bands on these types of atlas political maps work well, so it’s easy to think of them as simple and elegant. We soon found these tint bands are, in reality, quite challenging to produce given the array of geometric circumstances (i.e., variations in boundaries) they must accommodate. Using buffers or offset line symbols didn’t work because we got unwanted rounded corners and spikes in unexpected locations; even the representations symbol options available in ArcGIS 9.2, which were better, but still didn’t handle all of the geometric cases necessary for this effect to be achieved the way a professional cartographer would like to see it.
Examples of strategies that didn’t work: spikes and overshoots from offset line symbols and buffers that do not fill all the nooks and niches in the geometry’s perimeter.
Basically we were only thinking of boundaries as vectors and were therefore making no progress. So we started asking around for alternative solutions, and we ended up asked one of the programmers on the Geoprocessing Development Team at Esri. The first thing he asked was whether we were using raster or vector data as the input. From that moment on the solution was obvious; we knew Spatial Analyst has a Shrink tool, which would produce a polygon that could be used by the Erase tool to create a tint band. The only catch was the need to convert the boundary polygons to rasters and then convert the Shrink tool’s results back to polygons.
To do that, and to automate much of the work, we created a Script tool, written in Python, that automated the conversion of the polygons to rasters, the shinking, and the conversion of the result back to polygons. From that point, creating tint bands was an easy 4-step process—two steps for the wide bands, and the same two steps for the narrow bands. Here are the two steps:
- With your boundary polygons as the input, run the Expand or Shrink Polygons tool to create a shrunken version of each polygon.
- Use the Erase tool, with your boundary polygons as the input, and the shrunken polygons as the erase features to produce tint bands.
It’s up to you to decide what colors to use; you can add an attribute and calculate the colors you want, or use the Identity tool to assign the attributes from the original polygons to the tint bands. Here is an example of tint bands produce using this method: