dasymetric

A method of thematic mapping which attempts to portray a geographic distribution as a collection of homogenous areas, but also attempts to place the boundaries between those areas at their actual locations on the surface of the Earth. A dasymetric map can be used to show both quantitative (aka "volumetric") distributions and non-quantitative distributions.

Unlike a choropleth map, boundaries of the areas being shown do not follow collection unit boundaries. Unlike an isopleth map, a dasymetric map does not assume that the distribution surface (conceived by treating the data values at any point as a third dimension) is smooth.

In the early 1930's, American geographer John Kirtland Wright wanted to make a population density map of Cape Cod. He had a collection of population data by town, the only meaningful political unit in the area. Wright could have made an ordinary choropleth map: he could have divided the population of each town by its area, and filled in each town on a town boundary map with a shade matching that town's density, darker shades portraying more dense areas.

But Wright was not satisfied with such a technique. Cape Cod is long and narrow, and most towns stretch from the Massachusetts Bay side to the Atlantic Ocean side. At the time, the Cape contained large areas sparse population next to areas of very dense population. A choropleth map would have not been very informative about Cape Cod's actual population distribution.

If Wright had relied on his town population data alone, he would have had no choice but to make a choropleth map. A line drawn on a map implies that the cartographer has a reason for drawing it there. And so, with only town data, Wright would have had to make his areas follow town boundaries. Area boundaries anywhere else would have been a deception.

In order to make a finer map, Wright had to acquire more data. In this case, Wright used a land use classification map of Cape Cod to identify likely areas of denser population and likely areas of sparser population. Wright coined the term "dasymetric" to describe his new technique in the 1933 paper¹ he published on the subject.

Dasymetric maps are an important tool of cartography, and can show distributions to a high level of precision. However, they are much more costly to produce than choropleth and isopleth maps. The problem lies in acquiring enough ancillary data to justify the technique, then interpreting that data, and combining it with the geographical distribution you wish to show. Special care must be taken to avoid creating a map of only the ancillary data.

The following three diagrams attempt to map the same made-up distribution, one using a choropleth technique, one using a dasymetric technique, and one using an isoplethic technique.

Choropleth:

.-------------------------------------------------.
|*****|::::::::|*******|:::::::| . . . |::::::::::|
|*****|::::::::|*******/::::.--'. . . .`--.::::::"|
|-----|::::.---'-.****|:::::|. . . . . . .|-------|
|. . .|::::|#####\****|:::::/------+------|. . . .|
| . . `-.--'#####|****|----|:::::::|******| . . . |
|. . . .|#########`---|:::::\::::::|******/. . . .|
| . . . |#############|::::::\:::::|*****/. . . . |
|. . . .|-----.-------|:::::::\::::|*****\ . . . .|
| . . . /:::::|*******|::::::::\---'---.* \ . . . |
|. . . /::::::|*******|::::::::/::::::::\* \ . . .|
| . . /:::::::|*******|:::::::/::::::::::|-'\ . . |
| .  /::::::::|*******|::::::/::::::.----'. .\ . .|
|---/:::::::::|*******|:::::/:::::::|. . . . .\ . |
|  /----------|*******|----+---.::::| . . . . .`--|     
| / . . . . . |*******|. . |. . \:::|. . . . . . .|          
|/ . . . . . .|---.---' . .| . . `--| . . . .-----|
| . . . . . . |:::|. . . . |. . . . |-------|     |
|. . . . . . .|:::| . . . .| . . . .|       |     |
`-------------------------------------------------' 

Dasymetric:

.-------------------------------------------------.
|*******. . . . :::::::::::                    ## |
|******* . . . .::::::::::: . . . . . . . . . . . |
|*******:::::::::::::::::::::::::. . . . . . . . .|
|*****::::::#########***::::::::::. . . . . . . . |
| . . . . .##############::::::::::::::::::::. . .|
|. . . . . ###############:::::::::::###::::: . . |
| . . . . .#############**:::::::::::###:::::. . .|
|. . . . . ###########****::::::::::::#:::::. . . |
| . . . . .#########*****     :::::::::: . . . . .|
|      . . *************     :::::::::::: . . . . |
|     . . .************     :::::::::::::: . . . .|
|    . . . ::::::::::::     ::::::::::::::. . . . |
|     . . .::::::::::::::::::::::::****. . . . .  |
|    . . . . .::::::::::::::::::: . *** . . . .   |
|     . . . . . . . . .::::::::: . . . .          |
|     *. . . . . . . . :::::::::. . .             |
|     . . . . . . . . .                           |
|            . .#.                                |
`-------------------------------------------------' 

Isopleth:

.-------------------------------------------------.
|*******::::::::::::::::::. . .         . :::::***|
|*******:::::******::::::::. . . . . . . .:::::***|
|***:::::::*************::::: . . . . . . ::::::::|
|:::::::::***####*********::::::::::::::::::::::. |
|. .::::::**##########*****::::::::::::::::::: . .|
| . .:::::**##########*****:::::::::****::::: . . |
|. . . :::**##########****::::::::::****:::: . . .|
| . . .:::***########****:: .::::::::**:::::. . . |
|. . . ::::****####*****:: . . ::::::::::::. . . .|
| . . . :::*************::. . ::::::::::::: . . . |
|  . . . ::::********::::: . ::::::::::::::. . . .|
|     . . ::::::::::::::::::::::::::::::: . . . . |
|      . .:::::::::::::::::::::::***:::::. . . .  |
|     . . . . . . . . :::::::::::***::: . . . .   |
|      . . . . . . . . . .:::::::::::::. .        |
|     .:: . . . . . . . . . . . ::::. . .         |
|      . . . . ::::. . . . . . . . . .            |
|           . ::**::. .                           |
`-------------------------------------------------'

¹Wright, J.K (1933) A method of mapping densities of population with Cape Cod as an example. Geographical Review, 26: 103-110.

Diagrams inspired by Robinson, Arthur H. et. al., Elements of Cartography 5ed. 1985, John Wiley and Sons. Figure 14.8, p. 342 (Current version ISBN: 0-471-55579-7)