The dasymetric map is a method of thematic mapping, which uses areal symbols to spatially classify volumetric data. The method was defined and developed in 1911 by Benjamin (Veniamin) Petrovich Semenov-Tyan-Shansky and popularised by J.K. Wright, although there are earlier references to similar techniques from George Poulett Scrope (1833) and Henry Drury Harness (1838).
The term "dasymetric" was invented by Semenov-Tyan-Shansky using the Greek translation for "measuring density" (dasys – dense, metreo – to measure). Semenov-Tyan-Shansky defined dasymetric maps as maps "on which population density, irrespective of any administrative boundaries, is shown as it is distributed in reality, i.e. by natural spots of concentration and rarefaction."
Developed methods and approaches in dasymetric mapping include areal interpolation, filtered areal weighting using the binary method, filtering with land cover data, and cadastral-based expert dasymetric system.
Dasymetric maps are used instead of choropleth maps because they represent underlying data distributions more accurately. Choropleth maps and dasymetric maps differ in three main ways. First, dasymetric zones are generated using ancillary data while boundaries on choropleth maps use units used for more general purposes (such as U.S. county boundaries). Second, choropleth zones have varying levels of internal homogeny while dasymetric maps are designed to be internally homogenous. Last, choropleth mapping methods are standardized while dasymetric methods are under researched.
Considered a hybrid or compromise between isopleth and choropleth maps, a dasymetric map utilizes standardized data, but places areal symbols by taking into consideration actual changing densities within the boundaries of the map. To do this, ancillary information is acquired, which means the cartographer steps statistical data according to extra information collected within the boundary.
Like other forms of thematic mapping, the dasymetric method was created and historically used because of the need for accurate visualization methods of population data. Dasymetric maps are not widely used because of a lack of standardized dasymetric mapping techniques that are accessible to the public. This leads to methods which are highly subjective with inconsistent criteria. Although fields such as public health still rely on choropleth maps, dasymetric maps are becoming more prevalent in developing fields such as aerial interpolation and population estimation using remote sensing.
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