Dasymetric Map

A dasymetric map is a type of thematic map that improves upon choropleth maps by refining the way data is distributed over geographic areas. Instead of using administrative boundaries (such as counties or districts) to show data, it uses ancillary data (like land use or satellite imagery) to more accurately represent where people or other mapped features are actually located.

The term "dasymetric" was coined in 1911 by Benjamin Semyonov-Tian-Shansky, who first fully developed and documented the technique, defining them 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.

Key Features of a Dasymetric Map

1. Uses Additional Data – Unlike choropleth maps, it integrates extra data sources like land cover, population density, or satellite imagery.
2. More Accurate Representation – It removes uninhabited areas (e.g., water bodies, forests) and redistributes values to occupied areas.
3. Improves Visualization – It provides a better understanding of spatial patterns by showing real variations instead of using arbitrary administrative zones.

Example Process of Creating a Dasymetric Map

1. Start with a Choropleth Map – Example: Population density by districts.
2. Remove Uninhabited Areas – Exclude lakes, forests, or public lands where no people live.
3. Redistribute the Data – Adjust the population density to match inhabited regions only.

Comparison with Choropleth and Isarithmic Maps

• Choropleth Map – Uses administrative boundaries but may misrepresent data distribution.
• Dasymetric Map – Refines data using real-world geographic features for better accuracy.
• Isarithmic Map – Uses continuous lines (isolines) to represent gradual changes, like temperature or elevation.