Guide to processing maps from GADM to make cartograms

This is the process for preparing map data to generate cartograms.

Table Of Contents

• Step 1: Download map from GADM
• Step 2: Process map in Mapshaper
  • Step 2.1: Projection
  • Step 2.2: Simplification
  • Step 2.3: Export

Step 1: Download map from GADM

First, visit https://gadm.org/download_country_v3.html and select the country. Click on "Shapefile", which will download a zip file. You can also obtain the map from another website. Shapefiles, GeoJSON and TopoJSON are supported by Mapshaper.

Step 2: Process map in Mapshaper

Then, visit https://mapshaper.org/ and import the zip file. Click on the correct layer, which should be gadm36_[country]_1 for a map downloaded from GADM, as seen in the image below:

Step 2.1: Projection

You will need to know the desired projection for this map. Then, click on console (upper right corner) and type the following command to project the map, replacing [spatial_reference] with your chosen spatial reference system:

$ -proj +init=EPSG:[spatial_reference]

Check to make sure that the map is projected correctly.

Step 2.2: Simplification

Type the following command to find out the total number of vertices.

$ -simplify 100% stats

The unique coordinate locations tells you the number of vertices. Use the following equation to calculate the percentage required.

[percentage] = 100 * [desired no. of vertices] / [unique coordinate locations]

So, if you would like 50,000 verticies, then the percentage will be 100 * 50,000 / 573,076 ≈ 9 We recommend 50,000 vertices for an optimal balance between time, space and resolution.

Then, type the following command to execute the simplification, replacing [percentage] with the required percentage:

$ -simplify [percentage]% stats

Step 2.3: Export

In Mapshaper, export the map using the following command, replacing [country_name] accordingly.

$ -o format=geojson [country_name].json

Now, continue following the steps here to generate the cartogram: https://github.com/Flow-Based-Cartograms/go_cart#running-the-cartogram-generator