How to reduce GeoJSON file size with mapshaper.org

GeoJSON and shapfiles can be a few hundred megabytes, and that becomes a problem when you are trying to make interactive online maps where the webpage size should be small.

https://mapshaper.org/ to the rescue! No installation whatever, all inside a browser, yet super fast.

But there are some problems still if you use the converted files with plotly for making choropleths. Hence this guide:

Step 1: Import file

Go to https://mapshaper.org/ and import your GeoJSON file.

Step 2

Make sure "detect line intersection" is checked.

Step 3

If you map loads correctly, click "Simplify".

Step 4

Tick "prevent shape removal", use "Visvalingam / weigthed area" if you don't have other preferences and apply.

Step 5

Move the slider to any percentage. If your initial filesize is 100 MB and you slide to 1%, then output file will be 1% of 100 MB. Also click "Repair" after sliding.

Step 6

Now click export.

Step 7

Use whatever output format you please, but for compatibility with plotly and D3.js, you should include the output parameter gj2008 to export properly.

And you're done! yay