Robin's Blog

Setting limits for a choropleth layer manually with leaflet-choropleth

Following on from my last post on plotting choropleth maps with the leaflet-choropleth library, I’m now going to talk about a small addition I’ve made to the library.

Leaflet-choropleth has built-in functionality to automatically categorise your data: you tell it how many categories you’d like and it splits it up. However, once I’d set up my webmap with leaflet-choropleth, using the automatically generated categories, my client said she wanted specific categories to be used. Unfortunately leaflet-choropleth didn’t support that…so I added it!

(It always pleases me a lot that if you’re in a situation where some open-source code doesn’t do what you want it to do, you can just modify it – and then you can contribute the code back to the project too!)

The pull request for this new functionality hasn’t yet been merged, but the updated code is available from my fork. The specific file you need is the updated choropleth.js file. Once you’ve replaced the original choropleth.js with this new version, you will be able to use a new limits option when calling L.choropleth. For example:

var layer_IMD = L.choropleth(geojson, {
    valueProperty: 'IMDRank',
    limits: [1000, 5000, 30000],
    scale: ['red', 'orange', 'yellow'],
    style: {
        color: '#111111', // border color
        weight: 1,
        fillOpacity: 0.5,
        fillColor: '#ffffff'
    }
}).addTo(map);

The value of the limits property should be the ‘dividing lines’ for the limits: so in this case there will be categories of < 1000, 1000-5000, etc.

I think that’s pretty-much all I can say about this – the code for an example map using this new functionality is available on Github and you can see a live map demo here.

This work was done while analysing GIS data and producing a webmap for a freelancing client. If you’d like me to do something similar for you, have a look at my freelance website or email me.

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This post originally appeared on Robin's Blog.

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Categorised as: Academic, GIS, Programming