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Clustering Markers On Leaflet Maps

posted by Andy Maloney

So you’re creating an interactive map using Leaflet and have diligently added your 8107 markers to the map.

Leaflet - Too Many Markers!

Leaflet – Too Many Markers!

Uh-oh. Definitely slow and pretty much doesn’t provide a user experience. What do we do now?

There’s a great plugin for Leaflet called Leaflet.markercluster by Dave Leaver which will save us.

Leaflet Clusters

Leaflet Clusters

This plugin clusters the markers and shows the number of items in each cluster, and as we zoom it adjusts the clusters based on the current view. This not only makes the map easier for the user to understand, it’s also a lot more efficient.

If you followed my previous Leaflet tutorial, adding the clustering plugin is extremely simple. Let’s take a look.

Leaflet.markercluster has some cool capabilities. By default, hovering the mouse over an area outlines the area like this:

Leaflet Clusters - Hovering

Leaflet Clusters – Hovering

Clicking inside an area zooms in and shows you the next level of clusters:

Leaflet Clusters - Zoom In

Leaflet Clusters – Zoom In

When a cluster only has one item in it, it shows it as a marker:

Leaflet Clusters - Zoom In Again!

Leaflet Clusters – Zoom In Again!

And of course, like the last example, clicking a marker shows more information about the location:

Leaflet Popup

Leaflet Popup

For this example, I’m going to create a map showing the airports and train stations of the world. I grabbed a data set from OpenFlights.org which gives locations and other information for airports and train stations. I downloaded the CSV, converted it to JSON, and collapsed it a bit since it’s rather large.

This example consists of three files and two images:

  • index.html – the page itself
  • maps/leaf-demo.js – JavaScript to create the map and populate it with markers
  • maps/markers.js – a list of markers with their latitude & longitude as well as other info about the location to display in the popup when you click a marker
  • maps/images/pin24.png and pin48.png – the pin to display (one for normal display, one for retina)

(The files used in this example are available on github.)


Let’s start by taking a look at the HTML. This is pretty much the same as the example in the last post and should be fairly straightforward.

The only significant changes here are that I added two stylesheets and one javascript for leaflet.markercluster. If you want to customize the look of the cluster indicators, you can override the stylesheets.

The Leaflet JavaScript

The first part of this is the same as the last example. We create the map, set up the tiles, and create our icon.

And here are the changes needed for adding clustering:

Line 22 creates a cluster group. There are several options which can be passed here to customize it.

Next, for each element in the JSON data, we create a marker on the map, set the popup for it to display additional information, and add it to a new layer in the cluster group.

Finally in line 39 we add the cluster group as a layer on the map.

That’s it. We’re done.

The JSON Data

As I mentioned above, I downloaded a data set from OpenFlights.org and converted it to JSON data. I made this into a JavaScript file which just sets a variable. This lets us include this file as straight Javascript and to use the markers variable as-is.

Final Result

Try it out!

I hope this is useful to someone out there in and amongst this series of tubes we call The Internet.

(The files used in this example are available on github.)

As always, if you have suggestions on how to improve my code or make things easier, please comment below or feel free to contact me.

Update 12 July 2016: MapQuest discontinued the tile set I was using, so I switched to OpenStreetMap’s tiles. Note that these are useful only for testing. They say on their site:

Apart from very limited testing purposes, you should not use the tiles supplied by OpenStreetMap.org itself.

Update 12 July 2016: Updated example to use Leaflet 0.7.7 (no code changes, just including newer version of javascript).


  1. Comment by Rajavelu:

    Thanks for your detailed explanation about marker clustering in openstreetmap with leaflet.
    I understood as, currently marker clustering are works based on distance, right?
    I want to cluster markers based on region, for example cluster all markers in a particular country or state.
    Can anyone help me!

  2. Comment by abhijit:

    This is awesome. Thank you so much…

    • Comment by Andy Maloney:

      Glad you found it useful!

      • Comment by abhijit:

        It saved my day for sure. Thank you. I stumbled into an issue and if you know off the top of your head please feel free to advise.

        I am adding a link on each of the popups and then ajax loading another page into a div when the link is clicked. Once the ajax load happens, the leaflet map becomes unresponsive. At first I thought it was due to my code, but I removed the clustering part and it works fine.

        I am attaching the code below. I am not asking you for a full fledged solution but if you have seen this behavior or have an insight, please feel free to share:

        I am attaching an event handler to each of the items with class ‘link’ like this:

        I am ajax loading a different page in the createDialog method like this:


  3. Comment by Saen:

    What needs to be changed if we want to convert this example to work with Leaflet v 1.0.0 beta 0.2 ?

  4. Comment by Saen:

    Have you tried any heatmaps ?

  5. Comment by Claude Jean:

    Thank you very much for your tutorial very helpful. Can you plz tell me what wrong with my code. It only works when I have one point. When I try to loop more than one nothing is displayed.

    • Comment by Andy Maloney:

      I would suggest opening it in Chrome and using the developer tools to set a breakpoint on the for loop to check that point.length is returning what you think it should return. Then step through that code to see what’s not working as expected.

  6. Comment by Diouck:

    I try to apply your tutorial on api

    how you made ​​to incorporate the data in this leaflet or transform the data size geojson
    Thank you

    • Comment by Andy Maloney:


      I’m not sure I understand your question.

      You have some data which includes coordinates – are you asking how to access the coordinates within that data structure? Or are you asking how to extract the coordinates and put them in a GeoJSON file?

  7. Comment by Dhamo:

    Instead of the aggreated count as number , i want to fill the colour in that geo polygon like choropleth

    How to do that?

    Thank you

  8. Comment by JOE:

    Hi, how do i get rid of the hover recangle thingies… thanks

  9. Comment by Dinesh kumar krishna:

    I am using cluster group. But how to fix cluster expand level. Suppose default cluster group loads with continent level then click. It expand to deeper level and when third click on cluster group again it expand to country level. I want to fix three expand level of cluster group.

    I tried freezeAtZoom() but it stops the cluster group from expand even from first level..

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