Chris Essig

Walkthroughs, tips and tricks from a data journalist in eastern Iowa

Leaflet formula: Counting markers within a radius

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ssThe last project I worked on at The Courier was a comprehensive guide to mental health and disability services in NE Iowa. For the project, we first wanted to explain the MHDS system to our readers and how it works. It’s a relatively new system with a lot of moving parts and no good way to search for services. We really wanted to help residents and families in need of these services, so we designed everything — from the wording we used to the way items were laid out — to be focused on our readers.

After laying out the details of the new system, we wanted to allow our readers to search for services near them. A map seemed like an obvious choice; but we wanted to do more than just have them search for their address and plot a dot on the map.

(As a side note, we used Ben Welsh‘s jquery-geocodify plugin to geocode the addresses because it’s awesome.)

Instead, we wanted to show readers how many providers were close to them, as well as how to get ahold of the service provider and directions to the facility. For the former task, we used the following process:

First, the providers are plotted on the map. Then, we ask the reader to type in their address, which is plotted on the map using jquery-geocodify. They also have a second option for radius, which will be used to determine how many providers are within whatever radius they have given. They have four options: 5, 10, 25 and 50 miles.

Then, as the markers are getting plotted on the map, we run a few Javascript functions to calculate how many markers on the map are within the radius.

I’ve pulled out just this part of the mental health project and open-sourced it. You can view the full code over at my Github page.

The demo uses data on mental health providers, which is contained in this JSON fileand plots them on the map based on each provider’s latitude and longitude coordinates.

Here’s a quick run through the Javascript that makes all this run. It’s contained within the project’s script.js file. We’ll start from the bottom of the file and move our way up.

Note: This walkthrough assumes a decent understanding of Leaflet. I’m also using Leaflet-Awesome Markers and Font Awesome in this project.

First, we’ll add the base map tiles to our map. We’ll also  add the json_group to the map, which is a FeatureGroup we will create later in the file:

// Base map
var layer = new L.StamenTileLayer('toner-background');
var map = new L.Map('map', {
    center: new L.LatLng(42,-93.3),
    minZoom: 4,
    maxZoom: 10,
    zoom: 6,
    keyboard: false,
    boxZoom: false,
    doubleClickZoom: false,
    scrollWheelZoom: false,
    maxBounds: [[33.154799,-116.586914],[50.190089,-77.563477]]
// Add base layer to group
// Add our markers in our JSON file on the map

Next, we’ll add the code for jquery-geocodify. For the onSelect function, we will grab the location of reader’s address and then call a geocodePlaceMarkersOnMap function, which we will create later:

// jQuery Geocodify
var maxY = 43.749935;
var minY = 40.217754;
var minX = -96.459961;
var maxX = -90.175781;

var search_marker;
var search_icon = L.AwesomeMarkers.icon({
    icon: 'icon-circle',
    color: 'green'

    onSelect: function (result) {
        // Extract the location from the geocoder result
        var location = result.geometry.location;

        // Call function and place markers, circle on map
    initialText: 'Zip code, city, etc...',
    regionBias: 'US',
    // Lat, long information for Cedar Valley enter here
    viewportBias: new google.maps.LatLngBounds(
        new google.maps.LatLng(40.217754, -96.459961),
        new google.maps.LatLng(43.749935, -90.175781)
    width: 300,
    height: 26,
    fontSize: '14px',
    filterResults: function (results) {
        var filteredResults = [];
        $.each(results, function (i, val) {
            var location = val.geometry.location;
            if ( > minY && < maxY) {
                if (location.lng() > minX && location.lng() < maxX) {
        return filteredResults;

Next, we’ll loop through our providers.json file, which has a variable of json_data, and create a marker for each object in our array. We’ll then add it to our json_group FeatureGroup, which is defined at the top of our script file:

// This loops through the data in our JSON file
// And puts it on the map
_.each(json_data, function(num) {
    var dataLat = num['latitude'];
    var dataLong = num['longitude'];

    // Add to our marker
    var marker_location = new L.LatLng(dataLat, dataLong);

    // Options for our circle marker
    var layer_marker = L.circleMarker(marker_location, {
        radius: 7,
        fillColor: "#984ea3",
        color: "#FFFFFF",
        weight: 1,
        opacity: 1,
        fillOpacity: 0.8

    // Add events to marker
        // What happens when mouse hovers markers
        mouseover: function(e) {
            var layer_marker =;
                radius: 8,
                fillColor: "#FFFFFF",
                color: "#000000",
                weight: 1,
                opacity: 1,
                fillOpacity: 1
        // What happens when mouse leaves the marker
        mouseout: function(e) {
            var layer_marker =;
                radius: 7,
                fillColor: "#984ea3",
                color: "#FFFFFF",
                weight: 1,
                opacity: 1,
                fillOpacity: 0.8
    // Close event add for markers

// Close for loop
}, this);

The changeCircleRadius function is called anytime the radius is changed on the page. It first calls the pointsInCircle function, which we will define later, and then sets the radius of the circle using Leaflet’s setRadius function for circles.

Leaflet uses meters, so we will need to convert miles to meters using the milesToMeters function, which will be defined later. The mile’s value is contained within the DIV with the id of radius-selected. We’ll grab the value, which is the radius in miles, convert it to meters and then set the radius of the circle using Leaflet.

// Change circle radius when changed on page
function changeCircleRadius(e) {
    // Determine which geocode box is filled
    // And fire click event

    // This will determine how many markers are within the circle
    pointsInCircle(circle, milesToMeters( $('#radius-selected').val() ) )

    // Set radius of circle only if we already have one on the map
    if (circle) {
        circle.setRadius( milesToMeters( $('#radius-selected').val() ) );

$('select').change(function() {

The geocodePlaceMarkersOnMap function is called every time the reader enters an address into our geocoder and presses enter. It passes with it the location of their address, which we get from jquery-geocodify. With this function, we will create both a circle and a marker on top of the given address.

First, we set the view of the map based on this location and remove the circle on the map if there already is one. Then we actually create a circle and set how large it is based on the radius set by the user.

We then remove the marker if it’s already on the map and create a new one. When we create a new marker, we’ll set it to draggable and set its dragend method to reset the view of the map every time the reader drags the marker somewhere on the. It then calls the pointsInCircle function, which we will define next. Finally, we’ll call the same pointsInCircle function and add the marker to the map.

// This places marker, circle on map
function geocodePlaceMarkersOnMap(location) {
    // Center the map on the result
    map.setView(new L.LatLng(, location.lng()), 10);

    // Remove circle if one is already on map
    if(circle) {
    // Create circle around marker with our selected radius
    circle =[, location.lng()], milesToMeters( $('#radius-selected').val() ), {
        color: 'red',
        fillColor: '#f03',
        fillOpacity: 0.1,
        clickable: false
    // Remove marker if one is already on map
    if (search_marker) {
    // Create marker
    search_marker = L.marker([, location.lng()], {
        // Allow user to drag marker
        draggable: true,
        icon: search_icon

    // Reset map view on marker drag
    search_marker.on('dragend', function(event) {
        map.setView( ); 
        circle.setLatLng( );

        // This will determine how many markers are within the circle
        pointsInCircle( circle, milesToMeters( $('#radius-selected').val() ) );

        // Redraw: Leaflet function

        // Clear out address in geocoder

    // This will determine how many markers are within the circle
    // Called when points are initially loaded
    pointsInCircle( circle, milesToMeters( $('#radius-selected').val() ) );

    // Add marker to the map
// Close geocodePlaceMarkersOnMap

The pointsInCircle function is the last function called when one of two things happen: the user enters an address or the user drags the marker on the map. Its purpose is to find out how many markers on the map are within the circle we created and then display that number in the legend.

Fortunately, Leaflet has a really handy distanceTo method, which determines how many meters are within two points. It uses the two points’ latitude and longitude coordinates to find this out.

With the pointsInCircle function, we first capture the lat, long of the circle we’re putting on the map. We then loop through our providers.json file, find the latitude and longitude coordinates of each object and determine how far it is from our circle using the distanceTo method.

We then decide if that amount is higher or lower than the radius set by the user. If it is lower, we add it to a counter. When we’re done, that counter will equal the number of markers within the selected radius. It will be put on the map under the “Results” header.

Lastly, we will determine how many results we have and use the correct wording on the page. If we have one result, we’ll use the word “provider” (singular) in the map’s legend. Otherwise, we’ll use “providers.”

// This figures out how many points are within out circle
function pointsInCircle(circle, meters_user_set ) {
    if (circle !== undefined) {
        // Only run if we have an address entered
        // Lat, long of circle
        circle_lat_long = circle.getLatLng();

        // Singular, plural information about our JSON file
        // Which is getting put on the map
        var title_singular = 'provider';
        var title_plural = 'providers';

        var selected_provider = $('#dropdown_select').val();
        var counter_points_in_circle = 0;

        // Loop through each point in JSON file
        json_group.eachLayer(function (layer) {

            // Lat, long of current point
            layer_lat_long = layer.getLatLng();

            // Distance from our circle marker
            // To current point in meters
            distance_from_layer_circle = layer_lat_long.distanceTo(circle_lat_long);

            // See if meters is within raduis
            // The user has selected
            if (distance_from_layer_circle <= meters_user_set) {
                counter_points_in_circle += 1;

        // If we have just one result, we'll change the wording
        // So it reflects the category's singular form
        // I.E. facility not facilities
        if (counter_points_in_circle === 1) {
            $('#json_one_title').html( title_singular );
        // If not one, set to plural form of word
        } else {
            $('#json_one_title').html( title_plural );
        // Set number of results on main page
        $('#json_one_results').html( counter_points_in_circle );
// Close pointsInCircle

The final piece is to add the variables we’re using to the top of the script file. We’ll also add the milesToMeters function, which converts miles to meters.

// We'll append our markers to this global variable
var json_group = new L.FeatureGroup();
// This is the circle on the map that will be determine how many markers are around
var circle;
// Marker in the middle of the circle
var search_marker;
// Marker icon
var search_icon = L.AwesomeMarkers.icon({
    icon: 'icon-circle',
    color: 'red'

// Convert miles to meters to set radius of circle
function milesToMeters(miles) {
    return miles * 1069.344;

I hope this walkthrough helps fellow Leaflet users! If you have any questions, leave them in the comments.

Happy coding.


Written by csessig

June 22, 2014 at 1:32 pm

5 Responses

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  1. your miles to meters conversion number is incorrect: should be 1609.344 not 1069 . thank you for sharing and good luck!


    June 24, 2015 at 6:45 am

    • Thanks for catching that! I’ve updated my code on Github and on this page to reflect the change.


      June 24, 2015 at 8:46 am

  2. Good morning can help me with some tutorial, where I can draw a marker anywhere on the map and by a combo box can choose a distance and I can draw a circle by seeing the names of markers that are in that circle, something like that


    November 24, 2016 at 4:09 pm

  3. […] couple of years ago, I open-sourced a project that allows you to count how many markers are within a circle using Leaflet. As part of the […]

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