How to Display Feature Collection GeoJson with Leaflet’s Marker Clusters

I enjoyed playing with Leaflet’s marker cluster plugin this weekend while writing a new demo, Demo 5, that shows how to load feature collection GeoJson into a Leaflet map layer.

Leaflet Marker ClusterIt was surprisingly easy to setup. First, I updated my GeoJson utility project to create feature collection GeoJson loaded with 768 of my runs from Garmin Connect. The features in the feature collection are summaries. They contain the same properties as the activity detail GeoJson from Demo 4, except that they use starting latitude/longitude instead of a polyline.   Next, I generated individual feature GeoJson files for each run with the full polyline. Finally, I added the new GeoJson files to my website at http://www.exploringspatial.com/activity/.

A Backbone Collection, Activities.js, uses AJAX to fetch the activity summaries. The collection is passed into the ActivitiesMapLayerView.js. The code in its render function is simple:

render: function() {
    var _self = this;
    geoJsonLayer = L.geoJson(this.collection.toJSON(),{
        onEachFeature: _self.onEachFeature
    });
    this.map.fitBounds(geojson.getBounds());
    this.activitiesLayer = L.markerClusterGroup();
    this.activitiesLayer.addLayer(geoJsonLayer);
    this.map.addLayer(this.activitiesLayer);
    this.map.on('popupopen', function(event) {_self.onPopupOpen(event);});
    $('.returnToSearch').on('click', '.returnTrigger', function(event){
        _self.onReturnToSearch(event)
      });
}
  1. L.geoJson – Create a map layer by passing in the activity collection JSON along with a onEachFeature function (see below).
  2. L.markerClusterGroup – Create a marker cluster map layer.
  3. addLayer – Add the GeoJson layer to the marker cluster group.
  4. addLayer – Add the marker cluster group to the map.

The last two lines of code bind a listener to the popupopen event, and add a listener for the activity detail page event that returns to the main page.

The onEachFeature function is used to bind a popup to each feature marker on the map. It does some date and number formatting and then passes HTML into layer.bindPopup.

onEachFeature: function(feature, layer) {
    var date = new Date(feature.properties.startTime);
    var triggerId = feature.properties.activityId;
    var msg = [];
    msg.push(feature.properties.name);
    msg.push('Start: ' + date.toLocaleDateString() + ' ' + date.toLocaleTimeString() + '');
    var dist = Math.round((feature.properties.totalMeters * 0.000621371)*100)/100;
    msg.push('Dist: ' + dist + ' mi');
    msg.push('Go to Activity');
    layer.bindPopup(msg.join(''), {maxWidth: 200});
},

The interesting bit happens in the onOpenActivity and renderActivity functions when a user clicks the “Go to Activity” anchor in the popup. The onOpenActivity function instantiates an Activity Backbone model and invokes its fetch function to go get the polyline for the selected activity id. The renderActivity function is passed in the success property of the AJAX call.

onOpenActivity: function(event, popup) {
    var location = popup._latlng;
    this.map.closePopup(popup);
    // Capture the current center and zoom to restore map later
    this.originalCenter = this.map.getCenter();
    this.originalZoom = this.map.getZoom();
    this.activity = new Activity({activityId: event.target.id});
    var _this = this;
    this.activity.fetch({
        success: function () {
        _this.renderActivity();
    }
});
        },

When the polyline is successfully returned from the server the renderActivity function creates a new activity map layer to swap with the marker cluster map layer. A feature GeoJson map layer is added to the map, along with the start and end markers.

renderActivity: function() {
    $('.returnToSearch').show();
    if (this.map.hasLayer(this.activitiesLayer)) {
        this.map.removeLayer(this.activitiesLayer);
    }
    var props = this.activity.get('properties');
    this.map.fitBounds([
        [props.get('minLat'), props.get('minLon')],
        [props.get('maxLat'), props.get('maxLon')]
    ]);
    var style = {
        color: '#FF0000',
        weight: 3,
        opacity: 0.6
    };
    this.activityLayer = L.geoJson(this.activity.toJSON(), 
        {style: style}).addTo(this.map);
    var polyline = this.activity.get('geometry').get('coordinates');
    var startPoint = polyline[0];
    var endPoint = polyline[polyline.length - 1];
    this.activityStart = L.marker([startPoint[1], startPoint[0]], 
        {icon: this.startIcon}).addTo(this.map);
    this.activityEnd = L.marker([endPoint[1], endPoint[0]], 
        {icon: this.endIcon}).addTo(this.map);
},

Once the user clicks Back to Search, this is the block of code that restores the marker cluster layer. The activity feature layer, start, and end markers are removed (not shown), and then the marker cluster layer is added back to the map and the original center and zoom are restored.

 this.map.addLayer(this.activitiesLayer);
 if (this.originalCenter != null && this.originalZoom != null) {
    this.map.setView(this.originalCenter, this.originalZoom, {animate: true});
    this.originalCenter = null;
    this.originalZoom = null;
 }

That’s it! Be sure to go to the demo, http://www.exploringspatial.com/#demo/5, and click around.

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Rendering a GeoJSON layer in Leaflet

The addition of a GeoJSON layer showing a Garmin activity polyline completes my example of using Leaflet with Bing, Google, and OSM base layers.

Static GeoJSON Leaflet map with Google, Bing and OSM base layers.

Static GeoJSON Leaflet map with Google, Bing and OSM base layers.

I did a lot of work to get to this point. Some tasks were completely unrelated to the demo, like converting my website to a single-page Backbone application. Other tasks were behind the scenes, like writing a utility to convert Garmin FIT files into GeoJSON. A great deal of my time was spent recreating the Garmin-style map controls to switch between Google, Bing, and OSM. Eventually, I want to add the Google bicycle path layer and fix a bug I found today on the menu controls, but that can wait.

Once I was ready to add the polyline and custom pin icons, coding was a breeze. All that needed to be done was to extend L.Icon for the custom pin icons and drop the polyline GeoJSON into the L.geoJson function as shown below:

    var ActivityMapLayerView = Backbone.View.extend({

        initialize: function(args) {
            this.map = args.map;
            var CustomIcon = L.Icon.extend({options: {
                iconSize: [33, 50],
                iconAnchor: [16, 49]
            }});
            this.startIcon = new CustomIcon({iconUrl: 'media/pin_start.png'});
            this.endIcon = new CustomIcon({iconUrl: 'media/pin_end.png'});
            this.render();
        },

        render: function() {
            var props = this.model.get('properties');
            this.map.fitBounds([
                [props.get('minLat'), props.get('minLon')],
                [props.get('maxLat'), props.get('maxLon')]
            ]);
            var style = {
                color: '#FF0000',
                weight: 3,
                opacity: 0.6
            };
            L.geoJson(this.model.toJSON(), {style: style}).addTo(this.map);
            var polyline = this.model.get('geometry').get('coordinates');
            var startPoint = polyline[0];
            var endPoint = polyline[polyline.length - 1];
            L.marker([startPoint[1], startPoint[0]], {icon: this.startIcon}).addTo(this.map);
            L.marker([endPoint[1], endPoint[0]], {icon: this.endIcon}).addTo(this.map);
        }

    });

The GeoJSON is passed into L.geoJson along with style properties to control the color, weight, and opacity of the polyline. That is all it took!

Now it is time to being exploring the plethora of Leaflet plugins supporting interactive maps. I’m looking forward to getting started.

Converting Garmin FIT activities into GeoJSON

My demo accomplished what I wanted to do with Leaflet regarding Bing, OSM, and Google base maps. The demo is at http://www.exploringspatial.com/#demo/4. The only base map plumbing that remains is to figure out how to add Google’s bicycle path layer.

The next step is to display the activity polyline as a Leaflet layer instead of adding it directly to Google, Bing, or OSM maps via the vendors’ map APIs. With Leaflet, the base map is simply the background for the vector graphics displayed on a Leaflet map layer in the foreground.

Using GeoJSON with Leaflet

First, I needed to produce GeoJSON as described here: http://leafletjs.com/examples/geojson.html. GeoJSON is a JSON standard for geometry types (see http://geojson.org). The geometry types are:

  • Point
  • LineString
  • Polygon
  • MultiPoint
  • MultiLineString
  • MultiPolygon

Geometry types may be combined with additional properties using Feature or FeatureCollection. Leaflet has a map layer type that can consume Feature JSON or FeatureCollection JSON.

Garmin Activity FIT File in –> GeoJSON out

Modern Garmin fitness devices store data in a compressed binary format called the FIT protocol (see http://thisisant.com).

My first task was to create a utility to read Garmin activity FIT files and produce Feature GeoJSON. The code is on GitHub: https://github.com/smitchell/garmin-fit-geojson.

I used my 2012 Little Rock Marathon activity to test the GeoJSON utility. The FIT SDK decodes the activity FIT file. The GeoTools FeatureJSON class outputs the GeoJSON.

There are two steps to creating GeoJSON using the GeoTools API:

  1. Define a feature type (schema) for the feature properties.
  2. Build the feature following the feature type definition.

Defining a Simple Feature Type

The SimpleFeatureTypeBuilder class is used to layout the schema for the Feature.

public SimpleFeatureType getFeatureSchema() {
    final SimpleFeatureTypeBuilder simpleFeatureType = new SimpleFeatureTypeBuilder();
    simpleFeatureType.add("geom", LineString.class, DefaultGeographicCRS.WGS84);
    simpleFeatureType.add("name", String.class);
    simpleFeatureType.add("activityId", Long.class);
    simpleFeatureType.setName("activity");
    simpleFeatureType.add("activityName", String.class);
    simpleFeatureType.add("sport", String.class);
    simpleFeatureType.add("startTime", String.class);
    simpleFeatureType.add("totalMeters", Double.class);
    simpleFeatureType.add("totalSeconds", Double.class);
    simpleFeatureType.add("minLat", Double.class);
    simpleFeatureType.add("minLon", Double.class);
    simpleFeatureType.add("maxLat", Double.class);
    simpleFeatureType.add("maxLon", Double.class);
    return simpleFeatureType.buildFeatureType();
}

Building the Feature

The SimpleFeatureBuilder builds the SimpleFeature following the feature schema definition.

public SimpleFeature buildSimpleFeature(final FitActivity fitActivity) {
    final SimpleFeatureType featureSchema = getFeatureSchema();
    final SimpleFeatureBuilder builder = new SimpleFeatureBuilder(featureSchema);
    builder.set("activityId", fitActivity.getActivityId());
    builder.set("sport", fitActivity.getSport());
    builder.set("startTime", fitActivity.getStartTime());
    builder.set("totalMeters", fitActivity.getTotalMeters());
    builder.set("totalSeconds", fitActivity.getTotalSeconds());
    final Coordinate[] polyline = fitActivity.getPolyline().toArray(
        new Coordinate[fitActivity.getPolyline().size()]);
    final Geometry geometry = simplifyLineString(polyline);
    builder.add(geometry);
    final Coordinate[] boundingBox = generateBoundingBox(geometry);
    builder.set("minLat", boundingBox[0].y);
    builder.set("minLon", boundingBox[0].x);
    builder.set("maxLat", boundingBox[1].y);
    builder.set("maxLon", boundingBox[1].x);
    return builder.buildFeature("0");
}

The final result, formatted for readability, can be found here: http://www.exploringspatial.com/feature.json

This is a partial listing:

{
  "type": "Feature",
  "geometry": {
    "type": "LineString",
    "coordinates": [
      [
        -92.2639,
        34.7473
      ],

... many points omitted.

      [
        -92.2668,
        34.7484
      ]
    ]
  },
  "properties": {
    "activityId": 155155867,
    "sport": "RUNNING",
    "startTime": "2012-03-04T14:02Z",
    "totalMeters": 42453.58984375,
    "totalSeconds": 15162.140625,
    "minLat": 34.73176879808307,
    "minLon": -92.34434505924582,
    "maxLat": 34.78602569550276,
    "maxLon": -92.25817699916661
  },
  "id": "0"
}

Now I’m ready to create a Leaflet GeoJSON layer to display the polyline!