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Deals with overlapping markers in Google Maps JS API v3, Google Earth-style

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Overlapping Marker Spiderfier for Google Maps API v3

Ever noticed how, in Google Earth, marker pins that overlap each other spring apart gracefully when you click them, so you can pick the one you meant?

And ever noticed how, when using the Google Maps API, the same thing doesn’t happen?

This code makes Google Maps API version 3 map markers behave in that Google Earth way (minus the animation). Small numbers of markers (yes, up to 8) spiderfy into a circle. Larger numbers fan out into a more space-efficient spiral.

The compiled code has no dependencies beyond Google Maps. And it’s under 3K when compiled out of CoffeeScript, minified with Google’s Closure Compiler and gzipped.

I wrote it as part of the data download feature for Mappiness.

There’s now also a port for the Leaflet maps API.

Doesn’t clustering solve this problem?

You may have seen the marker clustering library, which also helps deal with markers that are close together.

That might be what you want. However, it probably isn’t what you want (or isn’t the only thing you want) if you have markers that could be in the exact same location, or close enough to overlap even at the maximum zoom level. In that case, clustering won’t help your users see and/or click on the marker they’re looking for.

(I’m told that the OverlappingMarkerSpiderfier also plays nice with clustering — i.e. once you get down to a zoom level where individual markers are shown, these markers then spiderfy happily — but I haven’t yet tried it myself).

Demo

See the demo map (the data is random: reload the map to reposition the markers).

Download

Download the compiled, minified JS source.

Please note: version 0.3 introduces a breaking change. The willSpiderfy(marker) and markersThatWillAndWontSpiderfy() methods have been replaced with the (similar, but different) markersNearMarker(marker) and markersNearAnyOtherMarker() methods.

How to use

See the demo map source, or follow along here for a slightly simpler usage with commentary.

Create your map like normal:

var gm = google.maps;
var map = new gm.Map(document.getElementById('map_canvas'), {
  mapTypeId: gm.MapTypeId.SATELLITE,
  center: new gm.LatLng(50, 0), 
  zoom: 6
});

Create an OverlappingMarkerSpiderfier instance:

var oms = new OverlappingMarkerSpiderfier(map);

Instead of adding click listeners to your markers directly via google.maps.event.addListener, add a global listener on the OverlappingMarkerSpiderfier instance instead. The listener will be passed the clicked marker as its first argument, and the Google Maps event object as its second.

var iw = new gm.InfoWindow();
oms.addListener('click', function(marker, event) {
  iw.setContent(marker.desc);
  iw.open(map, marker);
});

You can also add listeners on the spiderfy and unspiderfy events, which will be passed an array of the markers affected. In this example, we observe only the spiderfy event, using it to close any open InfoWindow:

oms.addListener('spiderfy', function(markers) {
  iw.close();
});

Finally, tell the OverlappingMarkerSpiderfier instance about each marker as you add it, using the addMarker method:

for (var i = 0; i < window.mapData.length; i ++) {
  var datum = window.mapData[i];
  var loc = new gm.LatLng(datum.lat, datum.lon);
  var marker = new gm.Marker({
    position: loc,
    title: datum.h,
    map: map
  });
  marker.desc = datum.d;
  oms.addMarker(marker);  // <-- here
}

Docs

Loading

The google.maps object must be available when this code runs — i.e. put the Google Maps API <script> tag before this one.

The Google Maps API code changes frequently. Some earlier versions had broken support for z-indices, and the ‘frozen’ versions appear not to be as frozen as you’d like. At this moment, the ‘stable’ version 3.7 seems to work well, but do test with whatever version you fix on.

Construction

new OverlappingMarkerSpiderfier(map, options)

Creates an instance associated with map (a google.maps.Map).

The options argument is an optional Object specifying any options you want changed from their defaults. The available options are:

markersWontMove and markersWontHide (defaults: false)

By default, change events for each added marker’s position and visibility are observed (so that, if a spiderfied marker is moved or hidden, all spiderfied markers are unspiderfied, and the new position is respected where applicable).

However, if you know that you won’t be moving and/or hiding any of the markers you add to this instance, you can save memory (a closure per marker in each case) by setting the options named markersWontMove and/or markersWontHide to true (or anything truthy).

For example, var oms = new OverlappingMarkerSpiderfier(map, {markersWontMove: true, markersWontHide: true});.

keepSpiderfied (default: false)

By default, the OverlappingMarkerSpiderfier works like Google Earth, in that when you click a spiderfied marker, the markers unspiderfy before any other action takes place.

Since this can make it tricky for the user to work through a set of markers one by one, you can override this behaviour by setting the keepSpiderfied option to true.

nearbyDistance (default: 20).

This is the pixel radius within which a marker is considered to be overlapping a clicked marker.

circleSpiralSwitchover (default: 9)

This is the lowest number of markers that will be fanned out into a spiral instead of a circle. Set this to 0 to always get spirals, or Infinity for all circles.

legWeight (default: 1.5)

This determines the thickness of the lines joining spiderfied markers to their original locations.

Instance methods: managing markers

Note: methods that have no obvious return value return the OverlappingMarkerSpiderfier instance they were called on, in case you want to chain method calls.

addMarker(marker)

Adds marker (a google.maps.Marker) to be tracked.

removeMarker(marker)

Removes marker from those being tracked. This does not remove the marker from the map (to remove a marker from the map you must call setMap(null) on it, as per usual).

clearMarkers()

Removes every marker from being tracked. Much quicker than calling removeMarker in a loop, since that has to search the markers array every time.

This does not remove the markers from the map (to remove the markers from the map you must call setMap(null) on each of them, as per usual).

getMarkers()

Returns an array of all the markers that are currently being tracked. This is a copy of the one used internally, so you can do what you like with it.

Instance methods: managing listeners

addListener(event, listenerFunc)

Adds a listener to react to one of three events.

event may be 'click', 'spiderfy' or 'unspiderfy'.

For 'click' events, listenerFunc receives one argument: the clicked marker object. You’ll probably want to use this listener to do something like show a google.maps.InfoWindow.

For 'spiderfy' or 'unspiderfy' events, listenerFunc receives two arguments: first, an array of the markers that were spiderfied or unspiderfied; second, an array of the markers that were not. One use for these listeners is to make some distinction between spiderfied and non-spiderfied markers when some markers are spiderfied — e.g. highlighting those that are spiderfied, or dimming out those that aren’t.

removeListener(event, listenerFunc)

Removes the specified listener on the specified event.

clearListeners(event)

Removes all listeners on the specified event.

unspiderfy()

Returns any spiderfied markers to their original positions, and triggers any listeners you may have set for this event. Unless no markers are spiderfied, in which case it does nothing.

Instance methods: advanced use only!

markersNearMarker(marker, firstOnly)

Returns an array of markers within nearbyDistance pixels of marker — i.e. those that will be spiderfied when marker is clicked. If you pass true as the second argument, the search will stop when a single marker has been found. This is more efficient if all you want to know is whether there are any nearby markers.

Don’t call this method in a loop over all your markers, since this can take a very long time.

The return value of this method may change any time the zoom level changes, and when any marker is added, moved, hidden or removed. Hence you’ll very likely want call it (and take appropriate action) every time the map’s zoom_changed event fires and any time you add, move, hide or remove a marker.

Note also that this method relies on the map’s Projection object being available, and thus cannot be called until the map’s first idle event fires.

markersNearAnyOtherMarker()

Returns an array of all markers that are near one or more other markers — i.e. those will be spiderfied when clicked.

This method is several orders of magnitude faster than looping over all markers calling markersNearMarker (primarily because it only does the expensive business of converting lat/lons to pixel coordinates once per marker).

The return value of this method may change any time the zoom level changes, and when any marker is added, moved, hidden or removed. Hence you’ll very likely want call it (and take appropriate action) every time the map’s zoom_changed event fires and any time you add, move, hide or remove a marker.

Note also that this method relies on the map’s Projection object being available, and thus cannot be called until the map’s first idle event fires.

Properties

You can set the following properties on an OverlappingMarkerSpiderfier instance:

legColors.usual[mapType] and legColors.highlighted[mapType]

These determine the usual and highlighted colours of the lines, where mapType is one of the google.maps.MapTypeId constants (or a custom map type ID).

The defaults are as follows:

var mti = google.maps.MapTypeId;
legColors.usual[mti.HYBRID] = legColors.usual[mti.SATELLITE] = '#fff';
legColors.usual[mti.TERRAIN] = legColors.usual[mti.ROADMAP] = '#444';
legColors.highlighted[mti.HYBRID] = legColors.highlighted[mti.SATELLITE] = 
  legColors.highlighted[mti.TERRAIN] = legColors.highlighted[mti.ROADMAP] = '#f00';

You can also get and set any of the options noted in the constructor function documentation above as properties on an OverlappingMarkerSpiderfier instance. However, for some of these options (e.g. markersWontMove) modifications won’t be applied retroactively.

Licence

This software is released under the MIT licence.

Finally, if you want to say thanks, I am on Gittip.

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