Google Maps v2: how to display various markers with the same coordinates

Question

I'm using Google Maps JavaScript API V2 to mark locations saved in an array. Markers with different coordinates are displayed well, but I have no idea how to display various markers with the same coordinates. I need it because an array of locations can have the same coordinates, but with a different description. What is the way to display those different descriptions?

The code where markers are added is:

var map;

    function loadEarth(mapdiv) {
        if (GBrowserIsCompatible()) {
            if(!mapdiv)
                return true;
            map=new GMap2(document.getElementById("map"));
            map.enableDoubleClickZoom();
            map.enableScrollWheelZoom();
            map.addControl(new GMapTypeControl());
            map.addControl(new GSmallMapControl());
            map.setCenter(new GLatLng(40.186718, -8.415994),13);
            }
    }
    function createMarker(point, number, description) {
        var marker = new GMarker(point);
        marker.value = number;
        GEvent.addListener(marker, "click", function() {
        var myHtml = "<b>#" + number + "</b><br/>" + description;
            map.openInfoWindowHtml(point, myHtml);
        });
        return marker;
    }

...

for(var i=0; i<gl_list.length; i++){
   var point = new GLatLng(gl_list[i].Latitude,gl_list[i].Longitude);
   description = "Visited place : "+gl_list[i].nome+" on : "+gl_list[i].data;
   map.addOverlay(createMarker(point, i + 1, description));
}

Thanks for your attention.

Answer 1

I very often have the exact same problem on my map and found myself worrying about the same multiple markers issues: which marker is on top? how does a user see them all? etc.

Eventually, I decided that the problem wasn't actually about markers; its really about a specific point on the map. From a user perspective, they could care less about the markers or any of the mechanics around the markers. All they really care about is the information associated with that location. From that perspective, the challenge is finding a way to provide the user a way to view the associated information in a straightforward way. So I decided to merge all of the markers that occupy the same location into a single marker that includes all of the information that matters to the user.

Step-by-step, here is the approach I use to solve the multiple markers in the same location problem:

Step 1: Sort the marker data to allow identification of the markers that occupy the same location:

gl_list.sort( function( a, b ) {
    var aLat = a.Latitude, bLat = b.Latitude;               
    if ( aLat !== bLat ) { return aLat - bLat; }
    else { return a.Longitude - b.Longitude; }
});

Step 2: Add a new function that creates a "special" marker for colocated members of the gl_list array:

var specialMarkers = null;

function createSpecialMarker( specialMarkers ) {
    var infoWinContent = "<table class='special-infowin-table'>";

    for ( var i = 0; i < specialMarkers.length; i++ ) {
        infoWinContent +=
            "<tr>" +
            "<td class='special-table-label'>" +
                "Visited Place [" + (i+1) + "]" +
            "</td>" + 
            "<td class='special-table-cell'>" +
                specialMarkers[i].nome + " on : " + specialMarkers[i].data +
            "</td></tr>";
    }
    infoWinContent += "</table>";

    var mrkrData = specialMarkers[0];
    var point = new GLatLng( mrkrData.Latitude, mrkrData.Longitude );
    var marker = new GMarker( point );
    GEvent.addListener(marker, "click", function() {
    map.openInfoWindowHtml( point, infoWinContent );
});

    return marker;
}

Step 3: Iterate over the marker data, identify groupings that have the same location, show them on the map using a special marker, and handle all of the marker data at other locations "normally":

for ( var i = 0; i < gl_list.length; i++ ) {
    var current = gl_list[i];
    var coLocated = null;
    var j = 0, matchWasFound = false;

    if ( i < ( gl_list.length - 1 ) ) {
        do {
            var next = assetData[ i + ++j ];
            if ( next !== undefined ) {    //just to be safe
                if ( next.Latitude === current.Latitude &&
                        next.Longitude === current.Longitude ) {
                    matchWasFound = true;
                    if ( coLocated === null ) {
                        coLocated = new Array( current, next);
                    }
                    else { coLocated.push( next ); }
                }
                else { matchWasFound = false; }
            }
            else { matchWasFound = false; }
        }
        while ( matchWasFound )

        if ( coLocated != null ) {
           var coLoMarker = createSpecialMarker( coLocated );
            if ( specialMarkers === null ) {
                specialMarkers = new Array( coLoMarker );
            }
            else {
                specialMarkers.push( coLoMarker );
            }

            i += --j;
            continue;
        }

        var point = new GLatLng(gl_list[i].Latitude,gl_list[i].Longitude);
        description = "Visited place : "+gl_list[i].nome +
                        " on : " + gl_list[i].data;
        map.addOverlay( createMarker( point, i + 1, description ) );
    }
}

The idea is to produce a single marker and let the InfoWindow convey the multiple pieces of information about the location that are important, ending up with something that looks like this:

Now I haven't run this actual code, but it is based on code that runs every day. This is more intended to give you a fairly detailed look at the approach and share a body of code that should get you pretty darn close to a usable solution, with the understanding that it may need a little tweaking and debugging.