[GIS] How to add markers to OSM base map in Google Maps API

google-maps-apiopenlayers-2openstreetmap

I am using the example from this webpage to display my maps:

http://wiki.openstreetmap.org/wiki/Google_Maps_Example#More_examples

(Example – Using Google Maps API v3 setting OSM as a base map layer)

However, I need to add markers but I get a blank page. See code below:

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8" />
    <meta name="viewport" content="initial-scale=1.0, user-scalable=no" />
    <meta name="apple-mobile-web-app-capable" content="yes" />
    <title>OpenStreetMap with Google Maps v3 API</title>
    <style type="text/css">
        html, body, #map {
            height: 100%;
            margin: 0;
            padding: 0;
        }
    </style>
</head>
<body>
    <div id="map"></div>

    <script type="text/javascript" src="http://maps.googleapis.com/maps/api/js?sensor=false"></script>
    <script type="text/javascript">
        var element = document.getElementById("map");

        /*
        Build list of map types.
        You can also use var mapTypeIds = ["roadmap", "satellite", "hybrid", "terrain", "OSM"]
        but static lists sucks when google updates the default list of map types.
        */
        var mapTypeIds = [];
        for(var type in google.maps.MapTypeId) {
            mapTypeIds.push(google.maps.MapTypeId[type]);
        }
        mapTypeIds.push("OSM");

        var map = new google.maps.Map(element, {
            center: new google.maps.LatLng(-37.372096000,143.919544000),
            zoom: 14,
            mapTypeId: "OSM",
            mapTypeControlOptions: {
                mapTypeIds: mapTypeIds
            }
        });

        var markers = new OpenLayers.Layer.Markers( "Markers" );
        map.addLayer(markers);

        markers.addMarker(new OpenLayers.Marker(lonLat));

        map.setCenter (lonLat, zoom);

        map.mapTypes.set("OSM", new google.maps.ImageMapType({
            getTileUrl: function(coord, zoom) {
                return "http://tile.openstreetmap.org/" + zoom + "/" + coord.x + "/" + coord.y + ".png";
            },
            tileSize: new google.maps.Size(256, 256),
            name: "OpenStreetMap",
            maxZoom: 18
        }));
    </script>
</body>

Please what am I doing wrong..

Best Answer

you are using OpenLayers markers. OpenLayers markers are used in OpenLayers Map api. Since you are using Google Maps Api, you need to use markers from google map api. so the markers sould be defined as

var markers = new google.maps.Marker() 
//see https://developers.google.com/maps/documentation/javascript/reference#Marker

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There is no way in OpenLayers to have vector data transform or reproject after a chance in base layer built into the library. OpenLayers Ticket #1249 is related to this, and this is one of the areas that we expect to improve in OpenLayers 3.0.

In the mean time, if you want this functionality, registering for a changebaselayer event and doing what you need to with that -- reprojecting the data, or whatever else makes sense -- is what you will need to do if you want to change the projection of your base layer / map.

OpenLayers – Fixing Image Layers Misplacement on Google Maps V3 and OSM

Hayley,

2nd UPDATE: Played around some more and got the radar overlay to work with OpenLayers. What I and a few others have referred to holds true here. The Ridge Radar gifs are really meant to go into Google Earth (though they might also play nice in Google Maps too). However, in OpenLayers, one does have to ensure that raster images are transformed to spherical mercator in order to display in Google Maps and Open Street Maps via OpenLayers. In order to do that, you have to geo-reference the raster to ESPG 4326 and then transform it to spherical mercator. Now I used GDAL to do the geo-referencing and also warping to spherical mercator. Here are the commands I used:

gdal_translate -of PNG -a_srs EPSG:4326 -a_ullr -127.62 50.41 -66.42 21.61 radar.gif radartemp1.png

gdalwarp -t_srs '+proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0 +y_0=0 +k=1.0 +units=m +nadgrids=@null +wktext +no_defs' radartemp1.png radartemp2.png

gdal_translate -of PNG -outsize 80% 80% radartemp2.png radartemp3.png

The first line geo-references the image to EPSG 4326 and also converts to a PNG. Just used to doing that but you should be able to keep GIFs (-of GIF).

The second line converts the image to EPSG 900913 via the proj.4 commands in single quotes. I had tried using 900913 and also 3785/3857 in gdalwarp but my version of GDAL didn't like those. Found the proj.4 commands worked. Key to this was the +nadgrids=@null option. I found that in our geo-referencing things worked fine with the transformation that we were using for our initial GM prototype. However, our images were "squished" when we tried to use them for OL. Discovered after a search and also in the reference that snapwire_org indicated that this is critical to getting the grid transformed into spherical mercator.

Now I found that in order to work with EPSG 900913 in OL, I had to use meters for the projection instead of the lat lon bounds and just transform everything between the two:

Declaring the map:

map = new OpenLayers.Map("map", {

    projection: new OpenLayers.Projection("EPSG:900913"),
    units: "m",
    maxResolution: 156543.0339,
    maxExtent: new OpenLayers.Bounds(
        -20037508, -20037508, 20037508, 20037508.34),
    displayProjection: new OpenLayers.Projection("EPSG:4326"),
    controls: [
        new OpenLayers.Control.Navigation(),
        new OpenLayers.Control.KeyboardDefaults(),
        new OpenLayers.Control.PanZoomBar(),
        new OpenLayers.Control.Scale(),
        new OpenLayers.Control.Attribution()
    ]

});

Now for the image overlay:

// create spherical mercator base layers

var googleLayer = new OpenLayers.Layer.Google("Google Physical", {type: google.maps.MapTypeId.TERRAIN, visibility: true});
var gstr = new OpenLayers.Layer.Google("Google Streets", {type: google.maps.MapTypeId.ROAD, visibility: true});
var osmLayer = new OpenLayers.Layer.OSM("OpenStreetMap");

NOW I've entered the correct numbers here after the 3rd GDAL transformation step above for your use in the new OpenLayers.Bounds line below

// create overlay with first image (GMoverlay is a global);

GMoverlay= new OpenLayers.Layer.Image("Radar Overlay","radartemp3.png",
     new OpenLayers.Bounds(-14206593.415,2465071.940,-7394741.764,6517585.728),   
     new OpenLayers.Size(847,484),
     {isBaseLayer: false, opacity: 0.4,
      projection: new OpenLayers.Projection("EPSG:900913")});

//add layers to map

map.addLayers([googleLayer, gstr,osmLayer, GMoverlay]);

Again, you can see in the Bounds, we're using meters:

new OpenLayers.Bounds(-14206593.415,2465071.940,-7394741.764,6517585.728)

You would get that info by running

gdalinfo radartemp3.png

The lower left and upper right coordinates of the transformed image are displayed in this command's output and then entered in the part above when declaring the new OL Image.

In going back and forth between meters and standard Lat/Lon, you have to use OL transforms like this one for setting the initial center:

map.setCenter(new OpenLayers.LonLat(-80, 40).transform(
        new OpenLayers.Projection("EPSG:4326"),
        map.getProjectionObject()),5);

You need to be careful here using transform on geometries as I discovered when grabbing a point on the map. This is also discussed in that link on spherical mercator and here's the relevant statement:

"Because all transforms are in place, once you have added a geometry to a layer, you should not call transform on the geometry directly: instead, you should transform a clone of the geometry:"

The referenced link that snapwire_org mentioned has a good deal of what I refer to here.

Also note that the size of the image is about a quarter the size of radartemp3.png. That's because you have to reduce the size number the farther out you zoom. So, you can't use 3700 x 1600 of the original image.

This really should be the answer you're looking for. I checked it myself in a test OL app with much of the pieces above and it projected spot on to what was being displayed at weather.gov. You still will have to do the geo-referencing and transformations. But GDAL is also open source and easy to install.

Tell you, it took a while to find that proj.4 option for our transformations. Then I pieced together several references which set up the map and image overlay in meters (including the one from snapwire_org) and then did transformations for points between 4326 and 900913. Worked like a charm then.

Good luck!

Best Answer

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