[OpenLayers-Users] OpenLayers with Google Maps - how to obtain degree lat/long?

[Eric Lemoine]

On Tuesday, November 17, 2009, plen <peteralen at earthlink.net> wrote:
>
> Hello,
>
> I am using OpenLayers 2.8 and based on multiple examples I found, I use the
> following to load a Google Map map layer:
>
> ------------------------------
> var options = {
>                 projection: new OpenLayers.Projection("EPSG:900913"),
>                 units: "m",
>                 maxResolution: 156543.0339,
>                 maxExtent: new OpenLayers.Bounds(-20037508.34, -20037508.34,
>                                                  20037508.34, 20037508.34)
> };
> map = new OpenLayers.Map('map', options);
> var layer = new OpenLayers.Layer.Google("Google", {"sphericalMercator":
> true});
> map.addLayer(layer);
> ------------------------------
>
> I do indeed get the google map layer displayed but what I ran into is that
> these options change how the OL API handles or deals with latlon
> coordinates.  For example, when I click on the map I get the following type
> of "coordinate" when calling the OL getLatLongFromViewportPX() function:
> (Lat: -39135.7584  / Lon: -273950.3093).  If I do a map.getExtent(), which
> typically returns the latlon for the SW and NE boundary corners, now returns
> a value like -381458.9896 / -722521.0097, -322984.6629 / -686366.2955.
> These are obviously not coordinates in degrees but some other value.  My
> application is expecting a decimal coordinate in degrees, which OL returns
> in the getgetLatLongFromViewportPX() and getExtent() function calls when
> google maps are not used.  Now the app does not work.
>
> I see that the "unit" value listed in the options is "m". Not sure if that
> is meters or some other value.  The OpenLayers.Bounds value also does not
> seem to be listed in a latlon coordinate.  In order to get my coordinate
> values in degrees, is there another set of options to use or is there some
> formula/algorithm that can be applied to what comes back from the OL
> functions in order to get the value I am looking for.
>
> Thanks for any insight - Peter

Yes, Google layers do not use the EPSG:4326 (lonl/at) coordinates
system. You can reproject lonlat, bounds and geometry objects to
EPSG:4326 using the transform method:

obj.transform(
    new OpenLayers.Projection("EPSG:900913"),
    new OpenLayers.Projection("EPSG:4326")
);

where obj is a LonLat, Bounds or Geometry object.

Cheers,


-- 
Eric Lemoine

Camptocamp France SAS
Savoie Technolac, BP 352
73377 Le Bourget du Lac, Cedex

Tel : 00 33 4 79 44 44 96
Mail : eric.lemoine at camptocamp.com
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