[OpenLayers-Users] SphericalMercator and SRS aliases epsg:900913, epsg:3857, epsg:3785, epsg:102113

[Bart van den Eijnden (OSGIS)]

Hi Anne,

in trunk this has been addressed, see:

http://trac.openlayers.org/ticket/2665

Best regards,
Bart

>   Hello list,
>
> OpenStreetMap and also some popular commercial mapservices (google,
> bing, yahoo) are using the so called sphericalmercator projection.
> OpenLayers has built-in support for this type of layer, inherited from
> OpenLayers.Layer.SphericalMercator.
>
> If you use a sphericalmercator layer as a baselayer, you may want to
> combine this with other layers. By default,  the openlayers
> sphericalmercator baselayer uses SRS projection-code 'EPSG:900913' . So,
> if you try to open an overlay, the overlay layer will be called with
> parameter SRS=EPSG:900913.
>
> EPSG Code 900913 happens to be a bogus code, based on the word 'google'.
> Perhaps due to the resemblance of '900913' and 'google' or due to other
> reasons, there are now several other codes in use for the same projection:
> EPSG:3857
> EPSG:3785 <= maybe because of a typo by someone somewhere?
> EPSG:102113
>
> Some map-services have stopped supporting SRS=EPSG:900913 and this
> raises a problem for openlayers sphericalmercator users. Instead of
> displaying a map, these mapservices will respond with something like 'no
> support for EPSG:900913'.
>
> The problem of overlaying sphericalmercator layers that do not support
> epsg:900913 can be (partly) solved as follows:
> Override the default srs code for sphericalmercator layers by explicitly
> resetting the projection before adding the layer to a map:
>
> var osmLayer = new OpenLayers.Layer.OSM("OpenStreetMap"); // osmlayer is
> now sphericalmercator with default epsg:900913
> osmLayer.projection = new OpenLayers.Projection ("EPSG:3857"); //
> osmlayer is now sphericalmercator with epsg:3857
> map.addLayers ([osmLayer]);
>
> Overlay layers will now be requested with SRS=ESPG:3857
>
> However, resetting the code for the baselayer projection, causes trouble
> for the controls for MousePosition and MeasureControl: distances and
> coordinates are no longer presented in the correct projection.
>
> I solved this by adding the following lines of code to file
> lib\OpenLayers\Layer\SphericalMercator.js
> OpenLayers.Projection.addTransform("EPSG:4326", "EPSG:3857",
>      OpenLayers.Layer.SphericalMercator.projectForward);
> OpenLayers.Projection.addTransform("EPSG:4326", "EPSG:102113",
>      OpenLayers.Layer.SphericalMercator.projectForward);
> OpenLayers.Projection.addTransform("EPSG:4326", "EPSG:3785",
>      OpenLayers.Layer.SphericalMercator.projectForward);
>
> OpenLayers.Projection.addTransform("EPSG:3857", "EPSG:4326",
>      OpenLayers.Layer.SphericalMercator.projectInverse);
> OpenLayers.Projection.addTransform("EPSG:102113", "EPSG:4326",
>      OpenLayers.Layer.SphericalMercator.projectInverse);
> OpenLayers.Projection.addTransform("EPSG:3785", "EPSG:4326",
>      OpenLayers.Layer.SphericalMercator.projectInverse);
>
> Questions:
> - is this the way to support aliases for 900913 and support
> MousePosition and MeasureControl at the same time?
> - if yes, shouldn't the above lines be part of the standard OpenLayers
> distribution?
>
> Anne Blankert
>
>
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>


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Bart van den Eijnden
OSGIS
bartvde at osgis.nl