[OpenLayers-Users] Change from OpenLayers.Layer.Google to OpenLayers.Layer.OSM

[Phil Scadden]

>
> My project has geoserver serving layers in EPSG:4326 projection.
Geoserver might store the layers in 4326, but it will serve them up in 
whatever the projection of your current baselayer is. This is very 
important to understand. Change the baselayer to one with a different 
projection and the projection of the map will change. As a result, it 
will fire off requests to geoserver asking for data in that baselayer 
projection.

>
> When I create a google layer:
>
>        var gmap = new OpenLayers.Layer.Google("Google Streets", 
> {numZoomLevels: 20});
>
> EPSG is 4326,
> and units are in degrees.

Actually,  if you use this, then your baselayer is 900913 and so your 
map will be 900913. Using 4326 as the actual "projection" doesnt make a 
lot of sense. 4326 is good to set as the maps displayProjection for 
mouse pointer but as a "projection" it has no features that a 
cartographer could love. 900913 isnt wonderful either but its far better 
than 4326. If you are going to google  OR OSM, then your map will be 
actually be in 900913. They are stored as image tiles and there is no 
easy way to reproject these on the fly to something else. In contrast, 
geoserver renders your data on the fly, into the requested image 
projection. That's easy for vector data. It can reproject raster on the 
fly now to but you might not like the look of it.

>
> var osm = new OpenLayers.Layer.OSM("osm", {projection: new 
> OpenLayers.Projection('EPSG:4326')});

When you create a layer, the  projection parameter is effectively a 
hint, telling openlayers what the NATIVE projection of the layer is. The 
map baselayer is what will determine the projection that is actually 
requested is. For OSM, this has no effect - you can only get 900913(3587)
> - Why is OSM layer projection EPSG:900913?
Because its a tile layer and cant be reprojected on the fly. (same as 
google).
> - Can I have a OSM baseLayer in a different projection? 
> The {projection: new OpenLayers.Projection('EPSG:4326')} argument 
> seemed to have no effect on the layer.
No.

If you really want a prerendered tile layer as base in something other 
than 900913, then you are probably better to create and store it 
yourself. That probably means hacking something like Tilemill or 
maptiler. However, I am pretty sure that you really dont want 4326 as a 
projection. My bible is Snyder's "Map Projections - A working manual".

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