[OpenLayers-Users] RE: Problems changing map projections

[Jon Blower]

(Apologies if this comes through twice, I don't think my first attempt worked.)

Hi,

In my application I would like to be able to switch smoothly between projections, particularly between lat-lon (EPSG:4326) and north polar stereographic (NPS, EPSG:32661).

I have noticed that the switch from lat-lon to NPS is very slow and have been trying to find out why.  Debugging in Firebug reveals that OpenLayers tries to load 35-40 tiles during the switching process.  Only the last 4 tiles are the ones that are needed to display the map in NPS, but the previous 35-ish are spurious.

Looking at the request strings, it seems that these spurious tiles have an extent that is more consistent with lat-lon than with NPS.  My code looks like this:

function changeProjection()
{
   var proj = ... // new projection code
   var newBaseLayer = getBaseLayerForProj(proj);
   // Set the parameters of the map
   map.setOptions({
       projection: proj,
       maxExtent: newBaseLayer.maxExtent,
       resolutions: newBaseLayer.resolutions,
       maxResolution: newBaseLayer.maxResolution
   });
   map.zoomToMaxExtent();
   map.setBaseLayer(newBaseLayer);
}

getBaseLayerForProj() finds a Layer object that is requestable in the new projection.  (Not all WMS servers can return imagery in NPS projection.)

There is another clue: zoomToMaxExtent() doesn't work as I expected.  Instead of zooming the map to an extent suitable for a NPS map (in this case, (-4350000,-4350000 -> 8350000, 8350000)), it zooms to a much smaller extent (-180,-140 -> 180,140).  The map only zooms to the full NPS extent after setBaseLayer().

It is the setBaseLayer() method that seems to trigger all the spurious tile-loading, and this is the slowest method in the above scheme.

So my guess is that the setting of the maxExtent of the map to the extent of NPS is being restricted by the extent of the currently-displayed base layer, meaning that the map does not zoom out to the correct extent.  So when setBaseLayer() is called, OL tries to load tiles using SRS=32661, but with BBOXes that are more appropriate to a lat-lon map.  Only after a large number of tiles have been loaded does OL "realise" that the extent of the map has changed, and finally loads the four tiles with the BBOXes I really wanted.

If I'm right, then I guess I need to find a way of changing the map's projection in an "atomic" way that doesn't trigger lots of tiles to be loaded during the process of changing the projection.

(By the way, I tried removing and hiding the current baselayer, changed the projection and then added a new baselayer afterwards.  But the setBaseLayer() method exhibits the same behaviour.)


Thanks in advance for any help, and sorry for the long email!

Jon


--
Dr Jon Blower
Technical Director, Reading e-Science Centre
Environmental Systems Science Centre
University of Reading, UK
Tel: +44 (0)118 378 5213
http://www.resc.reading.ac.uk