[OpenLayers-Users] Spherical Mercator 4326 Layers do not line up

[Rohan Parkes]

I've seen this problem on some Spherical Mercator maps, but generally they
should line up. I notice that in Singapore, Google Satellite and Bing street
are out by a large amount that looks like a failure to apply the WGS84 datum
when calculating degrees lat lon.

 

If the problem is a datum shift, it might be possible to correct it by
converting the coordinates from Spherical degrees to WGS84 degrees. I do
this in a static map generator, where the formulae I was using gave
spherical degrees instead of real WGS degrees. I converted the spherical
degrees back to metres (i.e. 900913 projection), using the reverseMercator
formulae from the OL SphericalMercator source file. Then I converted the
metres to proper WGS84, using the Proj4 library. This might be kludgy, but
it works. (If anyone knows a more elegant way of doing it, I'd be grateful.)

 

However, just one thing - you have the units defined as 'ft' in your map
global options. I think this should be 'm'.

 

Rohan Parkes 



From: users-bounces at openlayers.org [mailto:users-bounces at openlayers.org] On
Behalf Of B. Heath Robinson
Sent: Sunday, 15 August 2010 06:38
To: users at openlayers.org
Subject: [OpenLayers-Users] Spherical Mercator 4326 Layers do not line up

 

I am having a problem with a WMS layer lining up on top of a Google map.  It
appears to be about 15mi south of were it needs to be.  I am assuming this
is because of the fact that spherical mercator ignores the fact the earth is
an ellipse.  Anyway, I have included my openlayers code and my mapfile as
after spending hours search seems to be helpful.

I get the same results using either a shapfile or postgis data each with a
different projection.

I am using what I believe to be the best method.  If I use "reproject:true"
the map does line up, but that causes other known problems and is
deprecated.  Any help would be greatly appreciated.
---------Open Layers---------
function init(){
           var options = {
               projection: "EPSG:900913",
               units: "ft",
               maxResolution: 156543.0339,
               maxExtent: new OpenLayers.Bounds(-20037508, -20037508,
                                                20037508, 20037508)
           };
           map = new OpenLayers.Map('map', options);

           var gmap = new OpenLayers.Layer.Google(
               "Google Streets",
               {'sphericalMercator': true}
           );




-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.osgeo.org/pipermail/openlayers-users/attachments/20100819/9e82cc8f/attachment.html