[Qgis-user] OTF reprojection and +towgs84 parameters

[Standaert Arnout]

Hello list,

I have a problem with reprojections between the Belgian Lambert 1972 projection and the Google Mercator projection. More specifically, I'm trying to align a Lambert 72-based shape file with the Google Streets background maps provided by the new Openlayers plugin.

Here's an example shapefile and steps to reproduce the problem:
 * my example SHP file, in Lambert 72, can be found here: http://wwwald.be/dltemp/streets.zip
 * load it in QGis using "Add vector layer". The layer projection should be the one mentioned below in this email, with the TOWGS84 parameters (*).
 * set OTF projection on and project CRS to Google Mercator (EPSG 900913)
 * use OpenLayers plugin to to "Add Google Streets Layer"
 * set the OpenLayers zoom level to an integer value

After this, the layers should be aligned, but in my case the streets SHP layer is off by about 100m in the NWW direction.
A screenshot can be seen here: http://wwwald.be/dltemp/qgis_shot.png

Is it possible that QGis does not apply the "+towgs84" datum transformation in this case? 
In any case, if I define a custom CRS, identical to the Lambert 72 one without the TOWGS84 parameters, the streets layer does not seem to shift position visibly. So the "+towgs84" parameters don't seem to make the difference they should.

Can anyone shed some more light on this?

Thanks a bunch for any help,

Arnout


(*) There was a thread on the PROJ mailing list regarding the Lambert 72 projection, see http://n2.nabble.com/Belge-1972-Belgian-Lambert-72-31370-towgs84-parameters-td4425548.html. 
The correct projection string for the Lambert 72, with TOWGS84 parameters is confirmed there to be:

+proj=lcc +lat_1=51.16666723333333 +lat_2=49.8333339 +lat_0=90 +lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl  +towgs84=-106.8686,52.2978,-103.7329,0.3366,-0.457,1.8422,-1.2747 +units=m +no_defs

Uniting expertise from different fields of technology in a pleasant and stimulating environment enhances innovation and  the development of novel methods for sustainable production.
Have a look at the international congress 'Innovation for Sustainable Production 2010'
April, 18-21, 2010 - Bruges (Belgium)
http://www.i-sup2010.org
---
This e-mail, any attachments and the information it contains are confidential and meant only for the use of the addressee(s) only.  Access to this e-mail by anyone other than the addressee(s) is unauthorized.  If you are not the intended addressee (or responsible for delivery of the message to such person), you may not use, copy, distribute or deliver to anyone this message (or any part of its contents) or take any action in reliance on it.  In such case, you should destroy this message and notify the sender immediately.  If you have received this e-mail in error, please notify us immediately by e-mail or telephone and delete the e-mail from any computer.
All reasonable precautions have been taken to ensure no viruses are present in this e-mail and its attachments.  As our company cannot accept responsibility for any loss or damage arising from the use of this e-mail or attachments we recommend that you subject these to your virus checking procedures prior to use.