[Qgis-user] georeferencing

[M.E.Dodd]

Some more details.  
1. I have large 60mb .tiff (generated eventually from yesterdays .dxf file that I was trying to geocode and have given up for moment but will return to).  The .tiff has transparency.

2.  using:
QGIS version 1.9.90-Alpha QGIS code revision 9a9ab73 Compiled against Qt 4.7.1 Running against Qt 4.7.1 GDAL/OGR Version 1.8.1 GEOS Version 3.2.2 PostgreSQL Client Version 8.3.10 SpatiaLite Version 2.4.0 QWT Version 5.2.1
On windowsxp

3. used the georeferencer.  Identified points on the image and gave them the epsg:900913 x and y coordinates.  Actually I gps'd the 9 reference locations with survey grade gps and converted these data to the google reference system using http://twcc.free.fr/ which seemed to give reasonable results although not able to check accuracy other than plotting on google background.

4. chose transformation type: polynomial 1, resampling: nearest neighbour, no compression and target srs epsg900913 and load into qgis automatically.

5. the mean transformation error was 2.08 pixels which I think is pretty reasonable as I am not trying for extreme accuracy in this case.

6. then looking at the map with openstreetmap background the control points automatically appeared (I did not add them as a separate layer), they were in the correct place. But the map itself was shown about 19km further north than it should have been, I could measure exactly how far north and if its only the northing that is wrong.


-----Original Message-----
From: Andreas Neumann [mailto:a.neumann at carto.net] 
Sent: 01 March 2012 10:06
To: qgis-user at lists.osgeo.org
Subject: Re: [Qgis-user] georeferencing

 Hi,

 QGIS can reproject rasters on the fly. I believe from Version 1.7x on -  definitely with the trunk version.

 However, if you use Google maps or Openstreetmap through the OpenLayers  plugin you fixed to their projection.

 Reprojecting your own data to their projection should still work  though.

 If it doesn't, please provide more details - it may be a bug then.

 Andreas

 On Thu, 1 Mar 2012 09:53:50 +0000, M.E.Dodd wrote:
> I want to show a detailed map of buildings and vegetation over a 
> background image e.g. openstreetmap or google etc. Don't think qgis 
> can do on the fly transformation of rasters at moment so assume I have 
> to geocode the detailed map using epsg:900913 so it can be shown with 
> openstreetmap. Converted a set of fixed point locations from original 
> map into google's x and y then did the geocoding in qgis and plotted 
> over openstreetmap. The individual points then came out in the correct 
> place but the map itself was shown about 19km too far north. If I'd 
> had made a mistake in the x,y coordinates or the coordinate system 
> then the points should not be in the correct place so I am a bit 
> mystified as to what has happened and how to go about fixing it.
> The map I am trying to georeference is a .tiff with transparency 
> hoping that this would allow the details to show over the background, 
> however the 'transparent' part is showing up black at the moment so 
> some adjustment here needed too.
>
>  --
>  The Open University is incorporated by Royal Charter (RC 000391), an 
> exempt charity in England & Wales and a charity registered in Scotland 
> (SC 038302).

--
 --
 Andreas Neumann
 Böschacherstrasse 10A
 8624 Grüt (Gossau ZH)
 Switzerland
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