[Qgis-user] Georeference'd maps off by 200m

Tue Jul 28 19:02:44 PDT 2015

Hi,

I did reference the CRS of both source and output.  Source: EPSG:2032 NAD27 UTM Zone 18N and Output: EPSG:32618 WGS84 UTM.  The project properties are EPSG:4326 WGS 84.  When I use the georeferencer the resulting raster is still off by about 200m.  If I click on the raster and change it from WGS84 to NAD27 CRS, then everything appears to line up.  This is odd because I would expect the raster to be reprojected at WGS84.  I then used the Raster->Projections->Warp tool to change the Source SRS from WGS84 to NAD27 and the Output SRS to WGS84 and then reprojected the image.  The resulting GeoTiff lines up correctly.

It almost seems like the Georeferencer is not re-projecting between these two formats correctly.  I need to use the Warp tool to reproject manually.  Does this sound right?

Thanks,
Nick

> On Jul 27, 2015, at 10:33 PM, Nicolas Cadieux <nicolas.cadieux at archeotec.ca> wrote:
> 
> Hi,
> 
> When you use the georeferencer, you need to indicate the CRS of the the source file and of the the output file. Perhaps that step was not done properly.  I have had this type of problems in the past and it had to do with me changing  the CRS during the process.  Perhaps the output file CRS was changed and that change was not recorded.  Generally speaking, the module works and will reproject to anything.  Make sure the project CRS is the same as what the output should be for extra precaution. 
> 
> Nicolas Cadieux M.Sc.
> Les Entreprises Archéotec inc. 
> 8548, rue Saint-Denis Montréal H2P 2H2
> Téléphone: 514.381.5112  Fax: 514.381.4995
> www.archeotec.ca
> 
> On Jul 27, 2015 18:45, "Nick Papadonis [via OSGeo.org]	" <[hidden email] <x-msg://9/user/SendEmail.jtp?type=node&node=5217285&i=0>> wrote:
> I was able to get everything to line up correctly by changing the layer property CRS from WGS84 UTM to EPSG:26718 NAD27 UTM.  I was expecting the new raster to have WGS84 coordinates and the projected image with it.  It seems the coordinates are still referencing the NAD27 projection after the they should not.  Therefore I’m wondering if the Georeferencer pluging reprojects or not. 
> 
> Thanks 
> 
> > On Jul 27, 2015, at 6:19 PM, Nick Papadonis <<a href="/user/SendEmail.jtp?type=node&node=5217272&i=0">[hidden email]> wrote: 
> > 
> > Hi Folks, 
> > 
> > I’m trying to scan in a topographic map into a georeferenced raster so I can trace vector over it.  I have been playing with the plugin for a few hours and am having odd results. I’m hoping an expert here can point me in the right direction.  The issue is when I finally open the resulting TIFF file as a raster layer it appears the raster is 200m too far to the south.  It also appears this is a uniform error across the map.  The east and west line up fairly well.  I was suspecting something with the projection choices or coordinate systems. I figured the coordinate system translation code would have already had plenty of use in the community by now.  When creating the TIFF I used 10 coordinate points and Projective/Cubic.  I also tried 2-degree poly with similar results.  For the target SRS I used EPSG:32618 WGS84 UTM.  The raster source is EPSG:2032 NAD27 UTM Zone 18N. The two vector data sources I checked agree and are EPSG:4326 WGS 84.  I tried to create the TIFF in this format, however it ended up in the wrong coordinates when loaded as a layer. 
> > 
> > Any tips or tricks appreciated. 
> > 
> > Thanks, 
> > Nick
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