[Qgis-user] Problem georeferencing images in UTM

[Nicolas Cadieux]

Hi,
The utm grid is way off or we have the wrong epsg code.  If you geofence using 2 points (linear algo)  using a service like google earth, you will see the distortion.  The long lat coordinate are probably ok but I did not test them.

You can either use the long lat or use more points.  With 4 points and using the thin plate spline, I got good results.
Nicolas

> Le 17 mai 2018 à 12:42, Carlos Henrique Grohmann de Carvalho <carlos.grohmann at gmail.com> a écrit :
> 
> Thanks Nicolas, but the misalignment still persists. OTF is off, project has same CRS as data (epsg 29192). There is a ~400m difference in the N-S direction.
> 
> Carlos
> 
>> On Thu, May 17, 2018 at 1:28 PM, Nicolas Cadieux <nicolas.cadieux at archeotec.ca> wrote:
>> Hi,
>> Disable projection on the fly.  Make sure the project is in the same CRS as your maps.
>> Nicolas
>> 
>>> Le 17 mai 2018 à 12:14, Carlos Henrique Grohmann de Carvalho <carlos.grohmann at gmail.com> a écrit :
>>> 
>>> Hello all
>>> 
>>> I have two pngs of topographic maps from southeastern Brazil (a zip file with images and points is available here: https://www.dropbox.com/s/mexeh4be46pgsk6/images_georef.zip?dl=0)
>>> 
>>> I'm trying to georeference them, but with partial success only. In this image (https://www.dropbox.com/s/37xxqp4fkwpdyqh/utm_error.png?dl=0) you can see that the X coordinate is fine, but the Y coordinate of the two maps won't match.
>>> 
>>> Both maps use South American Datum 1969. I selected points near the edges, and used the UTM values. 
>>> 
>>> Interestingly, if I use Lat/Long, and place the points at the corners, using decimal degrees, the maps align perfectly.
>>> 
>>> 
>>> Any help is appreciated
>>> 
>>> 
>>> -- 
>>> Prof. Carlos Henrique Grohmann
>>> Institute of Energy and Environment - Univ. of São Paulo, Brazil
>>> - Digital Terrain Analysis | GIS | Remote Sensing - 
>>> 
>>> http://carlosgrohmann.com
>>> http://orcid.org/0000-0001-5073-5572
>>> ________________
>>> Can’t stop the signal.
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> 
> 
> 
> -- 
> Prof. Carlos Henrique Grohmann
> Institute of Energy and Environment - Univ. of São Paulo, Brazil
> - Digital Terrain Analysis | GIS | Remote Sensing - 
> 
> http://carlosgrohmann.com
> http://orcid.org/0000-0001-5073-5572
> ________________
> Can’t stop the signal.
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