[Qgis-user] Problem georeferencing images in UTM

[listsen at 0x906.com]

I haven't been able to open the dataset but going through the discussion
I would like to offer some assistance since I believe there are some
misconceptions here:

First, the data has nothing to do with the NAD27 / NAD83 datum's since
these are North American datum's and they don't apply I believe in South
America. Moreover, the separation between NAD27 to NAD83 is about 100m.
I think Carlos mention something like being 400m off.

Another point of confusion is about the projection of Google Earth.
Google is using a Web Mercator projection based on a perfect sphere.
This has nothing to do with the UTM that has already incorporate the
Earth Flattening values into the Northing and Easting equations.

If I find some time tomorrow, I will come back and give the proper
equations showing the difference between Google's projection and UTM.

-0x906 

On 2018-05-17 19:53, Carlos Henrique Grohmann de Carvalho wrote:

> Thanks for the input Nicolas 
> 
> These are official maps so the utm grid is ok. I figured that I was using a linear transformation, with the option to just create the world file, which works for control points at the corners (lat/long), since the boundaries of the map is defined by latitude/longitude. With UTM, this simple transformation won't work, but TPS will give the correct results. 
> 
> Carlos 
> 
> On Thu, May 17, 2018 at 7:28 PM, Nicolas Cadieux <nicolas.cadieux at archeotec.ca> wrote:
> 
> 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 [1]) 
> 
> 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 [2]) 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 [3] 
> http://orcid.org/0000-0001-5073-5572 [4]
> ________________
> 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 [3] 
http://orcid.org/0000-0001-5073-5572 [4]
________________
Can't stop the signal. 

  -- 

Prof. Carlos Henrique Grohmann
Institute of Energy and Environment - Univ. of São Paulo, Brazil 
- Digital Terrain Analysis | GIS | Remote Sensing -  

http://carlosgrohmann.com [3] 
http://orcid.org/0000-0001-5073-5572
________________
Can't stop the signal. 
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Links:
------
[1] https://www.dropbox.com/s/mexeh4be46pgsk6/images_georef.zip?dl=0
[2] https://www.dropbox.com/s/37xxqp4fkwpdyqh/utm_error.png?dl=0
[3] http://carlosgrohmann.com/
[4] http://orcid.org/0000-0001-5073-5572
[5] https://lists.osgeo.org/mailman/listinfo/qgis-user
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