[Qgis-user] Problem georeferencing images in UTM

[Carlos Henrique Grohmann de Carvalho]

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)
>>
>> 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.
>
>


-- 
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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