[Qgis-user] Problem georeferencing images in UTM

[Kirk Schmidt]

Hi Carlos:

I georeferenced 2 screen shots of your maps and obtained good results.  
I would have a close look at the coordinates of your ground control 
points and make sure you selected the utm grid intersection as a control 
point versus the corner of the mapsheet which is a lat/long grid. It is 
easy to make a simple mistake

Kirk



On 5/17/2018 8:53 PM, 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 <mailto: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 <mailto: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
>>     <mailto: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
>>         <mailto: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
>>>         <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
>>>         <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://carlosgrohmann.com/>
>>>         http://orcid.org/0000-0001-5073-5572
>>>         <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://carlosgrohmann.com/>
>>     http://orcid.org/0000-0001-5073-5572
>>     <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://carlosgrohmann.com/>
> http://orcid.org/0000-0001-5073-5572
> ________________
> Can’t stop the signal.
>
>
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-- 
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
RR # 1
Thorburn, NS
B0K 1W0
Tel (902) 922.3607
Email: kirk at nortekresources.com
Web: www.nortekreources.com

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