[Qgis-user] Problem georeferencing images in UTM

[Nicolas Cadieux]

> Le 18 mai 2018 à 00:28, <listsen at 0x906.com> <listsen at 0x906.com> a écrit :
> 
> 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)
>>>>>>  
>>>>>> 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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