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
> <[email protected]> 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
> <[email protected]> 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
> <[email protected]> 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
> <[email protected]> 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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