Hi,
NAD27 does touch the northern part of South America but you are correct
in saying that it does not extend in Brazil. I wrote the comments when
I was still trying to figure out where the maps were in the world. I
often get problems with NAD27 vs NAD83 grid in Canada as many maps have
been printed with the errors in NAD27 military grid reference system.
In those cases, error in much bigger than the standard 30 to 100 metres
error so that was my working hypotheses. People assume that adjoining
maps use the same CRS and that is often not the case in remote areas.
I was reprojecting the Google map image to the local CRS in order to
figure out the distortion and not using the WGS 84 Pseudo Merc
coordinates. It's important, like to say to know that there are
differences between the web merc and the various WGS 84 realizations.
I would like to see the numbers out of curiosity if you have them. I
often wonder why these projections were invented.
BTW, the problem was solved when we used the TPS transformation and not
the linear transformation.
Cheers,
Nicolas
On 2018-05-18 3:52 PM, Nicolas Cadieux wrote:
Le 18 mai 2018 à 00:28, <[email protected] <mailto:[email protected]>>
<[email protected] <mailto:[email protected]>> 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
<[email protected] <mailto:[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] <mailto:[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]
<mailto:[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]
<mailto:[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
<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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