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<p>Hi,</p>
<p>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.</p>
<p> 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.<br>
</p>
<p>BTW, the problem was solved when we used the TPS transformation
and not the linear transformation. <br>
</p>
<p>Cheers,</p>
<p>Nicolas<br>
</p>
<p> <br>
</p>
<br>
<div class="moz-cite-prefix">On 2018-05-18 3:52 PM, Nicolas Cadieux
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:F251D35C-16C4-4EA8-A093-F615AF503A3A@archeotec.ca">
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<div><br>
Le 18 mai 2018 à 00:28, <<a href="mailto:listsen@0x906.com"
moz-do-not-send="true">listsen@0x906.com</a>> <<a
href="mailto:listsen@0x906.com" moz-do-not-send="true">listsen@0x906.com</a>>
a écrit :<br>
<br>
</div>
<blockquote type="cite">
<div>
<p>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:<br>
<br>
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.<br>
<br>
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.<br>
<br>
If I find some time tomorrow, I will come back and give the
proper equations showing the difference between Google's
projection and UTM.<br>
<br>
-0x906</p>
<p><br>
</p>
<p>On 2018-05-17 19:53, Carlos Henrique Grohmann de Carvalho
wrote:</p>
<blockquote type="cite" style="padding: 0 0.4em; border-left:
#1010ff 2px solid; margin: 0"><!-- html ignored --><!-- head ignored --><!-- meta ignored -->
<div dir="ltr">Thanks for the input Nicolas
<div> </div>
<div>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.</div>
<div> </div>
<div>Carlos</div>
<div> </div>
</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Thu, May 17, 2018 at 7:28 PM,
Nicolas Cadieux <span><<a
href="mailto:nicolas.cadieux@archeotec.ca"
moz-do-not-send="true">nicolas.cadieux@archeotec.ca</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin: 0 0 0
.8ex; border-left: 1px #ccc solid; padding-left: 1ex;">
<div dir="auto">
<div> </div>
<div>Hi,</div>
<div>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.</div>
<div> </div>
<div>You can either use the long lat or use more
points. With 4 points and using the thin plate
spline, I got good results.</div>
<div>Nicolas</div>
<div>
<div class="h5">
<div><br>
Le 17 mai 2018 à 12:42, Carlos Henrique
Grohmann de Carvalho <<a
href="mailto:carlos.grohmann@gmail.com"
moz-do-not-send="true">carlos.grohmann@gmail.com</a>>
a écrit :<br>
<br>
</div>
<blockquote type="cite" style="padding: 0 0.4em;
border-left: #1010ff 2px solid; margin: 0">
<div>
<div dir="ltr">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.
<div> </div>
<div>Carlos</div>
<div class="gmail_extra"><br>
<div class="gmail_quote">On Thu, May 17,
2018 at 1:28 PM, Nicolas Cadieux <span><<a
href="mailto:nicolas.cadieux@archeotec.ca" moz-do-not-send="true">nicolas.cadieux@archeotec.ca</a>></span>
wrote:<br>
<blockquote class="gmail_quote"
style="margin: 0 0 0 .8ex;
border-left: 1px #ccc solid;
padding-left: 1ex;">
<div dir="auto">
<div> </div>
<div>Hi,</div>
<div>Disable projection on the
fly. Make sure the project is
in the same CRS as your maps.</div>
<div>Nicolas</div>
<div>
<div
class="m_5331241165982910871m_1909886288506383736h5">
<div><br>
Le 17 mai 2018 à 12:14,
Carlos Henrique Grohmann de
Carvalho <<a
href="mailto:carlos.grohmann@gmail.com"
moz-do-not-send="true">carlos.grohmann@gmail.com</a>>
a écrit :<br>
<br>
</div>
<blockquote type="cite"
style="padding: 0 0.4em;
border-left: #1010ff 2px
solid; margin: 0">
<div>
<div dir="ltr"><span
style="font-size:
12.800000190734863px;">Hello
all</span>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;">I
have two pngs of
topographic maps from
southeastern Brazil (a
zip file with images
and points is
available here: <a
href="https://www.dropbox.com/s/mexeh4be46pgsk6/images_georef.zip?dl=0"
target="_blank"
rel="noopener
noreferrer"
moz-do-not-send="true">https://www.dropbox.com/<wbr>s/mexeh4be46pgsk6/images_geore<wbr>f.zip?dl=0</a>)</div>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;">I'm
trying to georeference
them, but with partial
success only. In this
image (<a
href="https://www.dropbox.com/s/37xxqp4fkwpdyqh/utm_error.png?dl=0"
target="_blank"
rel="noopener
noreferrer"
moz-do-not-send="true">https://www.dropbox.com/s/37x<wbr>xqp4fkwpdyqh/utm_error.png?dl=<wbr>0</a>)
you can see that the X
coordinate is fine,
but the Y coordinate
of the two maps won't
match.</div>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;">Both
maps use South
American Datum 1969. I
selected points near
the edges, and used
the UTM values. </div>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;">Interestingly,
if I use Lat/Long, and
place the points at
the corners, using
decimal degrees, the
maps align perfectly.</div>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;"> </div>
<div style="font-size:
12.800000190734863px;">Any
help is appreciated</div>
<br clear="all">
<div> </div>
-- <br>
<div
class="m_5331241165982910871m_1909886288506383736m_2947533340930390789gmail_signature">
<div dir="ltr">Prof.
Carlos Henrique
Grohmann<br>
Institute of Energy
and Environment -
Univ. of São Paulo,
Brazil
<div>- Digital
Terrain Analysis |
GIS | Remote
Sensing - </div>
<div> </div>
<div><a
href="http://carlosgrohmann.com/"
target="_blank"
rel="noopener
noreferrer"
moz-do-not-send="true">http://carlosgrohmann.com</a></div>
<div><a
style="font-size:
13px; color:
#1155cc;"
href="http://orcid.org/0000-0001-5073-5572"
target="_blank"
rel="noopener
noreferrer"
moz-do-not-send="true">http://orcid.org/0000-0001-507<wbr>3-5572</a><br>
<div>________________<br>
Can't stop the
signal.</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
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</div>
<blockquote type="cite"
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margin: 0">
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<br clear="all">
<div> </div>
-- <br>
<div
class="m_5331241165982910871m_1909886288506383736gmail_signature">
<div dir="ltr">Prof. Carlos Henrique
Grohmann<br>
Institute of Energy and Environment
- Univ. of São Paulo, Brazil
<div>- Digital Terrain Analysis |
GIS | Remote Sensing - </div>
<div> </div>
<div><a
href="http://carlosgrohmann.com/"
target="_blank" rel="noopener
noreferrer"
moz-do-not-send="true">http://carlosgrohmann.com</a></div>
<div><a style="font-size: 13px;
color: #1155cc;"
href="http://orcid.org/0000-0001-5073-5572"
target="_blank" rel="noopener
noreferrer"
moz-do-not-send="true">http://orcid.org/0000-0001-507<wbr>3-5572</a><br>
<div>________________<br>
Can't stop the signal.</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
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</div>
</div>
</blockquote>
</div>
<br>
<br clear="all">
<div> </div>
-- <br>
<div class="gmail_signature">
<div dir="ltr">Prof. Carlos Henrique Grohmann<br>
Institute of Energy and Environment - Univ. of São
Paulo, Brazil
<div>- Digital Terrain Analysis | GIS | Remote Sensing
- </div>
<div> </div>
<div><a href="http://carlosgrohmann.com/"
target="_blank" rel="noopener noreferrer"
moz-do-not-send="true">http://carlosgrohmann.com</a></div>
<div><a style="font-size: 13px; color: #1155cc;"
href="http://orcid.org/0000-0001-5073-5572"
target="_blank" rel="noopener noreferrer"
moz-do-not-send="true">http://orcid.org/0000-0001-5073-5572</a><br>
<div>________________<br>
Can't stop the signal.</div>
</div>
</div>
</div>
</div>
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