Re: [Qgis-user] Georeference'd maps off by 200m

Tue, 28 Jul 2015 20:03:07 -0700 

Hi, 
Change the project properties to  EPSG:32618 .  Make sure you 
have CRS on the fly activated as extra precaution.  Then click on the 
source image (nad27) and then where it should be in 32618. (If you are adding 
the coordinates by hand, then you are using 2032. If you clicking on the 
screen, the coordinates are probably in 4326.) 
Look at the coordinate for the control point. The source should be in pixels 
and the second in 32618.  You can check by creating a shp file with points 
in 2032 then reprojecting it in 32618. 
If you have problems, use 2032 for your project CRS and try again then use warp 
if you absolutely need the map to be in another projection. 
Send the file with the control points if thing are not working for you. 
Nicolas Cadieux M.Sc. 
Les Entreprises Archéotec inc.  
8548, rue Saint-Denis Montréal H2P 2H2 
Téléphone: 514.381.5112  Fax: 514.381.4995 
www.archeotec.ca 
On Jul 28, 2015 21:58, "Nick Papadonis [via OSGeo.org]     " 
<[email protected]> wrote: 

        Hi, I did reference the CRS of both source and output.  Source: 
EPSG:2032 NAD27 UTM Zone 18N and Output: EPSG:32618 WGS84 UTM.  The project 
properties are EPSG:4326 WGS 84.  When I use the georeferencer the resulting 
raster is still off by about 200m.  If I click on the raster and change it from 
WGS84 to NAD27 CRS, then everything appears to line up.  This is odd because I 
would expect the raster to be reprojected at WGS84.  I then used the 
Raster->Projections->Warp tool to change the Source SRS from WGS84 to 
NAD27 and the Output SRS to WGS84 and then reprojected the image.  The 
resulting GeoTiff lines up correctly. It almost seems like the Georeferencer is 
not re-projecting between these two formats correctly.  I need to use the Warp 
tool to reproject manually.  Does this sound right? Thanks, Nick On Jul 27, 
2015, at 10:33 PM, Nicolas Cadieux < [hidden email] > wrote: Hi, When you 
use the georeferencer, you need to indicate the CRS of the the source file and 
of the the output file. Perhaps that step was not done properly.  I have had 
this type of problems in the past and it had to do with me changing  the CRS 
during the process.  Perhaps the output file CRS was changed and that change 
was not recorded.  Generally speaking, the module works and will reproject to 
anything.  Make sure the project CRS is the same as what the output should be 
for extra precaution. Nicolas Cadieux M.Sc. 
Les Entreprises Archéotec inc.  
8548, rue Saint-Denis Montréal H2P 2H2 
Téléphone: 514.381.5112  Fax: 514.381.4995 
www.archeotec.ca 
On Jul 27, 2015 18:45, "Nick Papadonis [via OSGeo.org ]     " <<a 
href="x-msg://9/user/SendEmail.jtp?type=node&node=5217285&i=0"
 target="_top" rel="nofollow" 
link="external" class="">[hidden email]> wrote: 
<blockquote style="border-left:2px solid #cccccc;padding:0 1em" 
style="border-left:2px solid #cccccc;padding:0 1em">

        I was able to get everything to line up correctly by changing the layer 
property CRS from WGS84 UTM to EPSG:26718 NAD27 UTM.  I was expecting the new 
raster to have WGS84 coordinates and the projected image with it.  It seems the 
coordinates are still referencing the NAD27 projection after the they should 
not.  Therefore I’m wondering if the Georeferencer pluging reprojects or not.
Thanks
> On Jul 27, 2015, at 6:19 PM, Nick Papadonis <<a 
href="/user/SendEmail.jtp?type=node&node=5217272&i=0">[hidden
 email]> wrote:
> 
> Hi Folks,
> 
> I’m trying to scan in a topographic map into a georeferenced raster so I 
can trace vector over it.  I have been playing with the plugin for a few hours 
and am having odd results. I’m hoping an expert here can point me in the right 
direction.  The issue is when I finally open the resulting TIFF file as a 
raster layer it appears the raster is 200m too far to the south.  It also 
appears this is a uniform error across the map.  The east and west line up 
fairly well.  I was suspecting something with the projection choices or 
coordinate systems. I figured the coordinate system translation code would have 
already had plenty of use in the community by now.  When creating the TIFF I 
used 10 coordinate points and Projective/Cubic.  I also tried 2-degree poly 
with similar results.  For the target SRS I used EPSG:32618 WGS84 UTM.  The 
raster source is EPSG:2032 NAD27 UTM Zone 18N. The two vector data sources I 
checked agree and are EPSG:4326 WGS 84.  I tried to create the TIFF in this 
format, however it ended up in the wrong coordinates when loaded as a layer.
> 
> Any tips or tricks appreciated.
> 
> Thanks,
> Nick
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