[Qgis-user] Re: "Freehand" raster georeferencing

James Card James.Card at calcad.com
Sun Jul 26 18:13:33 EDT 2009


On Sun, 26 Jul 2009 14:24:16 -0700, A. Marcia BARBOSA  
<anamarciabarbosa at gmail.com> wrote:

> Actually a "free-hand" georreferencing tool/option is something I have
> wished for more than once, too. For example, right now I'm trying to
> georeference some old maps whose limits were drawn by hand, naturally
> with little accuracy. It's quite difficult to find clear features
> whose coordinates truly match my own georeferenced maps, and none of
> my attempts with the georeferencer plugin yielded a good match. I wish
> I could just display a non-georeferenced raster over my map, give it
> some transparency, and then move, resize and rotate it freely, just
> using the mouse, until it was in place. And then save it with the
> georeferencing information.

That is exactly the process I used in georeferencing the image at  
<http://jdcard.com/Ripperdan/>. I used Google Earth to position the image  
and then calculated the details necessary for a world file from the data  
in the resulting KML file and the image properties.

We've done the same thing to find a close approximation for aerial photos  
that came to us without any georeferencing information. We knew  
approximately where they belonged and used Google Earth to make an initial  
alignment of them. After we created a world file we were able to import  
them into AutoCAD Map to align them more accurately and then rubber-sheet  
to match our data.

It takes some patience and persistence, and you get better after some  
practice, but I've found that it worked quite well to enable the use of  
old historical data that would otherwise remain only on paper.

-- 
James Card
California CAD Solutions, Inc.
209-578-5580 - Voice
209-521-6493 - FAX


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