rectifying scanned aerial photos

[Brian Culpepper]

> 
> Hi,
> 
> We've been using i.points and i.rectify to georef. scanned
> aerial photos.  Currently, our approach is to run i.points
> and to pick points off the screen, then locate the point on
> a quad-sheet, digitize it, convert the coordinates from l.l.
> to utm, and enter the "target" coordinates at the i.points
> prompt.  What I'd really like to be able to do is to pick
> the point on the screen, digitize the point off the map
> and have i.points take the input directly from the digitizer.
> Is this possible in grass?
> 
> Thanks,
> Tim
> 
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Tim,
	I believe an easier way to georeference your aerial photo
would be to digitize road intersections, building corners or something
"photo-identifiable" from the quad map. This would be done in a UTM
GRASS location, most likely the "target" location used with i.points.
The resulting vector map could be used with i.vpoints in an x,y location
to rectify your aerial photo. i.vpoints allows you to display a vector
file adjacent to your unreferenced aerial photograph and "mouse click"
corresponding points. When an acceptable number of evenly distributed
points are selected i.rectify2 can be executed to compute a 1st, 2nd,or
3rd order rectification with the rectified photo being written to the
target GRASS location.  I know I haven't done a very good job of explaining
the process but take a look at the man pages for: i.group, i.target,     
i.vpoints, and i.rectify2.

BTW, I have recently had trouble zooming into the vector map display in
i.vpoints, so if anyone has experienced similar problems and solved them
PLEASE let me know....

Hope this helps,

-- 
R. Brian Culpepper                                 Research Assistant
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Center for Advanced Spatial Technologies           (fax) 501 575-3846
Rm. 214 Ozark Hall                                (work) 501 575-6159
University of Arkansas                            brian at cast.uark.edu
Fayetteville, AR 72701
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All opinions are my own and do not necessarily reflect those of the
Center for Advanced Spatial Technologies or the University of Arkansas.