rectify a scanned map

Malcolm Williamson malcolm at
Fri Nov 19 17:43:53 EST 1993

Leonard Coop writes:
> I have scanned a map of the western US (lambert projection)
> and want to register it as lat-long.
> i.rectify does a poor job.  It looks OK at the edges or in the middle
> points but not both.  The error is as much as 200 miles.  The number of
> points I use does not affect error very  much.  Is the problem
> because i.rectify expects a more planar image (such
> as a satellite image)?  Do I need a way to convert the lambert
> (conical) projection into a cylindrical one?  Grass 4.0 does not seem to
> have a module to convert coordinate systems in this way.  How do we
> get a more accurate registration/rectification?
> This must be a problem solved every day somewhere, but we are stumped for
> now.

You raise several interesting points. What you really want to do is a projection
transformation, and i.rectify is not capable of this. If you were working with
vector data, you could use v.proj to do the transformation. If what you are 
really after is linework off of the scanned map, I would suggest transforming it
to vector *before* bringing it into GRASS (LTPlus can do this, if your scanning
software can't). 

However, if you are really after raster data, you will need to "rubber-sheet"
your data. In GRASS, your only option is to use i.rectify2 in GRASS4.1 (another
good reason to get 4.1!!), which can do 2nd- and 3rd-order transformations.

I just remembered something else. There was a posting on the grassu-list a 
couple of weeks ago regarding a contributed program called r.proj . If you 
didn't save this, I can forward it to you (the e-mail, not the program!).

Malcolm D. Williamson - Research Assistant       E-mail: malcolm at
Center for Advanced Spatial Technologies      Telephone: (501) 575-6159
Ozark Rm. 12                                        Fax: (501) 575-3846 
University of Arkansas              
Fayetteville, AR 72701

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