[GRASS5] raster georectification

Moritz Lennert mlennert at club.worldonline.be
Wed Jan 7 11:57:44 EST 2004

Kirk R. Wythers said:
> I have a set of scanned maps that I want to georectify. However I am
> unsure of the most efficient way to handle the three images (three
> color bands) associated with each image. I have imported them into a
> temporary xy location and can display with d.rgb (takes about 10
> seconds to display). I have grouped all with i.group, and used i.target
> to set up new target location and mapset. Now I am ready to use
> i.points. My question is this: Since it looks like all three element
> maps (rgb) need to have i.points run with associated coordinates,

Why do you need to run i.points three times ? Since they are grouped and
i.points and i.rectify work by group (you can determine which images in a
group you wish to rectify), you only have to use i.points once for
rectifying all images in a group.

> should the three elements of the color table me merged first in order
> to reduce the number of maps that need georeferencing, or is assigning
> coordinates to all three bands the standard approach? I read in "Open
> Source GIS: A Grass GIS Approach (Neteler and Mitsova) that one of the
> reasons that scanned color maps are broken into color bands is to
> reduce computational overhead and there by speed up image processing.
> Does it make sence to merge these maps first in order to reduce number
> of maps that need georectifying, or would I be more than paying for
> that time savings with very slow image processing?

It depends of what kind of processing you wish to do and what type of
image you have. Often one uses a specific band for specific analysis or
you can calculate indicators using the different bands, so it is generally
not advisable to merge them, not for speed reasons but for analytic

However the merged image could help you with i.points since sometimes it
is easier to identify a point on a merged image than on an individual


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