[GRASS5] raster georectification

[Glynn Clements]

Kirk R. Wythers wrote:

> > 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.
> 
> Perhaps I am misusing i.group. I started with 6 scanned maps, each of a 
> different geographic space representing adjacent townships and ranges 
> (essentially a 3X2 matrix of a single larger map). After importing into 
> GRASS, I was left with 3 color bands for each map (rgb). All 6 maps 
> need to be assembled into a single map with all color bands.  I defined 
> a single group with i.group. I now believe that I should have defined 6 
> separate groups with i.group (one for each of the 6 geographic 
> locations). So the question is: in what order should these steps be 
> completed?
> 
> 1. merge rgb bands into a single map for each of the 3 bands
> 2. georectify with i.points
> 3. overlay adjacent maps

If the 6 individual maps are just "tiles" of a single map, the logical
sequence is to patch them together first, then rectify the single
large map. Otherwise (i.e. if the individual maps use different
projections), you need to rectify them separately then patch the
results together.

The point which Moritz was making was that you don't need to rectify
the individual R/G/B bands separately. Each run of i.rectify will
simultaneously rectify all three bands (assuming that they have been
grouped).

You shouldn't actually need to merge the R/G/B bands at any point, and
doing so isn't desirable in most cases. When generating a composite
map, you usually have to reduce the colour depth, otherwise you end up
with massive colour tables which tend to make most operations
extremely slow (either that, or they just fail due to insufficient
memory).

The main situation where generating a composite map makes sense is for
a "preview" (particularly for programs which only allow the use of a
single image rather than separate channels).

E.g. you might generate a composite map (with a low colour depth) to
use when marking points in i.points, but you would actually rectify
the separate R/G/B channels (you might also wish to rectify the
composite map, or you could generate a new composite map from the
rectified channels).

-- 
Glynn Clements <glynn.clements at virgin.net>