[GRASS-user] Re: documentation of georectify tool in gis.m

[Moritz Lennert]

Michael,

On 11/01/07 16:19, Michael Barton wrote:
> Moritz, 
> 
> I'm afraid there is no FM to R. I've been so pushed to plug all the holes in
> making GRASS usable without X11, I just haven't kept up with all the
> documentation. I should probably relook at the mouseover help to make sure
> it is understandable

I don't think that you should necessarily be the one to do this as well. 
You have many other things to do, and I think that documentation is the 
one thing _everyone_ can help with.

Your description below is all I needed (thanks for taking the time), and 
I think something like this would be enough for most people. Should this 
go into the gis.m manual page, a separate georectify manual page, or on 
the WIKI ? I'm willing to format it to whatever is desired.


> 
> It is pretty straightforward (IMHO). 

It is once you understand it. One of my main problems was that I thought 
the "ref. map" was what you call the "base map" below, i.e. the one 
which is already georeferenced. As you can imagine I have problems 
loading that... ;-)

Another counter-intuitive (for me) feature is the fact that erasing 
works on the non-selected points. I would have expected the contrary 
(although I understand the reasoning...).

In terms of comfort, it would be great if the cursor jumped on to the 
next point after a click on the base map. Now, it automatically jumps to 
the "geographic coordinates" fields after a click on the reference map, 
but this does not happen when you click on the base map.

In addition, if you happen to click twice on the base map, the new 
coordinates get written after those that are already in the "geographic 
coordinates" field. In my eyes, if you have the cursor in the field and 
click on the map, the previous coordinates should be erased and the new 
ones written. This would make it easier if your hand slipped while 
trying to get a point on the base map.

After having done a georectification, closed the georectifier and then 
opening it again with a new group to rectify (but from the same 
origin-mapset), there are (logically) no more coordinates in the 
georectifier window, but the RMS values from the previous run are still 
visible. This could be a bit confusion. I would suggest setting them to 
empty at every new start of the georectifier.

Other than that, easy, fast and efficient !

Moritz


> Here are the steps.
> 
> Start up in the location you want the new map to be rectified INTO (not the
> xy location where it's from).
> 
> Open any georeferenced map(s), raster or vector, or some combination of
> multiple layers in a normal map display to serve as a base map for
> georectification.
> 
> Start the georectifier.
> -Decide if you are going to georectify vectors or rasters and check the
> appropriate radio button
> -Create a group if you don't already have one. This uses i.group for raster
> and its own routine for vectors (to create a group folder, etc). All maps
> that can be georectified with the same ground control points can go into the
> same group
> -Select a reference xy map that you can use to set GCP's
> -Start georectifying
> 
> When georectifying, you click on a GCP in the xy map and click on the
> corresponding point on the base map. You can also enter coordinates if you
> want. You can delete any GCP or exclude from computations. You can check the
> RMS error for all active points. This routine bypasses i.points and
> i.vpoints to create a points file. The points file can be used for rasters
> or vectors, since georectifying of both is supported.
> 
> When you are ready to georectify the map, pick the polynomial you want to
> use (dependent partly on the number of points you have), and press the
> button. This will use i.rectify for rasters and v.transform for vectors.
> AFAIK, 3rd order polynomial georectification is still broken in i.rectify.
> But the rest should work well.
> 
> The georectified map(s) are projected into your active location/mapset.
> 
> Hope this is helpful (and maybe the start of some docs)
> Michael
> 
> 
> On 1/11/07 4:18 AM, "Moritz Lennert" <mlennert at club.worldonline.be> wrote:
> 
>> Hello,
>>
>> I have some trouble using the georectifying tool in the GIS Manager.
>> Before asking any questions, I would like to RTFM. Is there any
>> documentation somewhere ?
>>
>> Moritz
> 
> __________________________________________
> Michael Barton, Professor of Anthropology
> School of Human Evolution & Social Change
> Center for Social Dynamics & Complexity
> Arizona State University
> 
> phone: 480-965-6213
> fax: 480-965-7671
> www: http://www.public.asu.edu/~cmbarton
> 
>