[GRASSLIST:685] orthorectification with Grass

[olivier.faivre at free.fr]

Hello,

I'm looking for a software helping me doing a good ortho correction on set of
aerian pictures (with the usage of a DTM) and building a single accurate
picture combining all the previous orthophotos.
The usage of this picture is for environmental analisys.

It seems that GRASS allows to do that task, but as i don't know anything about
GIS software and moreover GRASS,
so it's quite hard for me to try to do it by myself.
I go trough the handbook and tutorial, but i can't succeed in doing my ortho
correction....
So could you please help me....

I work on windows station, i've install GRASS 6.0, and it's working fine...

A) I create a location and a mapset.
Do i have to bother with the EPSG code or the coordinate system? or could i
choose whatever i want?

B) I import the scanned aerian photos with their fiducials marks (tiff files) in
grass using File/Import/Raster Map/ Multiple formats using Gdal
for each tiff files, this process creates 3 rasters (green, blue, red).
do i need to group them? if so, by which means?

If i want to display the tiff picture, i do it with Display/Display RG and
selection the 3 rasters link to my tiff file, then GIS/Region/Manage Region and
selection of one of the 3 rasters

C) I would like to import my DEM.
I have a topo database delivered from guys working on MathInfo on the same area,
and among these files, a file in Shape format with level lines.
I import it with File/import/Vector Map/Various format using OGR.

D) now, it's time to go on the ortho.photo process
1°) i select the group of images to ortho correct
2°) i select the imagery group target, selecting the actual location and mapset
3°) i need to select my elevation modem, but a raster file is required. my DEM
is a vector file.
i try to convert my dem vector file to a raster file using GIS/Map type
conversion/vector to raster (v.to.rast) but there are always missing parameters
or G_calloc out of memory
what have to be done?

4°) i should fixe camera parameters.
it seems that a camera reference file provided normally by the photograph
supplier should be required for this stage.
what to do if there is no such file? Is it still possible to fulfill a good
ortho correction without this file?
i read some parameters on my photo but have no idea about their meaning appart
from focal and diaphragme, perhaps some are interresting...:
- Wild 15/4 UA8-S No 13247 152,98 (if i read well the FAQ, it deals with the
reference of the camera...)
- FS200 1/400 f/5.6 FF--- EC---- SP- v/h.05197 00% dt232.1 ds018 26.9V -62mb
ER00 CAM525

5°) i should fixe parameters for image to photo transformation
I don't really understand what hane to be done and how it has to be done at this
stage.
Do i have to zoom the photo to mark with the mouse precisely some fiducial
points and them giving their coordinates?
If so, do i have to do only on 1 photo or on all photo or?
How do i give the coordinate of this fiducial points that i've marked? with the
usage of the famous camera reference file... If so and if i can't have this
file, what can i do?

6°) do nothing for exposure considering to be in vertical aerian photos case
7°) I should fixe parameters for photo to targer transformation
This stage is again not very clear for me...
Do i have to mark about 12 points on the various photos i have to ortho correct
and then mark these same points on my DEM using my mouse...

8°) don't know if something have to be done at this stage or if everything is
automatic

Is there anything else i need to know to fulfill my ortho correction?
I read on the FAQ somebody speaks about a map, required for the ortho.photo
process, that have to be scanned, contours digitized and georeferenced. I think
he also called this map 'topographic map'. Is it really necessary for the ortho
process? if so, what stage required that so called topographic map?

Sorry for this long email
Hope that somebody will be able to help me.
best regards

Olivier