Greetings
I have just received a task to Orthorectify very-high resolution images. I know that GRASS (LINUX) performs this (I have never tested) but my question is more “basic” and elementar: Does anyone has a link or a document that explains:
1- Required preprocessing before Orthorectification
2- How to do a Orthorectification
3- Relation between Error and DEM spatial resolution
Thanks
Kat
On Tue, Jun 21, 2011 at 10:50 AM, katrin eggert
<katrineggert1980@gmail.com> wrote:
Greetings
I have just received a task to Orthorectify very-high resolution images. I
know that GRASS (LINUX) performs this (I have never tested) but my question
is more "basic" and elementar: Does anyone has a link or a document that
explains:
1- Required preprocessing before Orthorectification
2- How to do a Orthorectification
3- Relation between Error and DEM spatial resolution
A good starting point is the documentation [0].
For orthorectification, you should use a recent version of GRASS
(6.4.2) and not 6.4.0 because the orthorectification procedure has
been recently overhauled and is creating artefacts in earlier versions
of GRASS, particularly for mountainous terrain.
There is no preprocessing required before orthorectification, apart
from the organisation of the imagery. The imagery to be orthorectified
should be in a xy location. A separate location with the target CRS
must exist and should hold reference imagery if you do not have Ground
Control Points.
The DEM must fully cover the target area and must have the same
resolution like the desired target resolution, resample if necessary
(bilinear is enough).
There will soon (within the next weeks I hope) be an article available
in Computer and Geosciences explaining the orthorectification
procedure in GRASS, entitled "Robust rectification of aerial
photographs in an Open Source environment".
HTH,
Markus M
[0] http://grass.osgeo.org/grass64/manuals/html64_user/i.ortho.photo.html
Hello Markus
Thank you for your answer. Just a few more questions 
For orthorectification, you should use a recent version of GRASS
(6.4.2) and not 6.4.0 because the orthorectification procedure has
been recently overhauled and is creating artefacts in earlier versions
of GRASS, particularly for mountainous terrain.
Thanks
There is no preprocessing required before orthorectification, apart
from the organisation of the imagery. The imagery to be orthorectified
should be in a xy location. A separate location with the target CRS
must exist and should hold reference imagery if you do not have Ground
Control Points.
1- My VHR image has already coordinates. Is this a problem?
2- Shall I apply Geometric Correction procedure before Orthorectification?
There will soon (within the next weeks I hope) be an article available
in Computer and Geosciences explaining the orthorectification
procedure in GRASS, entitled “Robust rectification of aerial
photographs in an Open Source environment”.
Is it possible tyo have acces to the draft version?
Thank you
Kat
katrin eggert wrote:
Hello Markus
Thank you for your answer. Just a few more questionsThere is no preprocessing required before orthorectification, apart
from the organisation of the imagery. The imagery to be orthorectified
should be in a xy location. A separate location with the target CRS
must exist and should hold reference imagery if you do not have Ground
Control Points.1- My VHR image has already coordinates. Is this a problem?
No. Sorry for the confusion, usually the imagery to be georectified
comes without any CRS and thus goes into a xy coordinate system. If
the imagery already has a CRS, you can use that, and source and target
location can be identical. To avoid confusion, mapsets should be
different.
2- Shall I apply Geometric Correction procedure before Orthorectification?
No, it would introduce more errors.
Markus M
Dear Katrin,
just to make sure you're on the right track: is your imagery aerial (taken
from an airplane) or is it satellite imagery? As far as I know, GRASS only
supports ortho-rectification for aerial imagery, but cannot ortho-rectify
satellite images yet, e.g. like Quickbird or Ikonos Imagery.
I had this issue a couple of months ago and had to switch to OSSIM (Open
Source Software Image Map [1]) for ortho-rectification of Quickbird Imagery.
Good luck!
Stefan
[1] http://www.ossim.org/OSSIM/OSSIM.html
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