[GRASS-user] Orthorectifying images

[Markus Metz]

On Tue, Jun 21, 2011 at 10:50 AM, katrin eggert
<katrineggert1980 at 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