[GRASS-user] Projecting and georectifying in GRASS
horning at amnh.org
Mon May 4 03:23:00 EDT 2009
I have a few questions about projections and image-to-image
georeferencing in GRASS. I'm using GRASS 6.4 from the weekly build a
couple weeks ago on my Ubuntu computer.
Is it possible to import an image and project it in one step? The method
I have been using is to run gdalwarp from the target GRASS environment
so the target projection is specified automatically but that creates an
image outside of GRASS that needs to imported in a second step. The
other method that I read about is to use two Locations, one for the
source and another for the target image. That also requires two steps.
It seems as if there should be a way to project an image as it is being
imported - maybe even as an option if the projection of an image to be
imported is different from the GRASS location but I haven't been able to
find it. Then again it looks like gdal isn't able to output GRASS images.
To do the image-to-image registration I want to take two images that are
in the same projection but not well aligned and warp one of them to
match the other. I was planning to use the Georectifier GUI for this but
have run into a problem. If I understand the functionally I should be
able to pick a source GCP by clicking on the XY map window and then
click on a corresponding point in a “normal map display” window to enter
the target GCP. By “normal map display” I assume the help file is
referring to the map window that open when GRASS starts up. If I load
the target image in the normal display before launching the Georectifier
GUI the image displays fine but after launching the Georectifier tool if
I try to zoom in on the image in the normal display or click on it to
select a target GCP the display canvas goes blank (white). If I try to
display an image in the normal display after starting the Georectifier
GUI the display canvas remains blank. Is this a bug or am I doing
Is it possible to use an RGB image for the image that is displayed in
the XY map window (the window holding the source image that launches
with the Georectifier GUI)? I could only seem to display a gray scale
image but maybe I am missing something.
Is there an easy way to enhance an image in a display window. It can be
convenient to rescale the pixel values to enhance certain objects for
picking control points but I'm not sure how best to do that. The only
way I'm aware of is using r.colors.
Thanks in advance for any help or advice.
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