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Topic: Tie points to GCPS problem

Hello,

I am trying to orthorectify and resample two SPOT 5 images (2006 and 2007). I begin with COSI-Corr.
I did the orthorectication/resampling step between my first image (2006) and a shaded DEM. It was ok. Then I selected 15 tie points between ortho2006 (the orthorectified one) and my second, raw image (2007). When I try to convert my set of tie points into GCPS, I got an error message if I use a DEM (I selected the real DEM, not the shaded one) :
"SYSTEM ERROR : TPOINTS_TO_GCPS--> Variable is undefined: TEXT."
If I do not select a DEM, no error message.
If I select 2006 image (not orthorectified) as my reference image, it works, but it says that my tie points are all outside the DEM...

An idea? Thank you in advance and have a nice day.

Florian

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Re: Tie points to GCPS problem

Hi Florian,

Are you sure all your tie-points are within the area covered by your DEM? if you use a DEM, you have to make sure the DEM covers the whole extent of your images.

Sebastien

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Re: Tie points to GCPS problem

Hi Sebastien,

Yes, I think it was the problem, I had one point outside the DEM. Thanks a lot for the reply!
Orthrectification/resampling step is now ok, but I have problems with correlation one. I will post in the right topic if I can't find a solution

Regards
Florian

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Re: Tie points to GCPS problem

What to do if there are no stable features to select image-to-image tie points? I have an ice stream that is bounded by ice on both sides. Furthermore, I have created a DEM of the area using an ASTER image (bands V3N and V3B) using the SILC software.

My biggest deal is just coming up with a logical work flow using the guidelines outlined in the Cosi-CORR user guide. At this point I am simply getting familiar with the process using ASTER images but would soon like to move to WorldView imagery where I know more questions will arise.

~Brandon

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Re: Tie points to GCPS problem

If you don't have any stable points in your image, you either cannot use tie points (and then rely on the absolute accuracy of the satellite metadata from the ancillary file), or you need to know the horizontal displacement at a few points in order to compensate for ground motion (the reason we have an 'offset' option in the tie-points to GCP function). If you don't have an a priori model of the motion of a few points, you simply can't use tie-points.

You're biggest pb with the WV imagery is that you'll be very sensitive to changes in elevation (unless you have nadir acquisitions, which I doubt), because you're not likely to have a DEM of great quality. Hence the displacements will be biased in the images epipolar directions. See Scherler et al 2008, Fig 2.
http://www.tectonics.caltech.edu/slip_h … SE2008.pdf

Cheers,
Sebastien

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Re: Tie points to GCPS problem

Sebastien,

Thank you for the information. Sticking with Worldview imagery, it sounds like it may be more trouble than it's worth, because you're correct, most of the images are about 10+ degrees off-nadir. Also, the images we're comparing do not have the same off-nadir angle. Is this a bigger artifact since WV is such high spatial resolution? What if I resampled to 5 or 10m res? You are also correct that the best DEM I have is a 15m ASTER DEM of the area.

~Brandon

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Re: Tie points to GCPS problem

Hi Brandon,

Well, it depends. Typically, the higher the resolution, the more difficult the data processing because you're a lot more sensitive to small variations of topography or of imaging geometry. High resolution is great if you need to be very accurate spatially or if the glaciers you want to look at are small. Otherwise, yes, you're better off using medium resolution imagery with larger footprints.

Note that the potential topo biases will remain even with medium resolution imagery as they mostly depend on the incidence angle differences. However, you're more likely to find close to nadir acquisitions with medium resolution images.

Sebastien