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Topic: AST14DMO File Usage and Tech Q/A

1. An ASTER product is distributed that is already orthorectified - AST14DMO from the LPDAAC. I've successfully generated a correlation output with vectors for locations in Namibia, and I get the correct general dune migration directions. However, I do not have confidence in the actual magnitudes of displacement yet because:
(a) uncertainty in the correct parameters for the frequency correlator (initial and final window sizes and step). I think one can achieve different results depending on the expected displacement of the dunes. I'm looking for about 1-5 pixels (15m/px).
(b) uncertainty in the orthrectification. Quite frankly, I do NOT feel the quality is high for ASTER DEMs, therefore, some displacements are clearly artifacts of the ASTER DEM errors.

(c) the ASTER pitch/roll/jitter error that has been discussed. This is an important technical point I'm hoping you can help me understand. I have the L1A data for the same scenes. I have generated ancillary data files successfully. What next? Destriping of the correlation file? If I am using two ASTER scenes, the destriping correction takes into account only one ASTER ancillary file. Do I need to the do the correction again using the second ancillary file? 
Lastly, I have subset overlapping data from the two AST14DMO files (a large area). Will this destriping process work using the L1A ancillary files?  In other words, does the data to be destriped have to be the same dimensions as the L1A granule/scene.

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Re: AST14DMO File Usage and Tech Q/A

Dear Stephen,

Thank you for your interest in COSI-Corr and for participating in this forum.

1- If you're expecting dune displacement to be around 5 pixels, you can use 32x32 pixels correlation windows. I would however suggest using multiscale 64-32 windows. Results should the same, but more robust against noise and decorrelation.

2- The ortho-rectification quality is always the heart of all solutions and problems. The best things is to be in a situation where the DEM quality doesn't matter much, and it's usually possible with ASTER images. You can have a look at figure 2 of this paper:
http://www.tectonics.caltech.edu/slip_h … SE2008.pdf
In short, if the images are taken with similar viewing angles and if the two images are orthorectified using the same DEM, potential errors will cancel out. With ASTER images, if the image footprint overlap very closely, it's usually a good indication that parallax effects will be minimum (you can try to quantify it roughly with the formula in the paper).

3- The automatic destriping method we're proposing assumes that you generated the ASTER ortho-images using COSI-Corr, starting from the L1A images. I am guessing it could still work even with a correlation deduced from the  ortho-ready images, but we've never tried, I can't guaranty the results will be good. For destriping, the idea is to chose areas were ground motion didn't occur, average them out in the direction of satellite jitter, and subtract it from the correlation. There are two ways you can go:
a- The easy way: the same file is used to determine the filtering model, and to apply the waveform subtraction. This would work only if you don't have dunes going all the way across (from East to West) your image. When the image appears in memory, you select a subset that encompasses all the height of the image, but that do not include any dunes (it's usually good practice to remove bad correlation points before doing this using the discard/replace values tool. Threshold SNR values less than 0.9 and EW and NS displacements that are too large to be physical. You can use the histogram in ENVI to help you determine where to cut the values) . I think most of this should be explained in the manual.
b- The more difficult way: if dunes are so widespread in your image that (a) cannot be applied, you need to mask out all moving dunes with NaN values in the correlation. You will then determine the correction model from a masked correlation, and you will apply it to the original correlation. The trick is to create a proper mask. If the dunes have largely moved, you can simply threshold all displacements and only keep the waveforms (assuming dunes have moved by a lot more than the amplitude of the waveforms to be removed). if you can't make this assumption, you'll have to mask things out manually...

4- Usually, images are acquired along similar orbits, hence wave artifacts are all aligned and you cannot distinguish whether artifacts come from one image, the other one, or a combination of the two. In that case, it doesn't matter which ancillary file you use, they will both produce similar destriping and will get rid of the waveform all in once. It's good to use the right ancillary if you have artifacts in several directions (CCD artifacts from SPOT and jitter from ASTER images, etc...), but in your case, you don't need to do it twice. Anyway, if you can't use the automatic destripe tool you can use the manual tool, you'll have to provide an average rotation angle to align the wave artifacts with the vertical.

5- We usually assume that the correlation file  to be destriped was produced by a pair of two L1A data. The destriping may not be as accurate if that's not the case. I'm not sure I really understand your question here..


All the best,

Sebastien

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Re: AST14DMO File Usage and Tech Q/A

Sebastien,

Thanks for your reply.

To clarify my last question: Let's say I were to generate an ancillary file from each of two L1A images that had overlapping but offset footprints. (Let's say they overlap by 1/2). Next, I orthorectify each image, and I subset the overlapping areas to create two new images that only contain the overlapping area. Will the destriping of those subsets be accurate using the L1A ancillary file even though the dimensions of the subset are now different that the original L1A? The dimensions of the subset images are also different now that the file has been orthorectified.

Last question for now: Is the only use for the ancillary file for destriping?

Thanks again for your response to my questions.

Cheers
Stephen

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Re: AST14DMO File Usage and Tech Q/A

Hi Stephen,

The destriping should work in your case, the correlation file does not need to have the same dimension as the L1A. For the destriping to be accurate, you of course need the ancillary file to belong to one of the images in the overlap.

The ancillary file contains the camera and orbital information for each line of the L1A image, in a format that is specific to COSI-Corr. It's primary use is for orthorectification and tie-points optimization. when you select the automatic destriping, the correlation is reprojected in an approximated focal plane geometry where wave artifacts are supposed to be oriented only across-track, easier for destriping. We need the ancillary data to perform this back-projection.

Cheers,
Sebastien