1 (edited by seven 2010-08-29 02:42:51)

Topic: Orthorectification via DEM vs Landsat 7

Hello,

I have a set of ASTER L1A data to determine rates of dune migration of an area.

My question is what would be the best way to orthorectify these? I have read through http://www.tectonics.caltech.edu/slip_h … SE2008.pdf and vaguely understand that errors and limitations associated with using a DEM for orthorectification. I have also read that using Landsat 7 15m Panchromatic band is also possible but haven't been able to find out more information on this, is it generally better to use a DEM?

My other question is: does the lower resolution of the DEM (e.g. SRTM) cause any issues when using it for 15m ASTER?

Many thanks.

Edit: Apologies if this is in the incorrect sub-forum!

2

Re: Orthorectification via DEM vs Landsat 7

Hi Seven,

Your question doesn't have a straight or unique answer, that's why you might be confused. In fact, it all depends on how you're planning on using the images and their particular purpose in your study. Let's consider several facts:

1- To avoid topographic effects (parallax) in the correlation, not only the images you're going to correlate will need to be well co-registered together, but in addition, they will have to be well registered with the DEM. If you have a mis-registration  between the DEM and the ortho-images, the two images you're trying to analyze won't register well. A good solution to ensure that the ortho-images and the topography are well co-registered is to optimize the GCP of the first image with the shaded topography. However, this operation can be sensitive and doesn't always work well if the DEM is not well resolved. As a rule of thumb, if the resolution difference between the 1A image and the DEM is more than a ratio of 3-4, this step won't work very well unless you have sharp topographic features that are well spread in your image. Correlation between a 15m 1A ASTER and a 90m SRTM may therefore not always work well. If the GCP optimization doesn't work (no convergence or large mean residuals), then you can "very carefully" select GCP manually, don't optimize them, and then produce the ortho ASTER this way. This corresponds to the traditional manual and tedious way for processing ortho-images.

2- Because correlation between the shaded DEM and the ASTER images may not work well (either due to the absence of sharp topo feature and/or because of the resolution difference), it is indeed possible to use a Pan Landsat image as first master image. However, Landsat images are not always accurately georeferenced. If the Landsat image you use as a master image is not well referenced with the DEM you want to use to rectify the ASTER images, optimizing the GCP with the Landsat image becomes useless as it won't achieve its primary goal: ensuring proper registration of the ASTER images with the DEM to minimize parallax artifacts. In this approach, you're using a Landsat image as a proxy for the ground truth.

3- Note that ASTER absolute metadata are not always poor (unlike most older SPOT 1-4 data). Hence, there's a possibility that even without using any GCP your image will registered well with the topography (or at least within a decent uncertainty).

4- If your goal is only to correlate ASTER images together, then I don't really think you need to use a Landsat image. You're certainly better off processing your images using GCP with a shaded DEM and try optimizing if it works. However, if your goal is to produce a time-series of images between a Landsat image and ASTER images, then I would suggest you indeed try to register the ASTER image to the Landsat image to avoid global mis-registration in the time-series.

5- In case you're using a shaded DEM to optimize your GCP, don't forget that only the master image is optimized this way. The second ortho should be optimized with respect to the first ortho to minimize registration errors in your time series.

6- If the sand dunes are moving quickly and you're only using 1 DEM made at a given date (e.g., SRTM), you will necessarily have topographic effects biasing your correlation. If 3D structures are largely moving (by several pixels), if you don't have access to 1 DEM per image and with all the DEM well registered, then you should try to select images with close to nadir acquisitions (within at most couple degrees from vertical) so that they are not sensitive to topographic changes. as a rule of thumb, most ASTER images sharing identical footprint will verify this condition.


Hope this helps,

Cheers,
Sebastien

3

Re: Orthorectification via DEM vs Landsat 7

Hi Sebastien,

Thanks very much for clearing some things up. I am correlating two ASTER images and was just using Pan Landsat for the initial master.

You mentioned the 3-4 ratio rule of thumb for resolution: I will attempt to use GDEM to overcome this, do you know if this is possible? I realise there are quality issues associated with GDEM (e.g. the accentuated pits/bumps problem).

I have created some ASTER ortho just using the ASTER satellite ancillary data (is this what you refer to by ASTER absolute metadata?) it works reasonably well, but there is a noticeable amount of error.

Just to clarify a couple of most likely obvious points, my workflow for the initial ortho was: Tiepoints Image to Image (Landsat to ASTER) -> Tiepoints to GCPS (No DEM selected) -> GCPS Optimization (No DEM selected) -> Orthoretification (No DEM selected). If I use SRTM/GDEM, I create tie-points with the DEM and the ASTER then in all the proceeding steps I select the DEM?

Thanks again.

4

Re: Orthorectification via DEM vs Landsat 7

Hi Seven,

From experience, the GDEM quality is very similar to SRTM 3arcs. Although the ground sampling of ASTER GDEM is better, the "resolution" doesn't seem much better. I think they will release another version of the GDEM next year, which should have a much better quality.

When I talk about the metadata absolute accuracy, it indeed means not using tie points or GCP. In any case, you should always use a DEM in your process if you want to be accurate, or you won't create an ortho-image, but only a georeferenced image, and these images cannot register well as you're not correcting for elevation and parallax.

Sebastien