Array NEST

Hi everybody,

Has someone tried to compare the accuracy of orthorectification procedure of NEST?

I've orthorectified a RS2 QP image using PCI Geomatica and NEST 4B-1, and using the same auxiliary (DEM, Orbital segment and Rational Functions, ...) data and parameters (pixel size, projection system, ...).

But when I superimposed my vector layer onto both products, the Nest one is misplaced 2-3 pixels in X, and 1-2 pixels in Y direction, while the PCI product has much more better accuracy.

Another question: does someone know how we can add some GCPs (manually selected) to the orthorectification procedure in NEST?

Thanks for your answer/comment in advance

Saeid

Hi Saeid,

>>Has someone tried to compare the accuracy of orthorectification procedure of NEST?

The Terrain Correction Operator implements a Rigorous Backward Range Doppler Approach (please refer to the help and help references if you want the details about the method) to geocode or orthorectify Radar Images.

The accuracy of final orthorectifed product depends on few parameters, like quality of the orbits, DEM accuracy and resolution, quality of timing information etc..

On the other side, from my point of view, the evaluation of the accuracy of orthorectified product is not a trivial thing at all and depends on what (and its accuracy) you are using to evaluate it, that is GCPs and their number or vector files.

Coming now to your question, of course we have done several tests to evaluate the accuracy and robustness of the procedure but again I can't say or report an absolute value in terms of final accuracy because what I said above

>>I've orthorectified a RS2 QP image using PCI Geomatica and NEST 4B-1, and using the same auxiliary (DEM, Orbital segment and Rational Functions, ...) data and parameters (pixel size, projection system, ...). But when I superimposed my vector layer onto both products, the Nest one is misplaced 2-3 pixels in X, and 1-2 pixels in Y direction, while the PCI product has much more better accuracy.

Well, I don't know how PCI Geomatica implements the Geocoding or orthorectification than I can't say anything about this.
Instead I can add that NEST does not use the Rational Functions approach.

Relating the misplacement you are reporting it is hard to comment. It would be interesting to know if you get always this off-set also with other images.

>> Another question: does someone know how we can add some GCPs (manually selected) to the orthorectification procedure in NEST?
If you mean to add point DURING the Terrain Correction operation, the Answer is NO way, since the procedure is fully automatic.

Best wishes,
Andrea

PS: the use of SarSIM TC (what Luis has reported in his email) is strongly affected by the DEM used and from the area covered by the image. I think that with current Spaceborne Radar sensor this approach can be avoided for geocoding the image.

Andrea,

Thanks for giving the practical hints.

I will read more about the theoretical background of NEST TC approach.

And I will test other images over this area to see the accuracy of them too.

All the best
Saeid