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I understand that the result of InSAR interferometry yields a LoS deformation and if deformation in the vertical or horizontal is to be computed, it should require additional gps observations at the site.

My first question is, could I simply solve for the components (vertical and horizontal) using geometry from the incidence angle?

Secondly, for DEM generated by repeat pass SAR images, i understand that the height obtain is relative. Can someone explain to me what does relative mean? Does it refer to the change in height from the first image to that of the second image? How then can I get an absolute height?

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    I'm not a great judge technically on this question but it looks to me like it contains multiple questions, making it too broad for our focussed Q&A format. If you have not already taken the Tour, I recommend that you do. – PolyGeo Jun 10 '15 at 9:29
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Indeed, the results of a single-geometry InSAR analysis yield Line of Sight deformations. When using a double geometry (both imagery in acscending and descending directions) the vertical component can be computed.

The horizontal deformation measurement is in that case sensitive in the East-West direction en far less sensitive in the North-South direction (due to the speed of the satellite). Of course, any measurement noise causing differences in the ascending and descending orbit results will be interpreted as horizontal deformation.

When using only one geometry, the vertical and/or horizontal deformation can only be computed when making stronger assumptions. In an area where horizontal deformation is very unlikely, e.g. a flat road on subsiding soil, one might make the assumption that only vertical deformation is present. When matched with GPS-measured deformation vectors, other assumptions can be made. Of course, the validity of those assumptions should be rigorously investigated on a case-by-case basis, with thorough understanding of the likely structural and non-structural deformation processes taking place on the site.

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