Masking low coherence areas before unwrapping



I am trying to create a clean displacement map but i can’t seem to find a way to weed out low coherence areas. I understand we can use the coherence raster to create a binary mask, but don’t know how to go about it. In the video demonstration of the subsidence mapping this part has been skipped.
Any help please!



Dear @amit
Right click on your displacment map and select band math then edit expression and put this:
Displacment (coherence >= 0.4)


Dear @mzohary , that worked like a charm! Didn’t expect it to be that simple. Thanks!


Your Welcome. Also selecting a higher rate of Multi look would leads to better coherency but you will lose resolution.


HI, i just saw what you explain @mzohary
but… do i need to load also the coherence band to the displacement band or is just put that little comand.?


thank you.



You need to have coherence and displacement within one product. Then you can mask out the low coherence areas with the statement posted above.
The easiest way is to simply mask them out in the properties of the displacement band (right-click) under valid pixel expression: coherence >= 0.3

If you want to write a new raster you can use the band maths:
IF coherence <=0.3 THEN NaN ELSE displacement
(you have to enter the correct names of your bands of course).
This gives you a new raster with the displacement and nodata values where the coherence is below the threshold.