Difference between points extracted from ascending and descending orbits

Dear colleagues,
I produced two interferograms from both type of orbits over the same region. I have some points
measured by GNSS inside one and the same area. I was surprised to see that after using them as mask pixels for extraction of the phase at the said points ino CVS file and reading them in GE the points didn’t overlap as I expected. I hope the the image illustrates well the situation.

Any ideas?

how did you import the points into SNAP (at which step)?

For both cases I imported a SHP file after the terrain correction.

And how did you export these points from SNAP again?

Sorry for my questions, I just want to reproduce the error.

After loading the product I imported the SHP via File-Import-VectorData-ESRI Shapefile and after that I opened the image file and with right click I use ExportMaskPixels

now I understand.
It can be related to the pixel size and registration of the two images. If they don’t belong to the same product, there might be a shift between the pixel centers.

What’s the spatial resolution of your rasters?

For the descending image QGIS is reporting -
Pixel Size 0.0001327709989928652827,-0.0001327709989928652827

for the ascending image
Pixel Size 0.000136813417771403117,-0.000136813417771403117

I expected that after TerrainCorr both rasters shall be the same since the DEM used is one and the same.

actually, the DEM is resampled according to the resolution of the input product, not vice versa.

If you want to go sure, you collocate both products and import the shapefile only once for the stack.

@ABraun What is the procedure for the collocate?

You find it here: It simply puts two rasters into one product and resamples the slave according to the master


@ABraun Is this procedure to be made before or after TerrCorr?

after the terrain correction.

OK. Thank you for this suggestion.

May be the problem is not with collocation. Find below the table I get for the points I need (SHP file) and the pints I get from SNAP. I can’t explain why.

I think this actually proves the point :slight_smile:
The Shapefile defines the location of the points at the centimeter scale but as a mask in SNAP, the pixel center is used as a reference.

That is good to know when extracting values. I guess it is more adequate to use pixel centers, but then the result is that it is not possible to map the same area with asc/desc orbits.

that’s true but in case of radar you can never know if the microwave interacts with the same surface structure or object when you are working with ascending and descending data.

I know that is tricky job, but let’s give it a try. I’d reply on the good geocoding. :slight_smile: