How to create time series of DEMs?

Hi friends,

Would you like to suggest me please?
I have to create time series of DEMs. The point of my research is to find how to find vegetation height changing during the growth season.
So, I have 5 SLC images taken in 2016. I tried different approaches.

  1. Use every single image as master and one image as slave. In such a case I can create different DEMs but, my boss told me that this way in not right. Differences of DEM’s caused, he said, by misalignment between images because all the interferogram pairs were coregistred to the different masters.
  2. So, I created a single superstack with one master. After that I exported images for each date from the stack using Raster->Data Conversion->Export Bands. From that point I have to create pairs from the 5 coregistred images I’ve got. Topographic phase removal in that case possible only if I delete unnecessary metada from the files (I had to delete any information about slaves, baselines). The problem is that after creating interferograms I always receive equal results for any pair of images. I think that all these DEMs correspond to the date of the master in my superstack.

So, I need, to coregister all my images to one master, somehow, and after that I must create different DEMs for all the dates I have. How to do it???

Additional question. I’ve tried to export my ifgs to SNAPHU without Topo phase removal. But in such a case SNAP can’t import them after SNAPHU, because unwrapped phase cannot be opened. Please advise, how to overcome it.

Many thanks!

First of all, I’m afraid you won’t get good interferometric results over vegetation with C-band.

The risk of misalignment is given, so using one master would be the better choice (AB, AC, AD, AE,…). As far as I understood it, getting interferograms between slave images is not yet possible in SNAP. Maybe this topic helps you:

You could first create different interferograms (AB, BC, CD,…) and then co-register as explained by lveci.

Just be aware that you will be seeing mostly atmospheric artefacts in the DEM time-series, plus vegetation quickly decorrelates so that there will be no phase-signal.

Thank you very much for answers!
I found out that SNAP doesn’t coregistrate interferograms at any step (right after preparing ifgs, after Topo, Goldstein, Import or Range-Dopler). So, I can’t use the method proposed by Iveci :((((

you could, technically, apply Range Doppler Terrain Correction on all Interferograms and then use Radar > Coregistration > Stack Tools > Create Stack

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Thank for your advice. Unfortunately, SNAP didn’t allow to do this.

what message do you receive?

Sorry, I went to deep into my Coregistration issues and I had to write more information.
Yes, technically I was able to create a stack of ifgs with different masters, as you had suggested. But after that, I have all the images in one stack and I can’t do something useful with them. I still see misalignment (on the Elevation_VH - Elevation_VH2) caused by the coregistration to the different masters.
If I try to coregistrate such ifg pairs SNAP shows me “{My ifg} already contains a band with the name Intensify ifg_VV_…”. I even tried to delete this band, but obviously, that action didn’t work.
I would like to use X-band imagery and something more powerful than SNAP, but so far I don’t have it.

did you save your product after deleting that one band?

Yes, I did. Now when I’m trying to Coregistrate them SNAP shows “[Nodeid: CreateStack] Real and imaginary master bands should be selected in pairs”. If I’m trying to delete the same band in the second ifg, I’m still seeing the same message.

That leaves you with the only option of generating DEMs of different pairs, geocoding them (Range Doppler Terrain Correction) and then building an average.

But still, InSAR vegetation heights is nearly impossible inf you don’t include polarimetric signatures, for example. But this is the field of PolInSAR and TomoSAR which can no longer be done in SNAP.

I wonder a bit how your supervisor thinks that might work at all…

Thank you very much for your support!!!

Probably, I will start to move in the PolInSAR direction…