External DEM for SLC data coregistration

Hello everybody,

i have a problem using the EXTERNAL DEM during the coregistration of two SLC data of Sentinel-1. The problem is that the slave image of the coregistered product is empty.

What I do is:

  • downoload .asc data that are lidar acquisition of 1 m resolution EPSG:32632
  • change the reference system from terminal with GDALWARP (new EPSG: 4326)
  • give the “EXTERNAL DEM” in input in SNAP’s coregistration

Can someone help me?

Still I have similar problem, but did you check up this post,

Source of the post

is the DEM smaller than the Sentinel-1 data?

Also here, what do you expect by using a DEM of this high resolution for coregistration? The DEM is only used to estimate the offset between both images based on terrain features which are also present in the raw radar data (no radiometric calibration applied).
I don’t think you need a DEM with 1m at this step already. Later, for Topographic Phase Removal or Terrain Correction it makes more sense.

Thank you for responding so quickly.

Yes the DEM is much smaller than the Sentinel-1 data. Could that be a problem?

You are right, for coregistration maybe a dem with 1m resolution is not necessary but then, when i will use the topographic phase removal, i suppose that i will have the same problem.

Nevertheless I will try using low resolution dem for coregistration and 1m dem for Topographic Phase Removal. I’ll let you know.

I used ACE30 for the BackGeocoding and later the 2m DEM (I reported in the other topic) and this worked fine.

DEM-based coregistration tries to find patterns which exist both in the DEM and the backscatter intensity. If the images are not of equal size or resolution these patterns cannot be found and no GCP are created and the coregistration fails.

I used SRTM 1sec (Autodownload DEM) for BackGeocoding and it works.
The workflow I I want to follow is that below:

(NB: I didn’t use the Graph Builder, I choose the desktop command for each step).
The problem is at Tophografic Phase Removal step. I will use a 1m DEM that is bigger then the subsett area, EPSG: 4326, nodata value: 0.0 . I’m think that something is wrong with my 1m DEM because if a use Topographic Phase Removal with EXTERNAL DEM Autodonload SNAP works.
I will send you a mail with my DEM, hope you can look up at it and suggest me if there is somethink wrong.

if you use the external DEM no AutoDownload is necessary. This only includes the DEM resources which SNAP can access by itself.
If the BackGeocoding works with ACE30 or any other DEM (SRTM, in case your study area is covered) you can still use the external DEM in the topographic phase removal. Do you get an error here?

yes, the problem is that when i use AutoDownload DEM SNAP works, but when i try to use my DEM that has a higher resolution (1m) than the AutoDownload one, SNAP doesn’t works. There is not a specific error but i have no output.

According to this answer for me, the high resolution less than 10 m brings an issue of processing or an error,

please have a look at this answer,

Source of the post

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but the stack was correctly generated by the BackGeocoding?

yes. In fact after BackGeocoding if I use AutoDownload dem for Topographic Phase Removal SNAP works…
That’s the reason why I suppose that the error is in the DEM that I provide (when I play Topographic Phase Removal with external DEM).

Hello everyone…

I have a problem using external dem for sentinel 1 SLC - pair S1 Tops registration. The error is negative array size exception.

My working area is East Antarctica. I am trying to measure ice velocity using interferometry, relatively new to this field.

I am using REMA_100m_dem for this region.

My data SLC products are:


Can anybody help?

is uit possible that the external DEM is stored in UTM coordinates? They can be negative in some areas.

SNAP needs it in geographic coordinates (lat/lon, WGS 84).

yes, sir

you are right, units are in meters and projection is polar stereo-graphic.

so do i need to re project raster?
any other alternative DEM that i can use in this region?

yes, you should reproject the DEM to WGS84 (EPSG:4326), for example with the gdal libraries

gdalwarp -REMA100m.tif REMA100m_reprojected.tif -s_srs EPSG:3031 -t_srs EPSG:4326

okay…let me try

thank you @ABraun sir for a quick reply

same issue mentioned here by @qglaude :confused:

I processed data Ace30 DEM as Geo-coding and terrain correction and REMA 200m DEM in streographic projection as in subtracting topographic phase removal …and this is the result i got as displacement.

it doesn’t look (technically) wrong to me.

@ABraun yes indeed…result looks good. may somehow i convert this relative LOS displacement to absolute displacement?

you find many discussions on this using the search function: