How to import an external DEM into SNAP?

Hello everyone,
I have created a mosaic of images from ALOS with a resolution of 12.5 m to calibrate may imagens of ALOS-2 (resolution of 10 m) and I don’t how to import the file into SNAP.
How can I do that? And if is possible to import it where can I access the file after import it?

Thank you very much!

Claudia Arantes

can you please speficy what you mean by “calibrate” the ALOS-2 images? Do you mean radiometric correction or geometric correction?

Normally, there there is no need to import external DEMs: Both the “Radiometric Terrain Flattening” and the “Range Doppler Terrain Correction” operators allow to select an external DEM, as long as it is stored in GeoTif format and projected to WGS84 (EPSG:4326).

Hello ABraun,

My ALOS-2 images are SLC, Stripmap, Fine. I am applying the following steps:
1 - Calibration to Beta 0
2 - Specke filtering
3 - Radiometric terrain flatening
4 - Range Doppler terrain correction
On steps 3 and 4 ask for a digital elevation model. And on this steps I’m trying to select a mosaic from Alos I have created. Are these steps correct?
Another question: Does it make much difference to use the SRTM-3sec or ALOS 12.5m that I had created is better?

Thank you very much,
Claudia

yes, these steps are correct and you can use external DEMs with the specifications I mentioned above (file type and projection).
If the spatial resolution is higher , the chance that (radiometric and geometric) topographic effects are corrected are clearly higher and you will have better SAR products.

But may I ask what you mean by ALOS 12.5 m and where you got it from? There is one larger misconception about this, but I’m not sure if it is the case here.

I’m doing the processing of SAR images with 10m of spatial resolution.
I did the download of DEM from ALOS images from VERTEX website (12.5m of spatial resolutuion). Is it not correct? Can I use then SRTM-3sec?

Unfortunately, this data not really offers a spatial resolution of 12.5 meter. For most parts of the world, the DEM contained in ALOS products fom Vertex is just SRTM data (30 m) which was up-sampled to a pixel size of 12.5 m. Please have a look here:

So, to answer your question: If you select SRTM 1Sec (AutoDownload), you already have the best quality which is freely available. I suggest to directly use it (instead of an external DEM, unless its quality is really better) to avoid unneccessary resampling.

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Thank you very much for your explanation!
I’ll use SRTM 1ArcSec.

Regards,

Claudia

ABraun,

In the Range Doppler Terrain Corrrection there is two options: SRTM 1sec grid and 1sec HGT. Which shoul I choose? My area is in Brazil (Amazonia Forest).
Thank you,

Claudia

Both SRTM 1Sec are the same data source.

  • SRTM 1Sec HGT (AutoDownload): This is the one you select if you want SNAP to check and download the DEM data (recommended).
  • SRTM 1Sec Grid: You only select this option if your internet connection is not stable or your firewall prevents SNAP from accessing the online resources. In such a case you can download/copy SRTM data manually, but this only works if the folder structure and file names are correct (not recommended).

grafik

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Thank you very much!
Regards,
Claudia

Hello ABraun,
In the option Radiometric Terrain Flattening I select Beta 0 but the result is Gamma 0? Why?
Thanks,
Claudia

Beta0 is a lower level of calibration, taking into account the calibration constant. Gamma0 furthermore includes the local incidence angle. Some more explanations on this are here: Radiometric & Geometric Correction Workflow