Dear Braun,
Thank you for your contribution. I think it requires further investigation. I also trust that the ortho of Sentinel 1 would be fruitful, as well as checking whether the shifts are linear and constant.
Google Earth uses orthometric, while auto-downloaded SRTM uses ellipsoid heights. So difference in vertical datum is probably main reason for mentioned elevation difference.
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Thank you for you contribution as well!!
Allow me to ask one more question.
I expect that roads in a SAR image would appear as black-ish stripes (with curves following the actual road). When I overlay a SAR image in either Google Earth or some GIS software+OpenStreetMap as basemap, I can locate those black-ish stripes but they are almost 10-20m far from the actual roads. This happens to the whole image. Is this a fault of the orthorectification procedure or is it based on how the actual SAR image is formed?
This is what I don’t understand.
this is certainly an error in the geocoding. It should be possible to have these black lines exactly over the roads in Google Earth. So it is not caused by the acquisition principle in general.
You do have point.
I down loaded a Sentinel 1 image, and i did exactly what you mentioned here.
(Why did you propose Terrain Flattening prior to RDTC? why not SAR-Sim provided that you said that SAR-Sim is more accurate?)
I saw that the blackish stripes do coincide with the roads on Google Earth.
Ultimately, it should be an ALOS 2 inherited problem.
Any thoughts?
Terrain Flattening is correction for radiometric distortions while RDTC or SAR SimTC is correction for geometric distortions.
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Thank you!
The Google Earth elevations (and their horizontal offset to your DEM) are not a very good reference, to be honest. What matters is if your SAR image is located correctly. Can you show an example of the vertical offset you experience?
Hello, yes!
Here follows a screenshot in which the (transparent) SAR image is overlaid on top of OpenStreetMap.
The orthorectified product is of:
- SRTM 1sec (Autodownloded)
- Bilinear DEM interpolation
- Ground Range pixel as suggested by SNAP.
- Projection used * EPSG:4326 - WGS 84
considering that the coordinate system of the basemap is
GCS_WGS_1984 / WKID: 4326 Authority: EPSG
bellow follows an example of the Sentinel 1 orthorectified product.
Pixel size 14m x 14m.
(Images are not scaled whith each other.)
Thank you very much!
Any thoughts?
What type of ALOS-2 aquisition mode are you using? Stripmap, Scan SAR?
Strip map: high sensitive quad pol 1.1
Any thoughts?
You can try the SAR Simulation Terrain Correction and check if it provides higher geo-accuracy
Are there any specific parameters to put to “ensure” good geo-accuracy?
more GCPs, smaller registration windows, more iterations, smaller GCP tolerance…
alright! i will test these based on your guidance!
Thank you very much! 
I have issues with orthorectifying ALOS-2 PALSAR-2 (Level 1.1) CEOS data in ERDAS Imagine 2018 (v 16.5.0 Build 1051) using a 1/3 arc second DEM . The DEM data is NAD83 (GRS80) with the vertical datum North American Vertical Datum of 1988 (meters) – NAVD88.) shareit app vidmate
What kind of issues? Can you please elaborate ?
However, I have not worked with the aforementioned software.