Graph Builder Subset and Terrain Correction Error Sentinel 1

CiaranEvans · June 1, 2020, 1:15pm

Edit

The error now appears to be when I Terrain Correct an image:

Read>Subset>Calibrate>Write - Works fine
Read>Subset>Calibrate>Terrain Correct>Write - fails

Hi there,

When I use the Subset Operator on its own in SNAP, I can subset an Sentinel 1 scene without issues (Using Geographic co-ordinates), however when I try to automate this using Graph Builder, I get:

The specified region, if not null, must intersect with the image's bounds.

Does anyone have any pointers as to how I can fix this?

I noticed that SNAP takes co-ordinates whereas GPT seems to take a WKT…

CiaranEvans · June 1, 2020, 1:18pm

Also just to add that the WKT is displayed in the correct location where the image footprint is.

ABraun · June 1, 2020, 1:41pm

have you tried creating the WKT outside SNAP, e.g. here: https://arthur-e.github.io/Wicket/sandbox-gmaps3.html

If you use this in the XML of the graph file, it should work.

CiaranEvans · June 1, 2020, 1:42pm

Funnily enough, that was the very website I used

ABraun · June 1, 2020, 1:43pm

are you working with GRD data?

CiaranEvans · June 1, 2020, 1:44pm

Yes, it’s strange as using the same co-ordinates for a top left, bottom right subset window in the SNAP operator works fine

ABraun · June 1, 2020, 1:52pm

any pre-procesing applied before this step?

CiaranEvans · June 1, 2020, 1:55pm

Hmmm peculiarly it seems to do it now. I have no idea why it was doing that.

There’s no pre-processing steps ahead

ABraun · June 1, 2020, 1:56pm

well, this good and bad in a way

CiaranEvans · June 1, 2020, 1:58pm

Right so, if I add terrain correction to the end, the processing dies.

So if my graph is Read > Subset > Calibration > Write (No problems)
But Read > Subset > Calibration > Terrain Correction > Write (Problem)

ABraun · June 1, 2020, 2:00pm

Did you maybe select a UTM coordinate system in the terrain correction?
This would explain why the geographic coordinates from the WKT are outside the expected bounds.

Edit: Nevermind, it should not make a difference if the terrain correction comes after the subsetting.

CiaranEvans · June 1, 2020, 2:01pm

Nope, set at WGS84

CiaranEvans · June 1, 2020, 2:08pm

Appears that if I try to Terrain Correct the product, I get the image bound error.

Guess subset isn’t where my issue is?

CiaranEvans · June 1, 2020, 2:10pm

Sorry for the image, copy+pasting from my VM is a bit broken!

ABraun · June 1, 2020, 2:13pm

is your region within the SRTM coverage?

CiaranEvans · June 1, 2020, 2:18pm

Oh wait, read the preview wrong sorry, it’s actually east of Guatemala city:

CiaranEvans · June 1, 2020, 2:43pm

@ABraun

I believe it’s how the Subset Operator in GPT works.

If I run Subset separately with SNAP (not GPT), then Radiometric Calibration, then Terrain Correction, it works fine…

ABraun · June 1, 2020, 2:55pm

hm, I have often used the GPT to make subsets in batch mode actually.

Just to go sure: There is an option to mask out all water areas in the Terrain Correction, but you probably disabled it because your data is entirely over water, right?

CiaranEvans · June 1, 2020, 2:56pm

Yep it is indeed disabled, I’m using an external DEM, which does work with the separate SNAP processes

CiaranEvans · June 1, 2020, 3:26pm

So I ran GPTs Subset (2 in the preview window) with:

Polygon ((-97.56358828861178267 14.96860616344341643, -97.56358828861178267 14.73100794671138303, -97.10260713307185654 14.73100794671138303, -97.10260713307185654 14.96860616344341643, -97.56358828861178267 14.96860616344341643))

And then ran SNAPs subset (3 in the preview window) with:

-97.56358828861178267 14.96860616344341643, -97.10260713307185654 14.73100794671138303

As you can see, SNAPs subset appears to be correct whereas GPTs still looks like it’s the full scene size. I assume that when it comes to Terrain Correction, the data is not mirroring the metadata?