Usage of External DEM, Checkbox "Apply Earth Gravitation Model"

[ercad]

Hello,

I think there still exists an error within some GUI dialogs, when an External DEM is chosen and the checkbox “Apply Earth Gravitation Model” initially appears. Microwave Toolbox 11.0.0

E.g. the GUI dialog “Radar/Interferometric/Products/Interferogram Formation”. If one selects “Subtract Topographic Phase” with an External DEM, the initially appearing checkbox “Apply Earth Gravitation Model” is not selected in the GUI, but when displaying the parameters (File/Display Parameters…), the parameter value is unexpectedly “true”. After 1x select/deselect of the checkbox, the parameter value is as expected “false”. This can lead to unwanted parameter setting when running the process with an external DEM (see screenshots below).

I found old posts, where this behaviour already was described for “Range-Doppler Terrain Correction”

A bug in S1 Toolbox, Range-Doppler terrain correction

Hi, I would like to report a bug in “Range-Doppler terrain correction”. It appears in “Range-Doppler terrain correction” operator. When “External DEM” option is selected the dialog window shows the option “Apply Earth Gravitational Model” as not selected, but actually it is set as selected in the algorithm. The bug is in RangeDopplerGeocodingOpUI function, lines 405-409.

EGM96 always being applied to external DEM in Terrain-Correction Operator

Hi, When using the Terrain-Correction operator, EGM96 is always applied to the External DEM even if the “Apply Earth Gravitational Model” checkbox is not checked. The cause is that in RangeDopplerGeocodingOpUI.java, the variable “externalDEMApplyEGM” is initialized as true while the checkbox “externalDEMApplyEGMCheckBox” is initially not checked. I come up with two solutions, but I don’t know which is better for the design of SNAP. Do you have any idea? Initializing the variable “externalDE…

So I started the actual SNAP 11 “Range-Doppler Terrain Correction” dialog and there it works correct. The initially appearing checkbox “Apply Earth Gravitation Model” is selected.

Searching more the forum, I found another old post concerning externalDEM and EGM:

SAR-Simulation Terrain Correction: Inconsistent results

Almost identical results are obtained when using External DEMs with different vertical reference (WGS84, EGM2008, EGM96). However, products produced with/without “Apply Earth Gravitational Model” in SAR-Simulation are misregistered by one 20m pixel east/west. We believe that externalDEMApplyEGM should not have this effect. Use of the same Copernicus GLO30 DEMs via auto download results in images shifted one 20m line south relative to the externalDEMApplyEGM=false images, and one 20m pixel east …

I therfore started also the actual SNAP 11 “SAR-Simulation Terrain Correction” dialog and there the same problem appears as above described for “Interferogram Formation”.

I took a quick look at the SNAP 11 source code. One difference I found is within the OpUI-method “private void enableExternalDEM(boolean flag)”. Within RangeDopplerGeocodingOpUI, last line of this method is “externalDEMApplyEGMCheckBox.setSelected(externalDEMApplyEGM)”. This line is missing in InterferogramOpUI and SARSimulationOpUI (and also within CoherenceOpUI).
I think this could be the reason for some reported problems in old posts and the following files should be revised if this is the case (I did no debugging):

• InterferogramOpUI.java
• SARSimulationOpUI.java
• CoherenceOpUI.java

Here the screenshots for the Interferogram-example:

[diana_harosa]

Thank you for reporting this issue.
JIRA ticket SNAP-3958 created.

[jun_lu]

The problem has been fixed and the fix should be in the next SNAP release. Thank you for pointing out the issue.