Upgrate Earth gravimetric recipes for SWOT data

[Esteban82]

This PR is only to upgrade the recipes to use the SWOT grids (in this link https://topex.ucsd.edu/pub/global_grav_1min_SWOT/) instead of this ones (https://topex.ucsd.edu/pub/global_grav_1min/).

In the four recipes I change the precision (or scale) and offset. I also change the SRC_FILE, REF and DOI.

I think that only important thing that I want your approval is for the SRC_TITLE of each recipe.

See discussion on #271

[joa-quim]

Given the large differences in some of the grid's scale factor, I would be more comfortable to see what we are loosing with quantization to uint16. Again, a simple test is to compare the original floating point values at some nodes (obtained with grdtrack) with those in the scaled uint16 grids.

[Esteban82]

Given the large differences in some of the grid's scale factor,

A comment on the grid factor. I believe that for earlier recipes, the scale was intended a range of 2^15 values. That is partly why I was so hesitant with this issue.

[joa-quim]

2^15 if for signed ints (1/2 negatives & 1/2 positives). 2^16 is for unsigned ints

[Esteban82]

I see, But I don't know why was set to signed ints. I don't know why we didn't saw that.

[joa-quim]

Maybe Paul checked that current scales were enough to maintain the FAA up to the microGal, but I'm not so sure about the deflections.

[WalterHFSmith]

Guys, I can see the thread of your conversation but I am not sure I understand what the original issue was. Dave Sandwell’s “img” files use two-byte signed integers for deflections of the vertical inmicroradians times 10, that is, 10^-7 radian, and gravity anomalies in milliGals times 10 (that is 10^-6 m /s^2). For vertical gravity gradient I think there is also a multiple of 10, maybe it is Eotvos times 10, but I would have to check. This scaling does not completely fill the maximum possible range of a signed two-byte integer (+/- 32767). W

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On Apr 16, 2025, at 3:20 PM, Joaquim ***@***.***> wrote: joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) Maybe Paul checked that current scales were enough to maintain the FAA up to the microGal, but I'm not so sure about the deflections. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because your review was requested. <#272 (comment)> <https://github.com/notifications/unsubscribe-auth/APUT6GLMXYLAV4Q5RTHPEGL2Z2UPTAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQGUZTEOBYG4> joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) <#272 (comment)> Maybe Paul checked that current scales were enough to maintain the FAA up to the microGal, but I'm not so sure about the deflections. — Reply to this email directly, view it on GitHub <#272 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/APUT6GLMXYLAV4Q5RTHPEGL2Z2UPTAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQGUZTEOBYG4>. You are receiving this because your review was requested.

[joa-quim]

Walter,

This issue started when we realized that the new SWOT grid has a larger amplitude (SWOT minimum is ~400 mGal lower than the V32 version). Though a bit surprising, that is not the issue. The issue was that with the new SWOT's range Federico had a doubt on what would be a good scaling factor ... and conversations started. See #271 for more details.

[WalterHFSmith]

OK, The only comment I can make is that the gravity field of the Earth has not suddenly developed anomalies with an extra 400 mGal. Rather what I think is happening is that there are bad data values at the edges of SWOT swaths and these are not being fully edited before SWOT is getting turned into gravity. w

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On Apr 16, 2025, at 3:46 PM, Joaquim ***@***.***> wrote: joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) Walter, This issue started when we realized that the new SWOT grid has a larger amplitude (SWOT minimum is ~400 mGal lower than the V32 version). Though a bit surprising, that is not the issue. The issue was that with the new SWOT's range Federico had a doubt on what would be a good scaling factor ... and conversations started. See #271 for more details. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because your review was requested. <#271> <#272 (comment)> <https://github.com/notifications/unsubscribe-auth/APUT6GIUQJI475QRWO27ZUL2Z2XQFAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQGU4TAOBWGY> joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) <#272 (comment)> Walter, This issue started when we realized that the new SWOT grid has a larger amplitude (SWOT minimum is ~400 mGal lower than the V32 version). Though a bit surprising, that is not the issue. The issue was that with the new SWOT's range Federico had a doubt on what would be a good scaling factor ... and conversations started. See #271 <#271> for more details. — Reply to this email directly, view it on GitHub <#272 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/APUT6GIUQJI475QRWO27ZUL2Z2XQFAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQGU4TAOBWGY>. You are receiving this because your review was requested.

[joa-quim]

Walter, we are not talking about outliers.

[WalterHFSmith]

Joaquim, Isn’t that location on the Antarctic ice? I have no idea what the true gravity field is doing there but if it went from -500 to +500 mGal over such a short distance, wouldn’t that be surprising, and probably not physical? Is there a real situation that could generate such an anomaly gradient? Anyway, I am not trying to argue about the truth of any particular anomaly, only to say that the gravity anomaly, if expressed in integers representing 10^-6 m/s^2, should easily fit in a two-byte signed integer. Highest regards, Walter

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On Apr 16, 2025, at 6:33 PM, Joaquim ***@***.***> wrote: joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) Walter, we are not talking about outliers. diffs_faa.jpg (view on web) — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because your review was requested. <https://github.com/user-attachments/assets/05c5a704-0b4f-4119-91af-3a0951d11f9d> <#272 (comment)> <https://github.com/notifications/unsubscribe-auth/APUT6GJCD5NX3SOXA7QEK7L2Z3LEJAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQHE3DKMZTGU> joa-quim left a comment (GenericMappingTools/gmtserver-admin#272) <#272 (comment)> Walter, we are not talking about outliers. diffs_faa.jpg (view on web) <https://github.com/user-attachments/assets/05c5a704-0b4f-4119-91af-3a0951d11f9d> — Reply to this email directly, view it on GitHub <#272 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/APUT6GJCD5NX3SOXA7QEK7L2Z3LEJAVCNFSM6AAAAAB3ITGDEOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZDQMJQHE3DKMZTGU>. You are receiving this because your review was requested.

[joa-quim]

Walter, you are right that it's on the Antarctica shore but I think you misunderstood what the figure represents. It shows the grid differences between the SWOT and previous V32 FAA's. I've added the coastline to make it clear.

only to say that the gravity anomaly, if expressed in integers representing 10^-6 m/s^2, should easily fit in a two-byte signed integer.

Yes, no doubt. The discussion was only on what new value (if any) should we use as scale for these new grids.

Easter truce in Washington?

Cheers

[joa-quim]

More I look at this more I think that this SWOT anomalies cannot be right. This is 3 times the amplitude of the anomaly across the Chile trench. @dsandwell

[dsandwell]

Thanks for find the anomaly. This is probably an ishelf where there is a step in the ocean height due to ice. V32 is probably more accurate so I would use V32 around Antarctica. THere are some other issues with SWOT gravity related to the outer edges of the swaths. It will take a while to fix all of these.

[joa-quim]

Glad to having contributed with something 👍

[Esteban82]

Well, back to the subject of scale. I agree that the resolution of the data should be higher than that of the measurement. But I think it would be convenient to use a scale that maximizes the number of values.

[Esteban82]

Given that a new release of the original gravimetric grids was added in #291, I think it makes sense to add new recipes for the SWOT gravimetric data rather than replacing the previous recipes. This preserves the existing data while extending support for the new dataset.

[joa-quim]

Yes, and do you know if the previous reported problem around some parts of Antarctica have been solved?

[Esteban82]

I’m not sure. The files in the repository (https://topex.ucsd.edu/pub/global_grav_1min_SWOT/
) appear to be the same as before, although there is one file that was updated about 6 days ago. So perhaps there will be some news soon.

[dsandwell]

Hi All, I did not know this capability existed. Is that the same as the GMTSAR command we use? (I’ll answer the SWOT gravity question in the next e-mail) if ($mode == 1) then gmt grdcut @earth_relief_01s $R -Gdem_ortho.grd else if ($mode == 2) then gmt grdcut @earth_relief_03s $R -Gdem_ortho.grd else echo "[ERROR]:Wrong DEM mode selected." endif If that is the same capability, we would be most grateful if you could add this global dem at one arcsecond. https://ieeexplore.ieee.org/document/10440392 I believe this one goes to the poles. Then we could change the commands in GMTSAR to use the Copernicus DEM. That would be wonderful!! David

On Jan 22, 2026, at 11:57 AM, Federico Esteban ***@***.***> wrote: Esteban82 left a comment (GenericMappingTools/gmtserver-admin#272) Given that a new release of the original gravimetric grids was added in #291, I think it makes sense to add new recipes for the SWOT gravimetric data rather than replacing the previous recipes. This preserves the existing data while extending support for the new dataset. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you were mentioned.Message ID: ***@***.***>

David T. Sandwell IGPP 0225 Scripps Inst. of Oceanography La Jolla, CA 92093-0225 Ph. 858 534-7109 Cell 858 663-9426 http://topex.ucsd.edu Express Mail Address: 8795 Biological Grade Room 1102 La Jolla, CA 92037

[dsandwell]

Hi All, Thanks for serving up the global gravity and other models in this way. Could you please send a link with instructions on how to use them? The V32 global marine gravity model is the best we can do using standard nadir altimetry (i.e., not SWOT), so there is no need to change that. There is a V33 version with additional nadir altimetry, but it has not really been published. The SWOT gravity is much better, but it does have some issues near Antarctica and along the edges of the swaths. There are three SWOT gravity versions currently on our FTP site: SWOT01, SWOT02, and SWOT03. All were generated using the same approach described in Yu et al. (2024); the main difference is the amount of SWOT data included. A SWOT04 version is coming soon (in about 1–2 months). Yao and I are testing ways to address the issues noted above. For the existing SWOT01–03 models, the V32 model is used to fill the diamond-shaped gaps. For SWOT04, we plan to use V33 to fill the remaining coverage gaps. Hope this did not confuse you. Thanks, David

On Jan 22, 2026, at 11:57 AM, Federico Esteban ***@***.***> wrote: Esteban82 left a comment (GenericMappingTools/gmtserver-admin#272) Given that a new release of the original gravimetric grids was added in #291, I think it makes sense to add new recipes for the SWOT gravimetric data rather than replacing the previous recipes. This preserves the existing data while extending support for the new dataset. — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you were mentioned.Message ID: ***@***.***>

David T. Sandwell IGPP 0225 Scripps Inst. of Oceanography La Jolla, CA 92093-0225 Ph. 858 534-7109 Cell 858 663-9426 http://topex.ucsd.edu Express Mail Address: 8795 Biological Grade Room 1102 La Jolla, CA 92037