[postgis-users] Failing ST_Transform with Ross Ice Shelf polygon

Tue Nov 7 11:57:08 PST 2023

Hi guys,

What happens if the dataset is converted to 0-360 with ST_ShiftLongitude() then back again to +-180?

I'd expect that to come up with the +-180 standardised on one or the other, rather than having a dataset with both.

It is a shame that more recent versions of proj don't support 0-360 longitudes, as that would fix the issue in the Ross Sea (but only by moving it 180deg...)

Brent Wood

Principal Technician, Fisheries
NIWA
DDI:  +64 (4) 3860529
________________________________
From: postgis-users <postgis-users-bounces at lists.osgeo.org> on behalf of Paul Ramsey via postgis-users <postgis-users at lists.osgeo.org>
Sent: Wednesday, November 8, 2023 08:39
To: Marco Boeringa <marco at boeringa.demon.nl>
Cc: Paul Ramsey <pramsey at cleverelephant.ca>; PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] Failing ST_Transform with Ross Ice Shelf polygon

Hm, the current logic of the nudge would probably still not help your case, since your problem is both the coordinate being slightly out of bounds and also the change in sign. The nudge would move -180.0000000004 to -180, but it wouldn’t flip the sign.

On Nov 7, 2023, at 11:36 AM, Marco Boeringa <marco at boeringa.demon.nl> wrote:

Sounds interesting. I think many users of PostGIS would be really glad to see something like this implemented if it could reasonably be done. Haven't tried the double cast via geography yet, but seems fun thing to check and see the result.

Op 7-11-2023 om 20:31 schreef Paul Ramsey:
All that said…

It would be possible to “fix” this, but it’s a scary black box.
We already nudge geodetics back into place when casting from geometry to geography (interesting workaround, take your reprojected result and do a ::geography::geometry on it)

https://github.com/postgis/postgis/blob/42f04a29effdd9e8280c7aba17420ba306fc73f4/liblwgeom/lwgeodetic.c#L3351

For systems that we know are geodetics (and with modern proj we generally know that) we could apply the nudge to the outputs. It would make things slower (more logic) but it would only change those cases where the coordinates are in fact out of bounds by a very small amount.

P.

On Nov 7, 2023, at 11:22 AM, Paul Ramsey <pramsey at cleverelephant.ca><mailto:pramsey at cleverelephant.ca> wrote:

Nope.

It can be quite reasonably argued that the answer is correct, and the problem is treating EPSG:4326 (a geodetic coordinate system with angular units) as if it was a planar system with cartesian units (spoiler: it is not that). In angular units, -180.0000000004 is ridiculously close to 180.0. You aren’t complaining about the other coordinates, like where 175.123456789 is coming through as 175.123456788. Why not? It’s the same error! :)

I don’t know what it is about the math going through that fun CRS that is causing roundoff or even if it’s particularly large (I don’t think it is), but it is not at all unique to that system. You can generate data that is progressively offset from the original data doing nothing more exotic than going back and forth from WGS83 to UTM over and over and over.

ATB,

P

On Nov 7, 2023, at 11:16 AM, Marco Boeringa <marco at boeringa.demon.nl><mailto:marco at boeringa.demon.nl> wrote:

Thanks Paul,

But is there a more definitive solution in PostGIS / PROJ on the horizon in terms of future development? No one expects a perfectly valid geometry that just happens to hit the projection boundary of WGS1984 to come out garbled by doing a transform and back-transform to the original CRS. I realize there may be technical challenges here, but this will undoubtedly keep coming up many times in the future, and likely has in the past, by other confused non-expert users of PostGIS if nothing changes. It is really counter-intuitive to need to use stuff like ST_SnapToGrid, ST_ReducePrecision or ST_WrapX to "fix" something that goes right for 99.999% of all other data. It also makes any needed code more convoluted.

Yes, well, I know, storing data in WGS 1984 geometry may not be best practice with this kind of globe spanning data, but it works for most cases and I already cast to geography a lot to do stuff where geography is really needed.

Marco

Op 7-11-2023 om 19:02 schreef Paul Ramsey:

On Nov 6, 2023, at 3:39 PM, Paul Ramsey <pramsey at cleverelephant.ca><mailto:pramsey at cleverelephant.ca> wrote:

On Nov 6, 2023, at 3:33 PM, Marco Boeringa <marco at boeringa.demon.nl><mailto:marco at boeringa.demon.nl> wrote:

Well, yes indeed that is what is happening, 180 came out of the reprojection steps as -180. Full output geometry below. Is there any way to prevent this behavior?

Marco

Not really… Either snap to grid or reduce precision

https://postgis.net/docs/ST_ReducePrecision.html
https://postgis.net/docs/ST_SnapToGrid.html

will get you back onto the dividing line (note that it is at -180.00000000000014), but that won’t help in flipping -180 to 180. For your particular case, applying

https://postgis.net/docs/ST_ShiftLongitude.html

will fix it, I think, though not in generality

I think using

https://postgis.net/docs/ST_WrapX.html

would allow a more general purpose solution. At least one you have more control over.

P

P

[https://www.niwa.co.nz/static/niwa-2018-horizontal-180.png] <https://www.niwa.co.nz/>
Brent Wood
Principal Technician - GIS and Spatial Data Management
Programme Leader - Environmental Information Delivery
+64-4-386-0529

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