[gdal-dev] WMS supported SRS

Tue Sep 10 07:07:25 PDT 2024

Hi

  *   I meant to use CRSs only reported by GetCapabilities. I haven't seen that GetCapabilities returns so many (6782) possible projections...

I was meaning the same, CRSs reported in GetCapabilities. I used a search engine with phrase "service=WMS&version=1.3.0" and found in three minutes many examples, including this one kartta.hsy.fi/geoserver/asuminen_ja_maankaytto/ows?service=WMS&version=1.3.0&request=GetCapabilities<https://kartta.hsy.fi/geoserver/asuminen_ja_maankaytto/ows?service=WMS&version=1.3.0&request=GetCapabilities>. If you want to see more, install Geoserver on your computer and have a look at the default GetCapabilities.

Finding the best CRS from WMS GetCapabilities is guesswork. The server above is a Finnish one so probably EPSG:3067 is the most native CRS because that is what we tend to use in Finland. However, there is also one and only one additional BBOX for EPSG:3879 in GetCapabilities, but that would be somewhat odd selection for a country-wide CRS. It is also possible that for example some CRS of New Zealand does not really work in GetMap of the Finnish data even GetCapabilities says it will. But when WMS is configured to show a short CRS list then I would trust that they are all good to use. Just be aware that the list can also be very long.

If you start implementing the LIST_ALL_SRS option, it should be noted that the service level CRS list applies to all layers, and each layer can add more CRSs to the list. Maybe in theory there could be even "crs-list-of-service-root + crs-list-of-group-layer + crs-list-of-child-layer".

"7.2.4.6.7 CRS
Every Layer is available in one or more layer coordinate reference systems. 6.7.3 discusses the Layer CRS. In
order to indicate which Layer CRSs are available, every named Layer shall have at least one <CRS> element that
is either stated explicitly or inherited from a parent Layer. The root <Layer> element shall include a sequence of
zero or more CRS elements listing all CRSs that are common to all subsidiary layers. A child layer may optionally
add to the list inherited from a parent layer. Any duplication shall be ignored by clients.
When a Layer is available in several coordinate reference systems, the list of available CRS values shall be
represented as a sequence of <CRS> elements, each of which contains only a single CRS name."

-Jukka Rahkonen-

Lähettäjä: Michał Kowalczuk <michkowalczuk at gmail.com>
Lähetetty: tiistai 10. syyskuuta 2024 16.32
Vastaanottaja: Rahkonen Jukka <jukka.rahkonen at maanmittauslaitos.fi>
Kopio: gdal-dev at lists.osgeo.org
Aihe: Re: [gdal-dev] WMS supported SRS

Thank you for your feedback. My comments are below:

It means that there are tens of thousands WMS services which support 6782 different projections for each layer (checked from Geoserver version 2.25.3). I would not like them all to be reported as subdatasets.
I meant to use CRSs only reported by GetCapabilities. I haven't seen that GetCapabilities returns so many (6782) possible projections...

WMS standard also does not define any default CRS and the first one on the list in GetCapabilities does not need to be the best.
Therefore, the current form of the generating subdatasets names method for WMS driver is even more useless, if we have a non-default (not the best) projection for the data.

And the only good way to work with it is to parse XML and generate paths manually...

That's why I favor Even's option to introduce open option LIST_ALL_SRS=YES/NO.

Any more thoughts?

Michał Kowalczuk

wt., 10 wrz 2024 o 15:03 Rahkonen Jukka <jukka.rahkonen at maanmittauslaitos.fi<mailto:jukka.rahkonen at maanmittauslaitos.fi>> napisał(a):
Hi,

WMTS typically supports a rather small number of tilematrices and tiles are usually cached, so it makes a lot of sense to advertise the available matrices and utilize them. On the other hand WMS maps are created on-the-fly and there is very low technical cost on the server side to support however many projections. WMS standard also does not define any default CRS and the first one on the list in GetCapabilities does not need to be the best.

I guess that Geoserver is the most common WMS server in the world and amazingly many Geoservers run with the default settings. It means that there are tens of thousands WMS services which support 6782 different projections for each layer (checked from Geoserver version 2.25.3). I would not like them all to be reported as subdatasets.

I can see that ogrinfo and the OGC API Features driver report the list of supported projections. Test with ogrinfo OAPIF:https://ogc-api.nrw.de/inspire-us-feuerwehr -al -so shows "Supported SRS: OGC:CRS84, EPSG:25832, EPSG:25833, EPSG:4258, EPSG:4326, EPSG:3395, EPSG:3857, EPSG:3034, EPSG:3035"
Something similar might be an option for gdalinfo, but still about 6782 EPSG codes per layer is too much to be viewed by default. Maybe the CRS list could be reported under some metadata domain that user could read on demand?

-Jukka Rahkonen-

Lähettäjä: gdal-dev <gdal-dev-bounces at lists.osgeo.org<mailto:gdal-dev-bounces at lists.osgeo.org>> Puolesta Michal Kowalczuk via gdal-dev
Lähetetty: tiistai 10. syyskuuta 2024 15.15
Vastaanottaja: gdal-dev at lists.osgeo.org<mailto:gdal-dev at lists.osgeo.org>
Aihe: Re: [gdal-dev] WMS supported SRS

I found that there was a similar issue in 2021 without any specific answer:
https://www.mail-archive.com/gdal-dev@lists.osgeo.org/msg35549.html

I wonder if getting SUBDATASETS shouldn't return the result in a similar way as it does for WMTS service,
where GDAL generates paths to all layer/tilematrix/tilematrixset combination.
For example if layer is available in 3 SRS, GDAL produces 3 subdatasets, so using only SUBDATASE_NAME property user has access to all map services shared by WMTS server.
In my opinion, GDAL should use the same approach for WMS.

Does anyone object?
I will then submit a new feature request/bug report.

Best regards and have a nice day
Michał Kowalczuk

pon., 9 wrz 2024 o 16:17 Michał Kowalczuk <michkowalczuk at gmail.com<mailto:michkowalczuk at gmail.com>> napisał(a):
Hi,
Does GDAL provides a method to get supported spatial reference systems by WMS service?
GDALGetMetadata(SUBDATASETS) generate links only for the first (default) SRS from GetCapabilities, so this won't be helpful.
Working with WFS I can get supported SRS using OGR OGR_L_GetSupportedSRSList function.
Does GDAL offers something similar for WMS, or should I get this information from GetCapabilities XML by myself?

Regards,
Michal
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