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Add wiki page associating PROJ versions with key WKT features #154

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esilvia opened this issue Feb 20, 2025 · 2 comments
Open

Add wiki page associating PROJ versions with key WKT features #154

esilvia opened this issue Feb 20, 2025 · 2 comments
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@esilvia
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esilvia commented Feb 20, 2025

What is the issue about?

Committee efforts

Issue description

Multiple stakeholders have inquired about specific features of WKTv2 and want to know which version(s) of PROJ they need to use for specific features, such as EPOCH, geoid_model, and proper vertical CRS definitions.

Creating a wiki page that supplies specific versions of PROJ would facilitate adding support for LAS 1.5 and WKTv2 and help the community. Maybe this already exists somewhere in the OGC ecosystem?

Ezra noted in the 2025/02/20 LWG call that geoid_model is already supported to some extent in PROJ, but needs some significant additions (notably international geoids such as EGM08): https://github.com/OSGeo/PROJ/blob/77bb0cc0d2d80d14bcfc7921647fa39b144fe0ca/data/sql/customizations.sql#L463

@esilvia esilvia added the wiki Changes to be incorporated into the wiki label Feb 20, 2025
@EZChe-EZDataMD
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Context:
One challenge in describing vertical CRS in WKT is that the geoid models from NOAA/NGS do not have EPSG codes registered. In some practices, taking USGS 3DEP as an example, the data specification specifies how this can be tackled by appending the geoid name to the VERT_CS field in the WKTv1 as specified in LAS1.4. For example, VERT_CS[“NAVD88 height (ftUS) - GEOID18”]. It also explicitly mentioned that a user-defined entity will not be allowed such as "GEOID_MODEL[]". However, such workaround (changing the name of a CRS) could potentially invalidate the WKT.

Now the situation has changed. WKTv2 will be used in LAS1.5, and it can accept the field name called GEOIDMODEL.

Here is an example of Vertical CRS WKT with a geoid model provided by OGC standard of WKTv2 18-010r11:
VERTCRS["CGVD2013",
VRF["Canadian Geodetic Vertical Datum of 2013"],
CS[vertical,1],
AXIS["gravity-related height (H)",up],
LENGTHUNIT["metre",1.0],
GEOIDMODEL["CGG2013",ID["EPSG",6648]]
]

Although it still does not fully address the core issue that NOAA geoids do not have EPSG code, libraries like PROJ have related implementations to recognize the geoid based on the name in the name field of GEOIDMODEL[].

@EZChe-EZDataMD
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A lot of questions were also asked about recording epochs of the data coordinates and dynamic CRS.
In OGC standard of WKTv2 18-010r11, the name fields DYNAMIC[] and FRAMEEPOCH[] are used to define dynamic CRS while the data epoch is expressed via COORDINATEMETADATA[] and EPOCH[]. Here is an example:

COORDINATEMETADATA[
GEOGCRS["WGS 84 (G1762)",
DYNAMIC[FRAMEEPOCH[2005.0]],
DATUM["World Geodetic System 1984 (G1762)",
ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1.0]]
],
CS[ellipsoidal,3],
AXIS["(lat)",north,ANGLEUNIT["degree",0.0174532925199433]],
AXIS["(lon)",east,ANGLEUNIT["degree",0.0174532925199433]],
AXIS["ellipsoidal height (h)",up,LENGTHUNIT["metre",1.0]]
],
EPOCH[2016.47]
]

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