[PROJ] [gdal-dev] Static/Dynamic datum problems

[Cameron Shorter]

Thanks Evan, I'm switching this email to the proj email list as suggested.

In answer to Evan's question:
> Why ? I don't see a contradiction between those 3 statements, because as
you
> mentioned, WGS84 is dynamic.

I believe that the problem is that WGS84 is being used in webmapping
situations as if it were a static datum, locked to a fixed epoch, and in
other cases it is treated as if it were a dynamic datum.

This has led to the situation we have in Australia where:

WGS94 -transform-> WGS2020 (leads to a coordinate shift of ~ 1.8 metres)
WGS94 -null transform-> WGS84 (which was correct in 1994)
WGS2020 -null transform-> WGS84 (which will be correct in 2020)

I can see that I will need to write this up more clearly, which I'm in the
process of doing. I'll share when done (early next week).

Cheers, Cameron


On Fri, 21 Jun 2019 at 08:20, Even Rouault <even.rouault at spatialys.com>
wrote:

> Cameron,
>
> This would be more a topic for proj at lists.osgeo.org .
>
> >
> > Our Australian spatial data users are about to face a systematic mismatch
> > challenge when trying to use multiple static datums (GDA2020, GDA94) with
> > the dynamic datum (WGS84). At the moment, it is government agencies
> > grappling with the problem, but it is about to become a mainstream issue.
> >
> > Australia now has static datums for the years 1994 and 2020 and uses
> WGS84
> > (a time-dependent datum!), for web-mapping and web-services.  We
> recognise:
> > 1. Transforming from GDA94 to GDA2020 reflects Australia’s tectonic
> > movement of ~ 1.8 metres to the North East.
> > 2. GDA94 was defined as ‘equal to WGS84’ in 1994.
> > 3. GDA2020  was defined as ‘equal to WGS84’ in 2020.
> > All three statements can’t be accurate.
>
> Why ? I don't see a contradiction between those 3 statements, because as
> you
> mentionned, WGS84 is dynamic.
>
> > I believe this is a problem the whole world needs to address,
>
> I'm not sure to have understood what the problem you're facing to is
> exactly.
> Is it when combining data sources from several of those 3 CRS ?
>
> There are transformation paths between GDA94 and GDA2020 in the EPSG
> dataset,
> with static Helmert-based or grid-based transformations.
>
> With PROJ 6:
> $ projinfo -s GDA94 -t GDA2020 --summary --spatial-test intersects
> Candidate operations found: 5
> EPSG:8048, GDA94 to GDA2020 (1), 0.01 m, Australia - GDA
> EPSG:8447, GDA94 to GDA2020 (2), 0.05 m, Australia - onshore, at least one
> grid missing
> EPSG:8446, GDA94 to GDA2020 (3), 0.05 m, Australia - onshore, at least one
> grid missing
> EPSG:8445, GDA94 to GDA2020 (5), 0.05 m, Cocos (Keeling) Islands -
> onshore, at
> least one grid missing
> EPSG:8444, GDA94 to GDA2020 (4), 0.05 m, Christmas Island - onshore, at
> least
> one grid missing
>
> Mixing with data referenced to WGS84 is going to be more problematic,
> because
> of its dynamic nature indeed. So typically you will need to know if
> coordinates are expressed to one of the particular realizations of WGS84:
> CRS EPSG:7816: WGS 84 (Transit)
> CRS EPSG:7657: WGS 84 (G730)
> CRS EPSG:7659: WGS 84 (G873)
> CRS EPSG:7661: WGS 84 (G1150)
> CRS EPSG:7663: WGS 84 (G1674)
> CRS EPSG:7665: WGS 84 (G1762)
>
> The EPSG datset contains coordinate operations between those WGS 84 or
> equivalent ITRF realizations, and GDA94 or GDA2020
>
> To/from GDA2020:
> - EPSG:8049: "ITRF2014 to GDA2020 (1)'" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:8448: "GDA2020 to WGS 84 (G1762)" (Time-dependent Coordinate Frame
> rotation)
>
> To/from GDA94:
> - EPSG:6276: "ITRF2008 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6277: "ITRF2005 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6278: "ITRF2000 to GDA94 (2)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6279: "ITRF97 to GDA94 (2)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6280: "ITRF96 to GDA94 (2)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6313: "ITRF96 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6314: "ITRF97 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6315: "ITRF2000 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
> - EPSG:6392: "ITRF97 to GDA94 (1)" (Time-dependent Coordinate Frame
> rotation)
>
> So it seems to me that the needed operations are there. GDAL 3 and PROJ 6
> should be able to use them. What is probably more difficult is to figure
> out
> the WGS84 / ITRF realization and the epoch in which your coordinates are
> expressed.
>
> Even
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>


-- 
Cameron Shorter
Technology Demystifier
Open Technologies and Geospatial Consultant

M +61 (0) 419 142 254
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