[PROJ] MGA94/MGA2020

[Even Rouault]

Henning,

both GDA94 and GDA2020 are "static" CRS, that is coordinates are not 
time dependent in any of those, so it is expected that specifying the 
coordinate epoch in such transformations has no impact.

There are known time-dependent transformation between GDA94 and ITRF2014 
or GDA2020 and ITRF2014. Your GPS probably outputs coordinates in WGS 84 
(G1762) (or maybe another realization), but you may perhaps try to 
assume that ITRF2014 and that WGS 84 are equivalent for your purposes.

Then you can for example use

$ echo 300000 6000000 0 2020.5 | cs2cs -d 8 EPSG:28356 EPSG:9000

-36.12408288    150.77761389 -0.09245914 2020.5


You can also build a "ITRF2014 / Map Grid of Australia zone 56" 
projection by using 
"urn:ogc:def:crs,crs:EPSG::9000,cs:EPSG::4400,coordinateOperation:EPSG::17356"

(EPSG:9000 referes to the geographic CRS ITRF2014, EPSG:4400 to the 
easting/northing coordinate system, and EPSG:17356 to the conversion 
method for MGA56)

Then

$ echo 300000 6000000 0 2020.5 | cs2cs -d 8 EPSG:28356 
urn:ogc:def:crs,crs:EPSG::9000,cs:EPSG::4400,coordinateOperation:EPSG::17356
300000.44233932    6000001.44797768 -0.09245914 2020.5

(you might get slightly different results if the GDA94 to GDA2020 grid 
is present)

Even

Le 04/10/2021 à 21:16, henning at kaarta.com a écrit :
>
> Hello,
>
> I have a set of ground control points provided by a surveyor in 
> Australia that were created in June 2020.
>
> They are expressed in MGA94 / Zone 56 CRS, which I believe is EPSG:28356.
>
> I would like to convert these points to MGA2020 (EPSG:7856) and relate 
> them to actual data coming from a Trimble GPS today.
>
> Using PROJ7, I am able to get transformed values between MGA94 and 
> MGA2020 that reflect the displacement of the Australian continent 
> between those CRSs, e.g. (using some round numbers to illustrate here)
>
> echo 300000 6000000 0 | cs2cs +init=epsg:28356 +to +init=epsg:7856 -d 3
>
> 300000.435          6000001.421 0.000
>
> However, in comparing the results to current readings from the GPS,  I 
> see they’re off by what may be the continental drift that’s occurred 
> in the last 15 months – about 8 cm northeast.
>
> I know PROJ7 knows about time-dependent transformations, I just can’t 
> seem to figure out how to use that to get my GCP coordinates in 
> today’s MGA2020 grid. I’ve naively tried
>
> echo 300000 6000000 0 2020.5 | cs2cs +init=epsg:28356 +to 
> +init=epsg:7856 -d 3
>
> 300000.435          6000001.421 0.000 2020.5
>
> but it comes up with the same numbers.
>
> Do I need to construct a more elaborate pipeline, and is this even 
> something cs2cs can be used for?
> Any tips or tricks would be appreciated! As you might have guessed, 
> I’m pretty much a novice with PROJ7.
>
> Thanks,
>
> Henning Pangels
>
>
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