[PROJ] grid shift with GDA2020

[Pierre Chaponniere]

Thanks for the reply Greg, I'll elaborate a little on where I start from.

Initially, data is in WGS84 (EPSG 4326), we use PROJ API to transform
coordinates from WGS84 to a user selected transformation, in my case EPSG
7854
My problem is that GDA2020 is expressed from GDA94 using the above
mentioned transformation files.
I'll therefore need to 'jump' from WGS84 to GDA94 and from GDA94 to GDA2020
in one step, most probably using a pipeline - not sure how though...and if
possible...a sort of two-transform in one go.

And in more general terms I was questioning how PROJ is doing the link
between CRS codes and grid shift transformation files - where can I find
this information ?

Offsets that I observe from the data comparing ground truth points (in EPSG
7854) and my data (supposedly in EPSG 7854 but I doubt as I don't think it
is considering the transformation grid) are in the order of 80m to the NNE
from where it is supposed to be.

Thanks for your help !
Best regards,
Pierre C




Le mer. 29 juin 2022 à 22:24, Greg Troxel <gdt at lexort.com> a écrit :

>
> Pierre Chaponniere <pierrechapo at gmail.com> writes:
>
> > i am looking at transforming data into GDA2020 MGA zone 54 :
>
> I am far from a GDA expert but it does not make sense to talk about
> transforming data into a CRS without specifying the CRS the data is
> already in.
>
> > +proj=utm +zone=54 +south +ellps=GRS80 +units=m +no_defs +type=crs
>
> This is a bare projection, and GDA2020 zone 54 is a projection based on
> a datum. One is a way to take lat/lon into x/y, and the other is
> something you can use to transform to/from other datums.
>
> > But it seems this projection is associated with a transformation grid
> >
> >
> https://www.icsm.gov.au/datum/gda-transformation-products-and-tools/transformation-grids
>
> The projection is not, but GDA94 to GDA2020 transformations are, as I
> read that page.  (That makes senes to me; we have a similar situation
> in the US among former/current/future national datums.)
>
> > I have therefore dowloaded the corresponding .gsb transformation files
> >
> > https://github.com/icsm-au/transformation_grids
> >
> > However when I apply the transformation, it appears the process does not
> > use the transformation files and generate data with planimetric and
> > vertical offsets.
>
> You didn't explain offset from what and why you think the result is
> wrong.
>
> > How can I make sure the transformation grid are applied and what file
> makes
> > the link between proj code and transformation files - is this stored in
> > proj.db ?
>
> Almost certainly, label the input data as being in the CRS it is
> actually in, and find a CRS for the GDA2020 zone that labels it as being
> not just in UTM, but UTM GDA2020.  At least then you'll be asking proj
> to do the right thing.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/proj/attachments/20220630/caad92ae/attachment.htm>