[PROJ] Static/Dynamic Webmapping Problem version 2.0

Duncan Agnew dagnew at ucsd.edu
Wed Jul 17 17:24:49 PDT 2019

```No, "same point" means "same monument set in the ground". If I understand
things properly, a measurement in 1994
(which is analyzed in IRTF and then converted to GDA94) will have some XYZ
value, call it XYZ94. And a measurement
in 2020 (analyzed in ITRF and then converted to GDA2020) will have XYZ
numbers that have the same values as XYZ94.
Of course the 2020 data, in GDA94, will show a different coordinate for the
point. The equivalence of the numbers is very close
only when data from an epoch is in the reference frame (fixed to ITRF) for
that epoch.

You are probably right about ATRF: I just assumed that it would be set up
as NGS is setting up the new NA frame: a
frame that moves relative to ITRF so that coordinates of places on the NA
plate will, in the NA frame, stay the same, as most surveyors prefer. To
get these coordinates out of your data you do need to have the epoch (to
use the right parameters
for the moving NA frame)--but so long as you and everyone else does that
with their data, everybody will get the same numbers for a given monument,
whenever they make the measurement. I would have thought this would be
perfect for
Australia, since it doesn't deform hardly at all.

On Wed, Jul 17, 2019 at 12:16 PM Even Rouault <even.rouault at spatialys.com>
wrote:

> > So if you start out in 1994 and keep measuring the same point in GDA94,
> its
> > coordinates will change and get increasingly far from their initial
> value.
>
> When you say "same point", you mean a point at fixed coordinates in ITRF
> right
> ? (so contrary to common langage where when talking about "same point" we
> generally think of a point physically attached to the plate (not talking
> region subject to crustal deformation/earthquakes)).
>
> > In GDA2020 they will also change with time, but instead will approach the
> > GDA94 numbers measured in 1994, until in 2020 they will be the same--but
> > then start diverging again. The point is that the coordinate values
> > measured at a place in 1994, expressed in GDA94, will match the values
> > measured at the same place in 2020 expressed in GDA2020. Neither is
> > plate-fixed.
>
> > There will be a time-dependent reference frame known as the
> > Australian Terrestrial Reference Frame (ATRF) that is plate-fixed: in
> 2020
> > it will match GDA2020.
>
> Hum, my understanding of ATRF (based on
> http://spatialservices.finance.nsw.gov.au/__data/assets/pdf_file/
>
> 0018/217224/2017_Janssen_Position90_GDA2020_AUSGeoid2020_and_ATRF_explained.pdf
> <http://spatialservices.finance.nsw.gov.au/__data/assets/pdf_file/0018/217224/2017_Janssen_Position90_GDA2020_AUSGeoid2020_and_ATRF_explained.pdf>
> ) is that it is NOT plate-fixed. It is a realization of ITRF, so it is as
> Earth-fixed as ITRF is. Basically coordinates in ATRF at yyyy.yy should be
> pretty
> much the same as if expressed in WGS84(G1762)@yyyy.yy
>
> Even
>
> --
> Spatialys - Geospatial professional services
> http://www.spatialys.com
>
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