[talk-au] Talk-au Digest, Vol 110, Issue 2
neyfamily1 at gmail.com
Tue Aug 2 14:10:04 UTC 2016
It appears to be a lot of confusion regards the terms used here. As of 1
January, all coordinates (latitude and longitudes) will be determined on
the ITRF. Currently the ITRF2005. Transformations will be undertaken to
convert to local GDA (geodetic datum of Australia - latitude and longitude)
and MGA (map grid of Australia - north and east coordinates) using the
GRS80 ellipsoid and a single epoch, similar to how GDA94 was created. It
was actually based upon ITRF92. See further reading links below.
WGS84 in reality is not a datum like GDA94 (ie Geodetic Datum of Australia)
etc. but a reference frame or ellipsoid. There is technically no such thing
as WGS84 coordinates. But rather coordinates based upon WGS84. At present
the GRS80 and WGS84 ellipsoids are essentially the same. Meaning
coordinates computed from the WGS84 are the same as GDA94.
However, Over time the GRS80 and WGS84 ellipsoids are to be phased out. As
will any reference to GDA and MGA, which are plate based. The government
does not want to have to create new datum every 20 years or so.
The reference to gravitational models refers to the Geoid. A shape that
approximates sea level around the planet. The earth is more like a potato
than a ball in reality. Without getting too complex, we need a ball
(ellipsoid) to obtain coordinates, as we need a fixed centre. However,
since our heights were historically based on sea levels, as in AHD, we need
to determine the distance between sea level at any point (AHD 0m) and the
ellipsoid (the ball used to obtain coordinates). Gravity modelling can be
used to determine the seperation. At present, modelling is done on a 2' x
2' grid and positions in between interpolated. Hence the ellipsoid - Geoid
model, currently AUSGEOID09.
Incidentally, There were other ellipsoid prior to WGS84, such as Clarke
1858, used extensively in Australia in colonial days. Latitude and
longitudes from old maps therefore cannot be used directly on modern
mapping systems such as Google that use an entirely different ellipsoid.
Here are some of the ellipsoids used,
If you want to get right into the theory, look here at the GDA manual for
an in depth explanation. Way better than my brief.
In summary, it's getter to start thinking in terms of the ITRF, as this is
the only future coordination in Australia.
On Tue, 2 Aug 2016 at 10:02 PM, <talk-au-request at openstreetmap.org> wrote:
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> Today's Topics:
> 1. Re: Australia "changing coordinates" (Warin)
> Message: 1
> Date: Tue, 2 Aug 2016 20:43:15 +1000
> From: Warin <61sundowner at gmail.com>
> To: Andrew Davidson <theswavu at gmail.com>, talk-au at openstreetmap.org
> Subject: Re: [talk-au] Australia "changing coordinates"
> Message-ID: <a7dca9b5-e0c0-8faf-d6a3-ea1ece435c85 at gmail.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
> On 8/2/2016 3:37 PM, Andrew Davidson wrote:
> >> WGS84 is a datum fixed; as in dated 1984 and the data does not change.
> > I'm not sure that you've understood exactly what a semi-dynamic datum
> > is. Have a read of this:
> > http://ceur-ws.org/Vol-1142/paper6.pdf
> > particularly sections 3.4 and 3.5.
> I'm reading
> The document is dated in 1996 so more recent things are not in it. But I
> think the principles would hold.
> It lists;
> WGS 84 1 Jan 87 - 1 Jan 94
> WGS 84 (G730) 2 Jan 94 - 28 Sept 96
> WGS 84 (G873) From 29 Sept 96
> You will observe the refinements of WGS 84 (to reduce
> error/uncertainty). I would expect that suffix would be used in
> professional circles.
> The (Gxxxx) simply gives a refinement to WGS84, does not change the
> basis, and the refinements are getting smaller.
> This document also introduces the concept of 'uncertainties', a good move.
> Page 16 of 175 states in part
> "As of 2 Jan 94, a set of geodetic coordinates shall include a
> designation of the reference frame and epoch of the observations"
> Note that this is for any datum. I expect this date requirement will
> become more common and a requirement with earth centric datums.
> "In summary, the refinements which have been made to WGS 84 have reduced
> the uncertainty in the coordinates of the reference frame, the
> uncertainty of the gravitational model and the uncertainty of the geoid
> undulations.They have not changed WGS 84. "
> In section 3.5 the error is introduced in making the first transform
> from GDA94 to WGS84. Not clear if this transform was made at all, or if
> made was not to the correct date.
> The error could have been avoided by the correct transform in the first
> datum conversion?
> >> Or you have access to survey points/marks and their data in the local
> >> area.
> > LOL. Reminds me of this quote:
> > "You'll never find a survey mark on the ground with coordinates
> > referenced to WGS-84. If you do, it's wrong."
> A global datum will have more compromises than a local datum. For that
> reason alone I would expect survey marks to be in the local datum..
> e.g. NSW survey marks are GDA94 using MGA
> I also expect these will be translated to the new earth centric datum as
> a mathematical operation some time in 2017.
> I would expect that any local datum can be transformed into WGS84 with a
> sufficient level of accuracy for the task, if required (i.e. if the end
> equipment requires WGS84, or its refinements)
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> End of Talk-au Digest, Vol 110, Issue 2
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