[Qgis-user] OT: IRTF to GDA94 difference

[Chris Crook]

Just in relation to the use of the HTDP grids, in Australia you don't need to do anything so complicated :-)  On the other hand, if you come from New Zealand - as I do - then you definitely need to use an approach something like this if you want to use PROJ or almost any other GIS software to do the conversion between NZGD2000 and ITRF2008.

The relationship between GDA94 and ITRF2008 at any particular date is a standard 7 parameter transformation (3 x translation, 3 x rotation, plus scale).  PROJ, along with other GIS coordinate conversion software will handle this with a custom datum transformation string.

Cheers
Chris
________________________________________
From: Matt Boyd [mattslists at gmail.com]
Sent: 01 October 2014 18:26
To: qgis-user
Subject: Re: [Qgis-user] OT: IRTF to GDA94 difference

Thanks everyone,
this is a lot of help.

Matt

On Wed, Oct 1, 2014 at 4:55 AM, Even Rouault <even.rouault at spatialys.com<mailto:even.rouault at spatialys.com>> wrote:
Hi,

Related to that rather advanced topic, I think I should mention similar work
done in the past by Frank Warmerdam in proj.4 for NAD83 vs WGS84 :
https://trac.osgeo.org/proj/wiki/HTDPGrids.
Perhaps a similar approach can work for GDA94.

Best regards,

Even

Le mardi 30 septembre 2014 20:49:56, Chris Crook a écrit :
> Hi Matt
>
> ITRF is a global reference frame (or actually a series of reference frames
> such as ITRF2000, ITRF2005, ITRF2008, and shortly ITRF2013).  It is the
> coordinate system used for global system such as GNSS (global navigation
> satellite systems, of which GPS is the most well known).
>
> Each tectonic plate is moving relative to it.  So the movement of the
> Australian plate in this system amounts to about 7 cm per year.
>
> GDA94 is an Australian datum and is effectively defined in terms of the
> Australian plate, so it moves with it.  What that means a point in
> Australia has a fixed coordinate in terms of GDA94, but it's latitude and
> longitude in terms of an ITRF is steadily changing, equivalent to the 7cm
> per year.  In 1994 when GDA94 was originally defined the latitude and
> longitude were more or less the same as ITRF, which means now the
> difference is equivalent to about 20*7cm = 1.4m offset.  That is, if you
> confuse an ITRF latitude and longitude with a GDA94 latitude and
> longitude, you might be in error by this much.
>
> UTM refers to a series of Transverse Mercator with central meridians at 6
> degrees of longitude spacing.  That is to say it is a set of functions for
> converting latitude and longitude to and from easting and northing.  The
> UTM coordinate of a point therefore depends upon the latitude and
> longitude of that point, and as noted above, that depends on the datum
> (ITRF,GDA94) used to define the latitude and longitude.
>
> I haven't reread the specification for a while, but some (many) years ago
> the datum used for UTM was somewhat ambiguously defined to be the
> something like the most significant or dominant datum in the zone in which
> it applied.  However that may have changed to be more specific in these
> days of global reference frames.  If this definition remains then UTM
> zones over Australia could be in terms of GDA94.  However that seems
> unlikely, maybe someone with more current can confirm or deny!
>
> The main point is that if you care about accuracies of the order of 1m,
> then you need to know what datum you are using.
>
> The second main point is that the relationship between datums is time
> dependent (ie it is not the same in 1994 as 2014).  This is currently not
> handled by GIS coordinate system metadata, which only handles constant (in
> time) relationships between reference systems.
>
> Cheers
> Chris Crook
>
> From: Matt Boyd [mailto:mattslists at gmail.com<mailto:mattslists at gmail.com>]
> Sent: Tuesday, 30 September 2014 5:17 p.m.
> To: qgis-user
> Subject: [Qgis-user] OT: IRTF to GDA94 difference
>
> Hi fellow QGIS people,
> I'd like an easy (as in if it's too hard I probably won't have time to do
> it) way to confirm the drift on GDA94 compared to UTM. I've done a little
> digging and haven't been able to find anything except "Australia is moving
> approximately NE at a rate of 7cm per year and in 2005 it was 77cm".
> Having said that, I'm a little hazy on if UTM is the worldwide reference
> and how it's related to IRTF.
>
> Is there anywhere online that works this out automatically?
>
> Thanks
> Matt
>
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This message contains information, which may be in confidence and may be subject to legal privilege. If you are not the intended recipient, you must not peruse, use, disseminate, distribute or copy this message. If you have received this message in error, please notify us immediately (Phone 0800 665 463 or info at linz.govt.nz) and destroy the original message. LINZ accepts no responsibility for changes to this email, or for any attachments, after its transmission from LINZ. Thank You.