[Qgis-user] OT: IRTF to GDA94 difference

[Chris Crook]

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]
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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