[Qgis-user] OT: IRTF to GDA94 difference

[Even Rouault]

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]
> 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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