[Qgis-user] OT: IRTF to GDA94 difference

[Matt Boyd]

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>
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]
> > 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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