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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Matt<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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!<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Cheers<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Chris Crook<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Matt Boyd [mailto:mattslists@gmail.com]
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<b>Sent:</b> Tuesday, 30 September 2014 5:17 p.m.<br>
<b>To:</b> qgis-user<br>
<b>Subject:</b> [Qgis-user] OT: IRTF to GDA94 difference<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hi fellow QGIS people,<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Is there anywhere online that works this out automatically?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks<o:p></o:p></p>
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<p class="MsoNormal">Matt<o:p></o:p></p>
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