<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd"><html><head><meta name="qrichtext" content="1" /><style type="text/css">p, li { white-space: pre-wrap; }</style></head><body style=" font-family:'Sans Serif'; font-size:10pt; font-weight:400; font-style:normal;">On Fri, 12 Jun 2009, Delta Foxtrot wrote:<br>
> --- On Fri, 12/6/09, Liz <edodd@billiau.net> wrote:<br>
> > I'll ask the offspring. He is 7/8 of a surveyor<br>
> > with one semester to go!<br>
><br>
> Can you also ask the bearing the drift is going, all I've been able to find<br>
> out is approx north or north north east, but haven't been able to find<br>
> anything on the net that clarifies it.<br>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br></p><p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br></p>adrian advises<br>
Making me drag out the Geodesy notes....<br>
Ok, here we go. This may be incredibly boring, but it was boring in<br>
class. When GDA94 was defined one of the driving factors was GPS and<br>
the need because of it for an Earth centred datum that matched WGS84<br>
closely. GDA94 takes the same origin as WGS84, the centre of mass of<br>
the Earth and the same semi-major ellipsiodal axis, but a tiny<br>
difference in the semi-minor to match the geoid over Australia better.<br>
This results in the original position difference between the 2. This<br>
positional difference is fairly irrelevant to any but Geodetic<br>
surveyors, as most people do not have equipment to measure that<br>
accurately and the general Cadastral surveyors use predetermined marks<br>
to reference their surveys to.<br>
For those who are not following at the moment, an ellipsiod is<br>
a lot like a circle, but has a varying radius. It is defined by an<br>
origin, the largest radius (semi-major axis) and the smallest radius<br>
(semi-minor). All Earth models assume it is an ellpsiod of revolution,<br>
around the semi-minor axis to create a flattened sphere. It is a purely<br>
mathematical surface. The geoid is a surface defined by being<br>
perpendicular to the line of gravity at every point. It also is a<br>
surface where gravity has the same force at any point on the surface. If<br>
you level something with a spirit level, it is level with respect to the<br>
geoid.<br>
Back to the datums, I don't have any current misalignment data<br>
between GDA94 and WGS84. GDA94 is fixed to the position of reference<br>
stations on the Australian continent as of 1st January 1994. At that<br>
point the difference between the 2 was regarded as irrelevant at the<br>
DGPS level, so it should have been within 0.3m. I have a document from<br>
late 2002 saying that the difference was negligible, the continental<br>
drift was approx 7cm per year across the continent, and at that point<br>
the 2 datums had diverged by a further half-metre. Assuming the same<br>
continental drift, it has diverged by another half-metre. So,<br>
currently, GDA94 and WGS84 are within 1.3m of each other. Who would<br>
know this? In Australia, the Intergovernmental Council on Surveying and<br>
Mapping (ICSM) is supposed to deal with datums. Someone in there should<br>
be keeping track on the shifts. The shift at a given point can be<br>
calculated quite accurately with a survey grade receiver and a few<br>
hours worth of recordings.<br>
On to continental drift. While I mentioned above that it is<br>
approx 7cm per year, I can do better then that. The International Earth<br>
Rotation Service is responsible for keeping track of various receivers<br>
that measure the differences between their locations by several<br>
different methods, each to at least centimetre position. The data from<br>
these stations is freely available, but a pain to find. By finding the<br>
released position data used as the basis for the establishment of<br>
consecutive International Terrestrial Reference Framework (ITRF) datums<br>
the continental drift can be calculated for any participating station.<br>
From the horrible to navigate SINEX files provided by the IERS, in 1997<br>
the continent was doing about 6cms/y at Canberra and Hobart, 7cms/y at<br>
Pearth, and 13cms/y at Darwin. In 2000, the values are almost<br>
exactly the same. As for what direction it is going, the<br>
IERS releases files in 3D X,Y,Z Earth centred co-ordinates. These are a<br>
pain to work with, and I can't tell you from them what the direction is<br>
in a nice easy co-ordinate system. I always thought that it was in a<br>
North to North Westerly direction, an azimuth of around 330 degrees. <br>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br></p>References:<br>
IERS: http://www.iers.org/MainDisp.csl?pid=163-253<br>
ICSM: http://www.icsm.gov.au/<br>
Going Geocentric, Version 1, October 2002. Queensland Government<br>
Department of Natural Resources and Mines for the ICSM.<br>
(I should see if I can put this cd and it's information somewhere that<br>
you can all read it. It is quite a good overveiw of GDA94, and also<br>
Datums in general.)<br>
Geodesy Notes, Geoff Sandford, 2008 and 2009.<br>
<br>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br></p>-- <br>
BOFH excuse #288:<br>
<p style="-qt-paragraph-type:empty; margin-top:0px; margin-bottom:0px; margin-left:0px; margin-right:0px; -qt-block-indent:0; text-indent:0px; -qt-user-state:0;"><br></p>Hard drive sleeping. Let it wake up on it's own...<br>
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