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[aprssig] GPS off track

Ray Wells vk2tv at exemail.com.au
Thu Apr 3 01:41:57 UTC 2008


Ben and others,

According to Geoscience Australia - we use GDA94, not WGS84, and this is 
why ...

quote ....
However since 1994, the Australian tectonic plate has been shifting 
approximately northeast at a rate of about 7cm per year. Because the 
whole of Australia is moving uniformly, this movement is not an issue 
when obtaining positions relative to other GDA94 positions. However, if 
a position is obtained directly from a global positioning system (such 
as GPS) the difference may be apparent. If the global position has a 
large uncertainty – such as a standard GPS point position with accuracy 
of about 10 metres – then it is still not an issue. But where an 
accurate global position is obtained – such as a global high accuracy 
differential GPS system with an uncertainty of a few decimetres – the 
difference is very relevant.
......etouq

More info here ...
http://www.ga.gov.au/geodesy/datums/cosys.jsp

The Geoscience Australia web site provides a host of information of 
mapping standards in Australia.

GDA94 - Geocentric Datum of Australia, 1994.

Map height are relative to Australian Height Datum - AHD

This is what I'm seeing on a random selection of paper maps here.

1:25,000 printed in 1980 to AGD 1966
AGD - Australian Geodetic Datum. To convert WGS72 to AGD66 add 5.6" 
(172m) to latitude, decrease longitude by 3.2" (85m), and decrease 
satellite height by 28m.

1:25.000 printed in 2000 to GDA 1994
GDA - Geocentric Datum of Australia 1994. To convert GDA94 to AGD66 
increase Latitude by 5.7" and decrease Longitude by 3.9". To obtain 
WGS84 height increase AHD (Australian height Datum) by 31m

I suppose a newer map would provide conversion to/from WGS84

We've had something like 20 different standards over the years, with the 
1858 (yes, 1858) Clark standard still appearing in archival material, 
and being used, as recently as 1999.

I have to wonder if New Zealand is faced with similar datum reference 
problems as we experience here in Australia.

Ray vk2tv

Ben Lindner wrote:

> Our maps here in Australia use the WGS84 datum
>
> Ben Lindner
> VK5JFK
>
>
> Stephen H. Smith wrote:
>
>>
>> What datum are the GPS units set for? If they are not set to WGS84 
>> (WORLD Geodetic System 1984) mode, you will see hundreds of feet error.
>> This is a constant issue in the U.K. where many paper and electronic 
>> maps use the British Ordinance Survey datum rather than the WGS84 
>> datum that is native and default on most GPS devices. Google Maps 
>> (used by both findu and aprs.fi) are aligned with the world-wide 
>> GPS-inspired WGS84 datum. Many British APRS users set their GPS units 
>> to use the UK ordinance datum to make posits line up on their own 
>> maps; only to see major errors when they view themselves on Findu or 
>> aprs.fi.
>> I would guess that Australia and New Zealand may also suffer from 
>> this problem of a "local" datum in use rather than the world-wide 
>> WGS84. (The Brits established numerous local datums during the 
>> centuries of the British Empire in places all over the world.)
>>
>>
>>
>>
>
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