[aprssig] GPS off track

[Michael Carey]

I come across this issue all the time at work. I work in marine 
electronics and datum confusion is very painful when you have fishermen 
with old paper charts in AGD66 and new electronic charts in WGS84. 
Ideally, whatever mapping system/software you use can take this into 
account... the problem is, many don't.
For land based stuff I use Oziexplorer which automatically sorts out the 
datum for you. Many of the maps I use with Oziexplorer are in AGD66, 
some are WGS84 and some are in GDA94 (the "new" Australian datum which 
is virtually the same as WGS84). You don't need to worry if the software 
has been written correctly. When I enter a waypoint Lat/Long in 
Oziexplorer, I have to specify the datum. If it differs from the map I 
am viewing, Oziexplorer changes it to suit. The position is still the 
same and although the Lat/Long changes depending on what map I load... 
it is still the same position.
APRS software is easier as it is all based on WGS84, the "GPS datum"... 
as long as you have your GPS set to WGS84!
Michael.
VK5ZEA


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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