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[aprssig] GPS off track

Stephen H. Smith wa8lmf2 at aol.com
Wed Apr 2 09:05:47 UTC 2008


Phillip wrote:
> Hi,
>      Have am using a Toshiba Laptop Satellite 2140CDS, AGW engine,, 
> UI-View
> 32, and a DRSI TNC2 as my mobile tracker
>
> I have 2 New GPS units a GPSmart 12 channel  and new today a Globalsat
> BU-353 SIRF powerd both of them
>
> Both these GPs give me an error of 55 mtrs East and 30 North in 
> Google  when
> checking with www.aprs.if   and www.findu.com
>
> I did a track tonight   (02/04/08 NZT)   zl2tze-15 with the new BU-353 it
> has me not packed at my correct address.
>
> On Saturday  ( 29/03/08)  I travelled to Nelson using the GPSmart GPS 
> this
> also has the same error.
>
> Is this a GPS , UI-View , an error in the Google maps  or is it that 
> these
> cheaper GPS units are not the great ???
>
> My correct address is zl2tze-4 on the Maps Google
> Any help on this will be appreciated    
> 73 Phillip
> ZL2TZE
>
>
>   

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.)




-- 

Stephen H. Smith    wa8lmf (at) aol.com
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