Different DATUMs on GPS's

[lanason]

Different DATUMs on GPS\'s

Having had my GPS since the Mercury meet up. I was playing with it and it takes ages to find the Sats. On talking to the guy in Piplers in Poole last weekend, he said to turn the WAAS off as it would be quicker and it was.

We then spoke about accuracy and he said it had different DATUMS - for charts it was set on WGS 84 and so were the charts so no problems - but on land I entered long and lat from Multimap which is based on OS landranger ..... he said that the different datums means that he long and lats are DIFFERENT and cannot be used between different datums.

So if I change the datum to say British Grid (OS) enter a long and lat and then change back to WGS for charts will it convert / correct the position ???

I though Lat and Long was a unversal system - didnt realise it needed to be referenced back to different datums !!!!

Must be similar to Imp Gallons and US Gallons ........

<hr width=100% size=1>Adrian
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[BrendanS]

Re: Different DATUMs on GPS\'s

Lat and Long are a universal system, up to a point. Each charting sytem uses a slighty different datum point, and none are entirely accurate if taken outside their area of use. Complex subject really - do you understand Mercator projections, and how no flat map/chart can accurately show a curved surface?

To answer the question, if you say enter a waypoint while in OS mode and read off an OS chart it will be accurate (up to a point, GPS is more accurate than the old surveys under any system). If you then change your GPS to WGS 84, then use a WGS 84 chart, the waypoint will be accurate on that chart (again, up to a point)

WAAS only works in the US, though a similar Euro satellite system is currently being tested. Using WAAS in the UK is likely to cause more errors than refine them , as the idea is that base stations interrogate satellites for local inaccuracies due to diffraction delays etc in the atmosphere, then compensate for them. Any compensations your GPS picks up in the UK will be wildly inaccurate, and so you should turn off WAAS as it will make readings worse not better.

<hr width=100% size=1>Err, let me know if Depsol enters the forum, I'll go and hide

 

[lanason]

Re: Different DATUMs on GPS\'s

Yes I understand the Mercator projections - but hadnt thought through the implications onto small scale maps and charts. Never having used lat and long on land having used OS grid refs. It came about when I went to pick my son up from a party - put in the Post code to Multimap and it gave me the Lat and long - so thinks I enter into GPS and find the place easy !!!! Thats when I found out the error !! Still found him though /forums/images/icons/mad.gif
It's the up to a point bit thats the issue - was about 1/2 a mile out !!!

<hr width=100% size=1>Adrian
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[ccscott49]

Re: Different DATUMs on GPS\'s

The only thing you need to remember is enter the correct datum from where the map was drawn, (written on it) into the gps, all will then be OK.

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