GPS error

[AuntyPodes]

Looks as though you've got it upside down Seriously now -
It's like buying a house - location, location, location - except for location read DATUM!
Don't know much about Gamin but with Magellan you have the option of using 2D or 3D. I usually use mine on 2D and set the altitude at actual mounting = 6 feet. I'm told that if you set this incorrectly you'll get a compensatory error in
position - which sounds reasonable.
A fortnight ago during a race I set mine to track my course which I later plotted (using
OziExplorer more of which later) on my PC. I was surprised to find that some of my
plotted positions were at an altitude of 60<70 feet - and it wasn't QUITE that rough!
I've raised this with Magellan and await their response with interest!

 

[bob_tyler]

Dear Old Harry,
You said: "I was also quite struck with the difference betwen the car speedo, and the GPS speed readings - with the speedo at 70 mph the GPS consistently read 64mph! 'Well officer, I wasnt speeding - according to the GPS I was only going...."

Firstly, car speedos can be 10% out and still be legal. If your tyres on the driven wheels are new the speedo could be about 3% slow or, if worn nearly to the limit, 3%-4% fast.

The only way to check a speedo accurately is with a stopwatch using the kilometre posts on a straight stretch of motorway. I did this on a Mondeo and found that at an indicated 70 I was doing 63! No wonder you get idiots who do not pull left and cruise in the outer or middle lanes in the low 60s. The Mondeo had to read 78 to get a true 70!!! This was carefully timed with a chronograph.

My present car (2000 miles) shows 73 at a true 70. When the front tyres are a bit worn I will expect to see 75 0r 76! I always do a chronograph check if on a clear straight motorway.

Best wishes,

Bob

 

[chas]

Welcome back!!

 

[oldharry]

Like I said Bob - I doubt it would stand as a defence. In any case I dont intend to try! Interestingly the speedo in my Van accords with Garmin to within 1.5mph at 65 (it wont do 70 - not safely anyway) It kept the kids happy though.

My main point is that the trace for the return journey was at times considerably more than the width of the motorway adrift from the outward journey trace - proving that GPS - or at least GPS a la Garmin 12 - can not be relied upon for close quarters navigation, the point at issue in the thread.