Quote:
Originally Posted by
deiaccord
If GPS was inaccurate it would be no good for telling you where you are
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Thats a very good point!
Quote:
Originally Posted by
deiaccord
The satalites may be a long way a way but by triangulalting the distance from up to 6-7 satalites at one time a gps unit can give you a pretty good indication of your actual land speed which is going to be quite accurate.
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Ah yes, but,,
Quote:
Originally Posted by
Old Git ray
Over a short distance it would be innacurate as sometimes the fix can be 10m out and if the next fix it gets a few seconds later is 10m away it could record you as doing some ridiculous speed even when standing still. the better ones average the speed out over a time for accuracy.
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and,
The statalites are controlled by the US military, and can be made very innacurate at the click of a mouse. I'm not sure about other countries ability but in the UK we also have WASS (Wide Area Augmentation System - bet you didn't know that one, eh ??) which is a land based system that can increase the accuracy to within a few feet. This is a system favoured by maritime GPS, but devices like the Zumo also support it. The downside of WASS (being land based) is you can often loose the signal if you are 'out of sight'
So, the GPS system bases your speed on time between two points, ergo it is more accurate the faster you go. However, I belive that the calcualtion is based on 2 dimensional movement (even though your GPS operates in a 3 dimensional plane), so descending a steep hill will give you a slower reported speed that actual speed (going up hill, your reported speed will be slower than you actual speed) If you want accuracy, then a Road Book is the way to go, but then over rough terrain when the wheel is spinning (off the ground), or your wheel circumference has changed (like you've used some tread) you again loose accuracy.
The problem is that the only real way to check your speed is to measure the time between points, but this is only average. To get real accuracy, you'd need something like a laser speed detector (even so this is still only an average over a very short distance). So here we are agaim, the Police have access to the right gear, but we don't - and we're the ones who'll get shafted as a result!
All of this is academic realy, the more accurate you become, the closer you have to watch your speed..........
