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GPS versus Speedo Accuracy

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HawkGTRider:
That's pretty interesting...thanks for the words of wisdom.

OldButNotDead:

--- Quote from: JimRRides on July 31, 2017, 11:09:26 pm ---Corollary to GPS accuracy for speed

In the last few years I have observed officers and prosecutors referring to the GPS recorded speed on the video from the patrol vehicles as proof of the defendant's speed. Rarely does the defendant challenge the assumption that the GPS speed is an accurate measure. It is NOT an accurate measure of the suspect vehicle nor the patrol vehicle, although it may be referred to as an approximation. This source of speed measurement has never (to my knowledge) been vetted by the courts in a manner similar to speed radar. If you or an acquaintance face an accusation based on this measure,  challenge it. (Note that some video systems record the readout from the actual speed radar unit and thus is a different issue. Our traffic units had this capability but not our routine patrol units.)


--- End quote ---
Never knew that was happening.  I do not believe GPS is a reliable source to write a ticket OR to defend your case either.  Also, NinjaBob, GPS has a lot of problems with vertical accuracy.  There is this thing called PDOP, Positional Dilation Of Precision which tells you how good your accuracy is from the geometry of the satellites.  It is always worse in the vertical.

HawkGTRider:
I can believe that quite easily. When I was in Death Valley years ago, the official elevation was below sea level. At that point, my gps was off by quite a bit. Other times, such as going across some high mountain passes in Colorado, etc., it's been close to the posted elevation at times and not particularly close at others.

Good info...thanks for sharing OBND.

BMWKeith:
I'm kind of late to this thread, but I'll post my observations and theory here now anyway.
I've also noticed the GPS speed being significantly lower coming out of turns than in straight lines, sometimes 10 or more mph slower.  Also, it is not very good for quickly establishing your speed as it changes, such as when accelerating away from a stop or getting on the expressway.
I believe that this a reflection of the accuracy of your position calculated by the GPS.  Every position that is calculated has what is called Circular Error of Probability (CEP).  This is essentially the circle that you will see displayed on your GPS when you zoom in really close on your position.  Your true position could be anywhere inside of that circle.  Every time it calculates, it has that error.  If it displayed your speed based on the distance per time of every position update it calculates, first, it could be difficult to read because the number would be changing so frequently.  Second it could potentially have variations of several mph from one update to the next because of where you actually are in that circle, whether ahead of or behind your true position.  Therefore, I suspect that the speed display is updated based on some number of points several samples apart, maybe 5 or 10, so that it minimizes the errors in the calculated speed due to the error in your actual position.  In a turn, the distance between points several samples apart is smaller than the distance traveled because it is calculating based on the cord length of the arc just traveled, instead of the sum of a series of points that almost form a smooth arc.  Hence, you see a lower speed on your GPS coming out of a tight hairpin turn than you see on your motorcycle speedometer.

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