# how accurate is a GPS for speed

i finally broke down and bought a GPS. put in the rob roy yesterday and went for a spin. i was cruising at betwen 4.5 and 5 mph, according to the GPS. top speed was 5.7 mph and leasurely paddling was around 4 mph. how accurate are these numbers?

how good

GPS readings are fairly accurate. I average 'em out for a distance and double check with some actual speed calculations.

If a kayak leaves Chicago at 0900…

steve

Very accurate
Keep in mind GPS speed is over the ground, you might be paddling at 6 mph but if you have a 2 mph current against you the GPS will show 4 mph.

GPS Speed
The cruising speed seems high and the top speed seems low to me. I estimate my cruising speed to be around 4 mph and my top speed is 7 mph using my GPS in my Pygmy Osprey HP.

Accurate - always?
I am not convinced of their accuracy if measuring relatively short bursts of maximum speed. There will be errors in position, compounded by a sampling frequency on the order of about a second (see your GPS manual). Furthermore the error in position depends on your location and geographical/topological factors.

Over a longer time period, like minutes, I suspect these errors average out and reasonable accuracy is gained.

But this is all layman’s babble - Can anyone tune in with some hard facts about accuracy? I imagine this thread has been repeated earlier.

Coffee

maybe -
I don’t have one, but a paddling friend does. I think you’re on the right track about it being more accurate over a longer time period than a shorter one. I remember him being surprised one night that it knew where we were (in the middle of nowhere) to an accuracy of 17’. If the “norm” is a longer distance than that, then how can it know exactly how fast you’re going in a very short amount of time?

I don’t know much about them either, just curious.

Accuracy
Just remain still in one position and check the speed in your GPS, if it’s zero it’s accurate.

rob roy vs. osprey
your top speed is going to be higher. that’s just a given when you figure in waterline length, beam, etc. the cruising speed probably was a bit high. it was probably more like a good workout speed. it was also on flat water with NO wind. but even paddling upstream on a slow-moving river i was nearing 4 mph for an indefinite time. wasn’t putting in any huge effort or trying real hard. i was really trying to keep an even pace just to get some numbers that will be accurate for me. not looking to beef them up for testosterone’s state.

mine reads within 3 MPH of
of my trucks speedo. However the sampling/short burst of speed makes sense. I noticed a delay of nearly a second delay reading on rivers with fast runs.

They are right on.
We have checked ours many times with the car speedometer.

Cheers,

JackL

Hope that speedo was checked
Not uncommon for them to read a couple mph optimistic at highway speeds.

Mike

depends
it depends on how many satellites it is tracking and whether WAS is turned on or not. However, with the newer units like the Garmin 76c the processor is very very fast and computes changes in speed almost instantaneously, so it is quite a bit more accurate that a flow meter type readout. Do remember it is speed over ground one is measuring though does not account for current, wind, waves, etc.

Depends part II
I feel that GPS accuracy is pretty darned good, but your max speed can be easily thrown off by a momentary signal lock loss (trees, hills, cliffs etc). How else do you explain me maxing out at 78 mph in a QCC500x?

Jim

Max speed…
The max speed can be anything.

One day I was paddling with a nubie and she said her GPS said that she hit over 8 kts. I told her the only way that would happen is if she threw it at me…

Drive your Chevette, watch the speedometer, and the GPS. You will see superb accuracy. Doh!

Accuracy of speed vs. position
Don’t get these confused. GPS positionaly acuracy is affected by several things - including accuracy of the quartz clock in the receiver versus the atomic clocks in the satellites, geometry of the satellites being tracked, and atmospheric conditions, among other things. Having a WAAS enabled GPS (which is receiving the corrections) dramatically increases the positional acuracy.

Generally speaking, velocity (speed) is much more accurate than positional accuracy BUT, it only works when you are underway. The faster you go the more acurate it will be. This is because speed is calculated by measuring the distance to the satellites between successive fixes on the GPS. If you have a GPS unit with dedicated channels that can track all satellite in view it will be better than an older unit with a few channels that uses techniques such as “fast tracking” (these are not too common anymore). In either case, if I were to set up an experiment to accurately measure speed I would make sure the receiver has been tracking satellites for about 15 minutes (fresh almanac), make sure there were no atmospheric anomalies or significant predicted satellite maintenance scheduled (check with the USCG NAVCEN ahead of time http://www.navcen.uscg.gov/ADO/GpsActiveNanu.asp ), make sure I had good reception (not usually a problem over water) and then, make sure I was moving. If you didn’t do any of this, except the moving part, on most days, with reasonable satellite reception, you could expect an inexpensive, hand-held, comsumer grade GPS to very accurately measure you speed.

~wetzool

Two words.
Holy shit!

Mach 1
My GPS once told me I achieved a top speed of 760mph while driving my truck.

Ken

I don’t doubt it …
When conducting a training once I had one display coordinates in Mississippi when I was actually near Albany, NY. Trust, but verify!

~wetzool