How accurate are GPS readings?

[DancetheDervish]

So my friends and I have been taking gps reads on every hill via garmens. (Forerunner 201, Nuvi, and forerunner 405). And they all differ slightly from eachother. How can you be sure of how accurate they are? Would that mean getting a radar gun or? What are your thoughts?

[Concrete Slave]

I play baseball and I own a Juggs radar gun and testing that against my garmin gps, the max speed shown by both were the exact same on 8 of 10 downhill tests.

[DancetheDervish]

I play baseball and I own a Juggs radar gun and testing that against my garmin gps, the max speed shown by both were the exact same on 8 of 10 downhill tests.

What model Garmin do you use?

[lollygag]

xactly.. model does matter.. it has to update once a second or faster.

[fullycustom88]

ive got a forerunner too, and im fairly confident in its ability to tell me my speed simply because... F*CKING SATELLITES BRO!

no, but in all seriousness, these things can determine where you are on earth down to +/- 0.5 meters from a satellite traveling hundreds, maybe thousands of miles per hour around us in space. thats accurate enough for me

[DancetheDervish]

yeah, I hear ya. thanks guys!

[Old Guy]

They differ slightly because your speed isn't going to be the same; there are too many variables - your weight, wheel durometer and diameter, tuck efficiency, etc. That said, GPS does have limitations that have been discussed extensively () in another thread. If you think of the hill that you're riding as a right triangle then you're riding the hypoteneuse, which is longer than the base, but GPS, being designed to regard the earth as an even surface, is measuring your speed relative to the base. That means that you traveled a longer distance than GPS realizes in the time for which it has calculated your speed, so it shows you a slightly slower max speed than you actually achieved.

[couch_i]

Nuvi is a Garmin Car GPS right? I have a Garmin and it's pretty much spot on with my cars speedometer. Never used it for longboarding though. I'm getting ideas though

[Concrete Slave]

My model is a Nuvi whichever version came out in 2009. I don't have my car at the moment or else I would look at the exact model number and what not.

[Clemson_Fan]

ive got a forerunner too, and im fairly confident in its ability to tell me my speed simply because... F*CKING SATELLITES BRO!

no, but in all seriousness, these things can determine where you are on earth down to +/- 0.5 meters from a satellite traveling hundreds, maybe thousands of miles per hour around us in space. thats accurate enough for me

Actually Relative to you GPS satellites are stationary. GPS speed accuracy breaks down pretty quickly.

[DancetheDervish]

They differ slightly because your speed isn't going to be the same; there are too many variables - your weight, wheel durometer and diameter, tuck efficiency, etc. That said, GPS does have limitations that have been discussed extensively () in another thread. If you think of the hill that you're riding as a right triangle then you're riding the hypoteneuse, which is longer than the base, but GPS, being designed to regard the earth as an even surface, is measuring your speed relative to the base. That means that you traveled a longer distance than GPS realizes in the time for which it has calculated your speed, so it shows you a slightly slower max speed than you actually achieved.

Wow very, very helpful. What is the most accurate way to judge speed then?

[Concrete Slave]

Wow very, very helpful. What is the most accurate way to judge speed then?

You could use one of the many physics formulas out there. But you would need Time, Distance, etc. So stop watch and other materials would be necessary. But probably the most accurate.

[fullycustom88]

You could use one of the many physics formulas out there. But you would need Time, Distance, etc. So stop watch and other materials would be necessary. But probably the most accurate.

my gps gives an elevation reading, so i would assume it takes the grade of the road into account when calculating ur speed

[Clemson_Fan]

Most accurate would be to set up two stations and measure the time it takes to get between the two. or the wheel spinning method is second best

You could use one of the many physics formulas out there. But you would need Time, Distance, etc. So stop watch and other materials would be necessary. But probably the most accurate.

[fullycustom88]

Actually Relative to you GPS satellites are stationary. GPS speed accuracy breaks down pretty quickly.

yea... stationary relative to me. but still, that suckers hauling ass through space

[Old Guy]

Wow very, very helpful. What is the most accurate way to judge speed then?

IMO, in spite of its limitations, GPS is the best method of affordable, durable speed measurement for skateboarding.

[Old Guy]

Actually Relative to you GPS satellites are stationary. GPS speed accuracy breaks down pretty quickly.

No, GPS satellites are not in geosynchronous orbit. I'm not sure how to address your second statement; on what do you base it?

my gps gives an elevation reading, so i would assume it takes the grade of the road into account when calculating ur speed

No, it doesn't; that's not what it's designed to do. A sophisticated system could do that, but it would be too large and too expensive to be a practical skateboard speedometer. GPS wasn't designed to measure elevation; it's a navigation system. Consequently, GPS elevation measurements can be up to three times as inaccurate as position measurements. If the estimated position error that your unit displays is ten meters, then the elevation error could be thirty meters. In spite of that, GPS has revolutionized elevation measurements because sophisticated systems can render position readings with an accuracy of less than one centimeter. Three times that is still really accurate for an elevation reading. Unfortunately, to render that accuracy in real time requires a system that you almost certainly can't afford, and you definitely couldn't carry.

[DancetheDervish]

IMO, in spite of its limitations, GPS is the best method of affordable, durable speed measurement for skateboarding.

What would you say the Margin of Error is on the readings? I clocked myself at 40.9 yesterday with a garmin Nuvi on pedestrian mode, is it safe to say i hit 40?

[NC longboarder]

yes. the gps said alittle over give it to yourself that you did

[DancetheDervish]

yes. the gps said alittle over give it to yourself that you did

Wooo!!!