Speed 19ft Accuracy

[+unicyclist]

My question is, is the speed as accurate? The range is usually 19ft. I try to see how fast i can go on my scooter and unicycle but somtimes its outragous. Is this known to be a problem? Thanks for replies

[+Team Cotati]

My question is, is the speed as accurate? The range is usually 19ft. I try to see how fast i can go on my scooter and unicycle but somtimes its outragous. Is this known to be a problem? Thanks for replies

Yes, the speed's range seems quite accurate.

[The_Brownies]

very accurate....

[+briansnat]

When my GPS is on the dashboard it usually matches or is within 1 mph of what the speedometer is showing.

Edited December 10, 2005 by briansnat

[+Isonzo Karst]

oh yes, especially the part where it tells you your max speed was 89 mph.. ya gotta believe it

[+unicyclist]

it shows me going 20mph but the closest ive gotten to that is 17mph with major wind. So that 20 is like yea right. Sure looks good when you brag to your friends.

[+Ed & Julie]

I drive a (police) patrol car at work with a calibrated speedo.

When I check the car's speed vs. the GPS, they are usually right on.

 

Ed

Edited December 10, 2005 by The Badge & the Butterfly

[+Alan2]

Is the accuracy the same when you drive at lower speeds around let's say 15-20mph?

[+Kai Team]

Speed is calculated as distance over time. If you're positional error is high and shifts dramatically and you're only measuring for a short distance, the calculation could be unrealistically high (or low). How fast you're moving is relevant only to the extent that it determines how far you move between calculations (in this sense, slow speeds would be less accurate than faster speeds, although I doubt it's a meaningful difference in most cases).

 

A theorectical example: you travel 10 feet from south to north. Your error was 10 feet to the south when you started, and 10 feet to the north when you finished. The GPSr calculates that you moved 30 feet when you've really traveled just 10 feet. Your calculated speed is therefore 3x actual.

 

This isn't likely in most situations. Even a 20 ft positional error is negligible if you're measuring, say, the speed of a car on a highway.

 

PS - it sometimes takes a second or two for your GPSr to get all the information it needs to accurately determine changes in position, and therefore distance, and therefore speed. So measurements when you first start moving are generally less accurate.

Edited December 10, 2005 by Kai Team

[+Alan2]

Hmmm. A 20 foot positional error even at 60mph (88 feet per second) is significant. I wonder if the program corrects for error by averaging over a number of seconds rather than taking instantaneous calculations.

Edited December 10, 2005 by Alan2

[+BlueDeuce]

I've checked four GPS's in three cars and there are usually 2-3 mph lower then the car's speed-o-meter.

 

I'll let other make the court battles before I trust'em

[+Kai Team]

Hmmm. A 20 foot positional error even at 60mph (88 feet per second) is significant. I wonder if the program corrects for error by averaging over a number of seconds rather than taking instantaneous calculations.

 

I suspect so, which may be why it takes a couple of seconds to get an accurate speed when you first start moving (based on my experience with two Garmins, where is speed is relevant because I have it set so that the built in electronic compass takes over below a certain speed - i.e. I've spent a fair amount of time looking at the speed indicator to know whether I need to hold the unit level!).

Edited December 10, 2005 by Kai Team

[+Kai Team]

I've checked four GPS's in three cars and there are usually 2-3 mph lower then the car's speed-o-meter.

 

I'll let other make the court battles before I trust'em

That's been my experience as well. Because the error is systematic (always in the same direction), I suspect this is deliberate on the part of the car manufacturers (from a liability perspective, it's better to have you going a little slower than you think rather than a little faster) and trust the GPSr over the car's speedometer.

 

However, I wouldn't want to argue it in court, given my experience explaining GPS technology to the uninitiated (e.g. flight attendants)!

[+Ambrosia]

Talking about court, I've mentioned this before....my husband got out of a speeding ticket because he printed out his gps history and showed it to the judge. It was a big difference in what the policeman had said (and my husband looked at his spedometer at the time, and the gps was within 1 mile of his spedometer).

[+Isonzo Karst]

We have our GPSr's handlebar mounted on the bikes. When biking forested trails the Garmin (which reports max speed) will report some amazing speed bursts! Of course, the track will show some bizarre leaps out into the woods as well. In the car where usually you have an open sky, I'd expect the speeds reported to be pretty good.

[+HaLiJuSaPa]

In my car vs. the speedometer the GPS has proven quite accurate. As an ultimate test, I had the cruise control set at 65 and it never read below 64 or above 66.

 

So speed is quite accurate. Now altitude on the other hand....

[+sbell111]

My experience has been that they are very accurate. In my old Oldsmobeast, I had a digital speedometer. It always matched (within a few tenths) the report of my GPSr. When I bought my old WJ, it was off. Shortly after I bought the Jeep, I upgraded the tires to the suggested 'optional' size. Once I made that change, speedo matched. With my new WJ, it is right on.

 

From my experience (not counting tunnels), the GPSr's comuted speed is good enough to use on the road.

[+D@nim@l]

I saw this in the "Strange But True" column by Rich and Bill Sones

 

Hippest car toys

Q. What’s the hippest, tekkiest device on your car?

 

<Irrelavent info on throttle by wire deleted>

 

"As for things that could be on cars, GPS (global positioning system)

receivers can give a car’s speed over ground to an accuracy of a few centimeters

per second, based on radio signals received from a constellation

of satellites. Since a car’s tires deform when the car accelerates and decelerates,

the GPS measurement is actually more accurate than the wheel speeds

at measuring vehicle speed. The tiny difference between these speeds is

known as “tire slip” and is a strong function of infl ation pressure, detectable

via GPS to an accuracy on the order of a psi or 2. “As my mom put

it, ‘Do you mean to tell me that satellites orbiting the earth can measure

my car’s speed better than sensors on the wheels?’ Exactly.”

[+GPSOkie]

My GPSr report faster slower speeds than my vehicle speedometer. The faster I go the larger the gap is between the GPS and my speedometer. At 65 on my vehicle, my GPS reports 61mph. I did a time test using mile markers and the GPS is correct. I have non-standard size tires so that may be causing the difference.

[+sbell111]

It should be noted that many cars have speedometers that are just plain wrong.

[+Sputnik 57]

Actually, the GPSr uses doppler shift to measure your speed, not distance over time. As a result, the speed shown is highly accurate, even at slow speeds. I understand that they are accurate to plus or minus .1 mph, but I can't recall the source of that statistic.

 

Car speedos are notoriously inaccurate. Most often, they report a higher speed than you are actually traveling. Car manufacturers can justify this by saying that they don't want to be liable for your getting a speeding ticket. Since the speedometer and odometer are linked, it has the added advantage of causing your warranty to expire more quickly.

 

Car speedometers also vary with tire size and inflation. If you put oversized tires on your SUV, and don't change out the speedometer gear, your reported speed may be 10% lower than your actual speed. Be careful out there.

[+Team Cotati]

I am very fortunate. I lucked out and got a Merican made car that seems to have a very accurate speedometer/odometer. It and my highly accurate Magellan GPSr are usually within 0.5 mph of each other. I would be very disapointed to learn that Buicks are manufactured by Magellan. ;-)

[+Anonymous']

It's very accurate. I think it's really cool to watch how fast you're going.

[MagicTogether]

Most vehicles use a gear driven sensor on the output end of the transmission to send a VSS (Vehicle Speed Signal) to either the onboard computer or to drive a mechanical cable. If the gear is the wrong size (usually described as number of teeth on the gear) then the rotation speed of the output will be inaccurate. The inaccuracy is linear and can be expressed as a percentage error. So, if the error is ten percent, then at 60 MPH the indicated error will be 6 MPH. At lower speeds the error is less obvious, for example at 20 MPH the indicated error will be only 2 MPH.

 

All this by way of explaining why you see a greater apparent error when comparing the GPS to the speedometer at higher speeds.

 

Whew...

 

MagicTogether - Alaska

[Really Old Guy]

Hi, I'm another "just recently got into this sport" person. May I ask, doesn't the cost of the GPS unit usually reflect the accuracy? Also, what do you guys recommend as one's second purchase of a GPS unit? I have a Quest, thinking I could use it for the car and on the trails. Now i don't think that dragging it around in the dust and dirt is what I really want to use it for. I've been looking at the Garmin Vista C. What do ya think?

[+Sputnik 57]

Hi, I'm another "just recently got into this sport" person. May I ask, doesn't the cost of the GPS unit usually reflect the accuracy? Also, what do you guys recommend as one's second purchase of a GPS unit? I have a Quest, thinking I could use it for the car and on the trails. Now i don't think that dragging it around in the dust and dirt is what I really want to use it for. I've been looking at the Garmin Vista C. What do ya think? 

Since this is really a new topic, you should probably start a new thread, maybe in the units and software forum, but since you asked . . .

 

Cost and accuracy are not related. Virtually any 12 channel WAAS-enabled GPS will be as accurate as any other under clear skies. They will all get you within about 20 feet of the cache. The rest is bells and whistles.

 

Some newer units seem to perform better under tree cover. For example, Briansnat reports that the Legend C outperforms the 60C under trees, and the Legend C is cheaper (but newer).

 

My directionally challenged daughter will be getting a Quest for Christmas. I couldn't resist voice prompts for the now heavily dicounted price, but it will be used almost exclusively in her car (unless her husband starts caching with it).

 

It would be hard to go wrong with the Legend C. Its a great unit.

Edited December 11, 2005 by Sputnik 57

[+unicyclist]

my thinking is that it would jump ahead 19 feet and then go back to accurate, 19ft in a second on a razor scooter is quite impressive

[+HaLiJuSaPa]

I know from mile markers that the speedometer on my other car is 5mph off and the GPS proved it.

 

Another thing that was cool, I took it while having my 2-year old girl in a stroller and learned that I stroll her at an average of 2.8 mph