Accuracy question.....MPH

[+hkps1]

What is the accuracy of the speedometer/'MPH function on a GPS.

 

I realize that signal strength would affect that function. But given a good lock what would the variance be on average.

[Vlad]

the popular opinion is that the gps is accurate to roughly .1 mph. there is a time lag so gps is not great for instantaneous readings, but it is very accurate with more or less steady-state speeds.

[+hkps1]

the popular opinion is that the gps is accurate to roughly .1 mph. there is a time lag so gps is not great for instantaneous readings, but it is very accurate with more or less steady-state speeds.

 

Thanks for the information.

 

And does your screeen name come from the title character of his 1897 novel Dracula based on an actual human being, or should we say inhuman being?

[planewood]

the popular opinion is that the gps is accurate to roughly .1 mph. there is a time lag so gps is not great for instantaneous readings, but it is very accurate with more or less steady-state speeds.

 

Thanks for the information.

 

And does your screeen name come from the title character of his 1897 novel Dracula based on an actual human being, or should we say inhuman being?

 

I've compared my 60cx by measuring time between mile markers on the freeway and it is more accurate than either of my two vehicles. Within 1 MPH, or less, of the actual MPH. Been looking for a 5 mile straight stretch so I can get a better test.

 

ps - 1 mile in 45 seconds = 80 MPH.

Edited February 28, 2007 by planewood

[+Sputnik 57]

Because the position accuracy of a GPS receiver is too low to measure speed as distance divided by time, the GPS unit actually uses Dopler shift to measure speed. It is very accurate (as noted, within .1 mph), but as noted, reports speed only once per second, and not continuously.

[Suscrofa]

Because the position accuracy of a GPS receiver is too low to measure speed as distance divided by time, the GPS unit actually uses Dopler shift to measure speed. It is very accurate (as noted, within .1 mph), but as noted, reports speed only once per second, and not continuously.

 

Are you sure of this ?? Any links to document it. I personally thought it did compute the speed based on displacmeent per time elapsed. Anyway it is way more accurate than vehicule speedometer and nice to calibrate the said speedometer.

[segler999]

No, about a year ago, there were a lot of discussions about the speed calculation algorithm used in gpsr. Garmin, for sure, and probably the others, too, calculate speeds using Doppler. They do not use dx/dt. Somebody posted an email reply from Garmin tech support about this.

 

You can confirm this by studying your tracks. Many times I have recorded a track from either a walking or driving or windsurfing session. I download the track into software that has the capability to display the track as a text file, such as Gartrip.

 

If you compare the numbers in the speed column with dx/dt calculations you can make from the numbers in the distance and time columns, you get different results.

[+Sputnik 57]

Because the position accuracy of a GPS receiver is too low to measure speed as distance divided by time, the GPS unit actually uses Dopler shift to measure speed. It is very accurate (as noted, within .1 mph), but as noted, reports speed only once per second, and not continuously.

 

Are you sure of this ?? Any links to document it. I personally thought it did compute the speed based on displacmeent per time elapsed. Anyway it is way more accurate than vehicule speedometer and nice to calibrate the said speedometer.

Here is one source, but heck, its the Internet. Who knows?

[+lals cache]

Newbee question time....Why would I want to calculate my speed w/ my GPS?

[+Sputnik 57]

About the only "real" use I have for it is when I'm in my Jeep. I have oversized tires, and I haven't changed out the spedo gear in the transmission, so my speedometer reports my speed at about 90% of my true speed. I know that, because when my speedometer says I'm going 60, my GPS says I'm going 66.

 

It is also pretty cool when I am flying, but just a matter of curiousity. There is a pretty big variation in speed depending upon winds aloft (the GPS is reporting ground speed, and not air speed). Most commercial airliners fly at about 500 mph at altitude, but travel much faster with a good tail wind.

[segler999]

Why speed?

 

I take an old Legend in a waterproof bag with me when I windsurf. For race training (alone) it is very helpful to have the gps report the max speed and distance covered. I archive all my race training sessions by downloading the tracks to a pc. I can then plot them on maps, etc.

 

Out on the water, when you are trying to tune your gear and stance for maximum speed, it is very nice to look over and read the speed off your gps.

 

Speed, max speed, and distance covered (on the Trip page) are also very useful for biking, kayaking, running, orienteering, adventure racing, hang gliding, jetskiing, and fishing, to mention a few.

[+Prime Suspect]

Newbee question time....Why would I want to calculate my speed w/ my GPS?

Because your car's speedometer is lying to you. Most are set to read a few mphs above what you're really traveling.

[+PFF]

The speedometer in my wife's 2002 Buick LeSaber reads 2 MPH faster than the vehicle is going. I have not told her, and she never gets a ticket! [Grin] There is a slight variance, over time, as tires wear, but it is not significant.

 

Oddly, in my 2000, 2002, and 2006 Impala's, the speedometer was dead on--as checked with the GPS and over a three-mile measured course. Ditto for my current vehicle, a 2006 LeCross.

 

Off-topic for a moment.....Tne thing which never has varied between vehicles has been the accuracy of the fuel gage. Whether it was a Ford, Chevy, Buick, or even my two Checkers, tests showed that half-full in the tank is exactly midway between 1/2 and 3/4 on the guage.

 

I suspect it has to do with the relationship between volts, current, and resistance, which is what the guage sending unit measures. (There is a squared number in formula.) This could be fixed by re-lettering the guage. But I suppose designers feel it wouldn't look good to consumers. The engineers probably chuckle whenever someone observes that the "bottom half" of the tank burns so much faster than the "top half".

 

-Paul-

[Hertzog]

Newbee question time....Why would I want to calculate my speed w/ my GPS?

It was useful to me on a recent trip to Canada. I didn't have to worry about the mental conversions to km and km/hr. Just turned on the GPSr, set it for metric and used the speed readout until I was confortable with the metric values.

[planewood]

Take a look at this video.

 

http://www.dailymotion.com/video/x157l2_bu...on-at-top-speed

[+TeamRJJO]

The GPS System Spec calls for 0.1 m/s accurracy with 95% certainty.

 

Peace,

Ron (TeamRJJO)

[+HaLiJuSaPa]

I have used the GPS to calibrate speedometers in our cars (Saturn L-series and Kia Sephia). It shows that the speedometer in the Saturn is "dead on" but that the one in the Kia is about 4mph too high (i.e. if it reads 65mph I'm really going 61).

 

I love to use it to see how fast I run, walk, bike, rollerblade, etc. I once took it with me on a rollercoaster to see how fast it was going.

[+piper28]

Newbee question time....Why would I want to calculate my speed w/ my GPS?

 

Because they don't generally let you on the bridge of the ship to check their speed gauges .

 

(Seriously, I'll set it to nautical miles when I'm on a cruise ship and use it to keep track of how far to go to destinations. You can often tell when the cruise ship isn't going to make it on time that way.)

[+gpsblake]

I've found the speedometer accuracy in a GPS unit to be very good except in very dense areas. If I am bicycling in a dense forest, the speed jumps all over the place and I've even had max speeds at 90 mph on my bicycle. But even with that, the overall average MPH seems to be really accurate no matter the circumstances. I guess it averages itself out over the long run.

 

One more thing. The quicker you are going, the more accurate and stable the speedometer is. To prove it, try walking at a steady speed with some tree cover and you'll see it constantly bouncing around.