GPS Speed and Distance

[Motard660]

I was just thinking today.....i am using a GPS. Although i got the sensors, i am using the GPS unit itself to track speed and distance. How accurate is the information comparing to other bycicle computers (not gps).

[Niner]

I was just thinking today.....i am using a GPS. Although i got the sensors, i am using the GPS unit itself to track speed and distance. How accurate is the information comparing to other bycicle computers (not gps).

 

Very accurate for distance. More than any wheel based censor.

[Minion]

Here are two comprehensive articles comparing GPS device accuracy:

http://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth.html

http://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth_09.html

 

The accuracy varies with the type of course and terrain you're travelling on. The GPS will be generally be less accurate than a calibrated wheel-based sensor.

[Capricorn]

u kidding right Niner? GPS is not accurate to the millimeter, but your speed sensor is based on your accurate assessment of wheel circumference, which would have expected deviations in millimeters, not meters or kilometers as with GPS.

 

btw: what is the stated accuracy of GPS these days? Every so often, they improve the allowable tolerances for commercial use of GPS.

[Minion]

Every so often, they improve the allowable tolerances for commercial use of GPS.

I think, for navigation devices, it's still around 3m. You can get survey GPSs with millimetre-level accuracy, but they need to be standing still for a while to reach that level.

Edited February 14, 2012 by Edman

[Capricorn]

3m? that's pretty impressive. back when it was intro'd, the commercial stuff was limited to an accuracy of something in the 100s of meters. only mil-spec stuff could get anything down under 5m.

[Minion]

3m? that's pretty impressive. back when it was intro'd, the commercial stuff was limited to an accuracy of something in the 100s of meters. only mil-spec stuff could get anything down under 5m.

Before 2000, the civilian GPS signal was intentionally degraded to 100m. This was turned off in that year and, given the prevalence of GPS use by airlines etc., I'd say it's highly unlikely it will ever be turned on again.

 

I may have been a bit incorrect in my previous statement. 3m is a best-case measurement where the GPS can see lots of satellites and can also receive additional signals from WAAS base stations (only in US). The 3m accuracy means that 50% of all reading will fall within a 3m radius circle. About 95% within 6m radius and 98.7% within 7.5m radius.

 

http://www.kowoma.de/en/gps/accuracy.htm

[ShaughnK]

Up till now I would have always said that a GPS will be damm accurate, but last weekend I did a Cane ride with a friend.

He has a Garmin Edge 500 and I use a Bryton 35 . We both hit the Go button as we rode out on a 38 Km course.

When we got back we compared distances travelled and there was a 500m distance difference. ie. mine read 38.7 Km and his 38.2 Km. Yet we had ridden together the whole way.

So not so sure anymore on accuracy.

[Motard660]

Well that was exacly why i bought the GPS. At a stage we were about 6 friends cycling together with cycling computers. After each ride, all of us had differant distances despite that we ride exacly the same route. That distances was from time to time as much as 1.5km. My whole idea with the GPS was to have more accurate speed and distance......seems to me I am back at square one then! :-)

[Minion]

Well that was exacly why i bought the GPS. At a stage we were about 6 friends cycling together with cycling computers. After each ride, all of us had differant distances despite that we ride exacly the same route. That distances was from time to time as much as 1.5km. My whole idea with the GPS was to have more accurate speed and distance......seems to me I am back at square one then! :-)

The problem with a wheel sensor is that it's only as accurate as the value you use for the wheel circumference. My personal experience is that the actual circumference is usually 10mm or so different from the standard values given in the manual. If all of you had correctly set your wheel sizes, you'd probably find the values are very similar.

 

My road riding experience is that my GPS distances are almost always within 50m/100km of the distance from the speed sensor.

[johnw]

@Motard660 use the sensors for maximum accuracy. As mentioned though, you need an accurate wheel circumference measurement. One method is as follows:

1.pump wheels to your usual riding pressure

2. Using a pen mark a spot on your tyre

3. find a flat area, eg garage floor and mark on the floor the starting point where the spot on your tyre is on the floor.

4. make sure you get on to your bike so that there is some weight on the tyres and then roll the wheel forward in a straight line for 2 revolutions. (you can just do 1 revolution but 2 gives a slight bit more accuracy).

6. Then mark the spot on the floor where the tyremark ends up.

7. Use a tape measure to measure the distance in MM and divide by 2 to get an accurate measurement of the wheel circumference and enter this into your bike computer.

[Caerus]

@Motard660 use the sensors for maximum accuracy. As mentioned though, you need an accurate wheel circumference measurement. One method is as follows:

1.pump wheels to your usual riding pressure

2. Using a pen mark a spot on your tyre

3. find a flat area, eg garage floor and mark on the floor the starting point where the spot on your tyre is on the floor.

4. make sure you get on to your bike so that there is some weight on the tyres and then roll the wheel forward in a straight line for 2 revolutions. (you can just do 1 revolution but 2 gives a slight bit more accuracy).

6. Then mark the spot on the floor where the tyremark ends up.

7. Use a tape measure to measure the distance in MM and divide by 2 to get an accurate measurement of the wheel circumference and enter this into your bike computer.

 

You could always just use a piece of string.

[GLuvsMtb]

Most GPS units come with Speed / Cadence sensor (or at least the Garmin units do). I do not have to set my wheel size as the unit measures the distance measured on the GPS and self-calibrates the cadence / speed measurement. My understanding is that this improves the accuracy of the system. Anybody in the know to comment on this?

[Capricorn]

You could always just use a piece of string.

 

you always use math.

[Caerus]

you always use math.

 

I use one of these

[Capricorn]

ooo, pretty beads. so is that what u need the string for? should make a purdy necklace...