# Accuracy: GPS vs Cycle Computer

I rode 2.49 miles just now, both according to my snazzy new GPS watch (Forerunner 205 by Garmin) and according to my cycle computer.

Here is the odd thing: The cycle puter gave a max speed of 15.9 while my GPS gave my max at only 14.4 – quite a discrepancy!

I’m guessing that I may have done something that caused the magnet to pass the sensor too quickly, but I didn’t idle the whole ride, and I didn’t hit too many big bumps, either.

Any guesses? And which would you trust, or neither?

GPS speed is often calculated over a longer time interval than the distance a cycle computer calculated it, so if you only hit a maximum speed for a very brief period, the GPS won’t catch it.

GPSs are good for knowing where you’ve gone and average speeds, but for maximum speeds, it’ll only count speeds that you keep going at for long enough to get a few points.

Joe

“A man who has one watch always knows what time it is; a man who has two is never sure.”

I would trust the cycle computer at lower speeds. Distance should be accurate with the GPS, but speed isn’t. At higher speeds the GPS gets a little more accurate, but there’s still a little bit of discrepancy.

That depends on the terrain, many GPS’s do not correlate climbs and descents into the total distance accuratley

I really do not understand this obsession with gadgets and digits - why does anyone really give a monkey’s **** whether their max speed is 14.something or 15.something, let alone spend loads of cash to measure it ?

Max speed, by definition, is only attained for a few revolutions of the wheel. Average speed is a much better indication of performance - if you really need one.

My only measures of performance are:

1. did I die ?
2. did I have FUN ?

Chris

Average speed over a period of time is useful because you can determine how long it’ll take you to get somewhere. If you’re using a uni for any sort of commuting, this can be a fairly important factor to consider.

Max speed is just novell and fun - let people have their fun

I thought we unicyclists were supposed to be geeks!

Spoken by a guy who has a cycle computer for his Coker… which has been sitting in his garage, somewhere, waiting for him to install it.

I’m not sure I see the point in discussing this, but in the off chance that you care:

Some people like certain things that are different from what other people like. They’re called ‘idiosyncrasies.’ I like to know just how fast I was able to spin the wheel. I also like gadgets. I also like to know why some gadgets give different readings than others. I guess those gadgets are ‘idiosyncratic.’

Furthermore, if I only needed to get an average speed, I would only need to know the elapsed time, by your thinking, since I could just know my distance and calculate from there. But sometimes my route changes, and sometimes my route is so up and down that an average speed reading doesn’t tell me whether I’m doing well for an uphill from one ride to the next.

Finally, I have already spent the cash on the geared uni, so a GPS watch is a relative drop in the bucket…especially since, other than the cost of unicycles, I spend less than \$20 a year getting to work. In fact, I would bet that you spend more getting to work each year than I do, even figuring in the cost of the geared uni AND the GPS watch, which together, over time, will amortize to under \$400 a year.

For me, this kind of thing enhances the enjoyment of unicycling or anything else I do. I love gadgets.

Cathy

The more precisely the position is determined, the less precisely the momentum is known in this instant, and vice versa.
–Heisenberg, uncertainty paper, 1927

makes sense even for this debate cycle computer(velocity measurment/momentum) versus GPS(position)

er… and dont go read the fine print…

Re: Accuracy: GPS vs Cycle Computer

On Wed, 5 Apr 2006 08:20:05 -0500, joemarshall wrote:

>GPS speed is often calculated over a longer time interval than the
>distance a cycle computer calculated it, so if you only hit a maximum
>speed for a very brief period, the GPS won’t catch it.
>
>GPSs are good for knowing where you’ve gone and average speeds, but for
>maximum speeds, it’ll only count speeds that you keep going at for long
>enough to get a few points.

My eTrex Vista measures speed in a different way than my measuring
positions, then calculate the distance, then divide by time. That
would be an inaccurate way of determining speed because the accurary
of the position is insufficient.

I forgot the details but I believe the way that the eTrex determines
speed has something to do with Doppler shift in the GPS signal. Max
speed (for my eTrex, again) is just the highest value of the thus
derived speed. But indeed I have found it’s not very reliable. I would
trust a cycle computer better, especially if I knew I hadn’t idled or
so.

Distance accuracy is another matter. Distance is calculated by the GPS
from point to point. If the points are spaced too far away, you may
have swerved in between so then distance is too low. But if there is a
‘spurious’ point, maybe due to bad reception, then the distance is too
high. Again, I would trust a cycle computer better.

I would ‘trust’ neither. I would use either or both as indicators only.

The cycle computer relies on the input of the circumference of the wheel then does its calculations based on its internal clock which will mean its accuracy on determining the velocity of the wheel will depend on those factors (and more?).

The GPS will rely on a good signal and then uses averaging algorithms to determine position and some more algorithms to determine velocity of the GPS unit and so the accuracy will depend on those factors (and more?).

With all the wheel snaking, loss of traction, UPD’s etc. the input to the cycle computer is not going to be accurate. With all the flailing arms & body, tree cover, UPD’s etc. the input to the GPS is not going to be accurate.

So neither will be ‘accurate’ but with some common sense judgements on the readings obtained and an average between them you would probably get the best indicator.

In the end if you’re consistent with what you use you’ll get a good idea of how well or otherwise you are doing in relation to previous rides. I intend to fix my cycle computers mount and fit it to my coker to use in conjunction with my GPS so I can see how my training is going, and ‘just for fun’

There is always the off chance that I might care, but I don’t.

I probably spend more in a week getting to work than you do in a year - I must try to get work closer to home.

If measuring your riding parameters on each trip really enhances your day, well fine and dandy. If I look at a map and estimate a trip at 15 miles, I will allow roughly 1.5hrs to do the ride (plus a bit for a beer/food/looking at flowers & views/listening to the birds/chatting to fellow hoomin beans) - if I get there earlier, great, if I get there later, I still had a great ride.

I enjoy measuring time by the passing of the seasons rather than down to one decimal point of the m/h, km/h, m/sec, but whatever lights your wick …

I’m probably out on a limb here, but you can keep your gadgets and digits, give me beer and birdsong any day.

However you choose to do it, one-wheeled, two-wheeled, GPS, phases of the moon, have immense fun.

Chris

I have the e-Trex Vista CX. On a recent hilly Coker ride my max speed was 20+MPH, I know for a fact I never got that fast. I did lose signal once due to terrain and/or tree cover. I think most of the time the max speed is pretty accurate but do get the occasional anomaly.

The cycle computer may not be 100% accurate either. On my bikes I usually measure the wheel circumference, which gets close. There are mile markers on the local cycling trail, I do 2-3 mile runs and adjust the cycle computer until it matches the mile markers.

But bikes don’t wobble, most of the time anyway.

I am curious on “ground speed” vs. “wheel speed” for the GPS in hilly conditions. When I ski I regularly hit a max speed of around 55 MPH, of course this is going downhill. Am I really going faster? If the GPS is calculating some type of “horizontal” speed, my “diagonal” speed would be greater. Any known data on this?

I think that depends on the GPS unit.

A decent GPS knows your verticle from sea level within 1m give or take. As such, if you’re going downhill, the GPS should always know that.

Smart software should be able to plot you in 3D, not just 2D… but it all comes down to the brains inside the things and there’s no way to really know without asking the manufacturer.

Re: Accuracy: GPS vs Cycle Computer

On Fri, 7 Apr 2006 09:44:06 -0500, bmemike wrote:

>A decent GPS knows your verticle from sea level within 1m give or take.

That’s too much honour for an ordinary decent GPS I think, at least
without WAAS. Usually, horizontal accuracy doesn’t get any better than
5 m, and vertical accuracy is worse than horizontal.