I recently received a GPS for my bicycle, but not made by Garmin. I regularly ride 14.4 miles. The GPS logged it at 13.45 miles. I have plotted the route with Bikely.com and a couple of odometers. All of them are real close to 14.4 miles. I have read an article about things that cause a GPS to vary in accuracy. Has anyone else had the experience of a GPS giving a trip mileage different from that indicated by other quite reliable means?

Yes. Depending on the sensitivity and algorithms in the GPS unit they can drop out (i..e. not receive) under tree cover, valleys and canyons, tunnels, bridges, and in urban canyons. When they drop out they will not record part of the distance you actually traveled.

Thanks for the reply. I ride in what is largely open country. The unit often acquires satellites from inside my house. The GPS sensitivity numbers on its reception are low, which is good and means better accuracy. I have used it mostly in the hours of darkness, so I am not able to watch the display for loss of satellites even though I have a penlight on my helmet and it moves in union with my glance. Nevertheless, I would have thought (or hoped) it might catch up when it acquires satellites after losing them and realize it has covered distance.

Have you thougth about vertical accuracy and distance. Most GPS are not good at measuring vertical changes. Sometimes for higher accuracy they incorporate secondary land based triangulation signal https://en.wikipedia.org/wiki/Differential_GPS . Also your software may not even be using the vertical dimension in calculating distance and thereby cutting off a significant amount of distance depending how many hills you have.

Eric

Eric,

Thanks for the information. My daughter gave me the GPS and she lives in Knoxville. She gave it to me while we were there about ten days ago. I used it there on a ride from the Lovell Road and Pelissippi Parkway area up to Guinn and Oak Ridge Highway. My residence is actually in Idaho, though.

Another idea: Some GPS' have "threshold velocities" below which they will not measure. If you are starting and stopping a lot then the GPS might not record. Try taking a longer, continuous ride in a car and comparing.

The other variable is the reference(s) that make(s) you think that the real distance is 14.4 miles.

I hope you had a nice visit to Knoxville and get the GPS to work. I hope you had good weather. The couple day before Thanksgiving were wonderful. I mostly commute to and from UT.
Eric

This GPS unit is marketed specifically for use on a bike. If I hold the unit in my hand and happen to move my arm quickly, it will register a speed.

As I mentioned, the 14.4 miles for my course is the figure I get from the mapping at Bikely.com and is consistent with readings I get from three odometers used on two different bikes. It is also consistent with what I get when I drive the route in a car.

GPS accuracy is relatively precise and usually is within a few meters. That vraiation is at any instant point so riding over some distance really negates the variations. In other words, you should have almost perfect accuracy.

The three reasons other stated - you may be climbing and descending, which the GPS isn't picking up, toy are losing signal in spots, or the receiver many have an "auto pause" setting for slower speeds that is shutting itslef off, are what comes to mind. Check your GPS manual or go to the manufacturers' web site and see if there's a setting to shut measurements/time at slow speeds.

Eric,

Thanks. I did not get to Knoxville until late Friday evening after Thanksgiving. My wife went several days earlier and enjoyed the nice weather. She says my disposition is better when I ride and said she was taking me to a bike store so I would have a bike to ride when I am in Knoxville. It was Monday afternoon before I was ready to climb onto the new Trek FX 7.2. I ride a road bike here in Idaho, but the Trek is good, too. Tuesday I went for this ride. The weather was not as warm as the previous week, but still warmer than what I had left in Idaho. Wednesday we had to travel back to Idaho.

Thanks for the reply. The 14.4 miles I ride have a few short hills. I think Routeslip figured I climb a total of 245 feet during the ride. My speed slows to about 6 - 7 mph on a couple of the hills. The rest of the time it is around 12 - 14 mph with highest speed of around 18 mph. I am thinking I will contact the manufacturer for suggestions.

I've always had near perfect accuracy on my Garmin handheld units on my bicycle when it comes to distance except if I am off-road in the woods. That includes the Etrex, Legend, Legend-Cx, and Etrex H.

Hills have almost no effect, even if you rode the entire ride at a 5 percent grade, it's still 99.6 percent accurate as if you were riding a straight line. Even riding at a 20 percent grade, it will would be 93.9 percent accurate as compared to a straight line.

To understand more what is happening you're just going to have to do something different and see what happens. If you go a different route is it still off by the same amount? If you go a longer distance is it still off by the same fraction or is it an offset (off by a fixed amount)? Does the difference depend on what route you take?

Generally speaking my Garmin Edge 305 is within about 1 part in 1000 agreement with my Cateye wired odometer.

The other thing to consider, that I don't think I saw this posted. A GPS can really vary in accuracy at the beginning of the ride as it is acquiring satellites. It can sometimes take a bit to settle into a good accuracy.

So if you are the type that turns the GPS on and immediately hits the road, you can introduce some significant variation. I usually Turn on my garmin and let it start acquiring and then go get dressed etc so it has some time to do its acquisition.

-D

Thanks for the suggestion. I do wait until the symbol indicates satellites have been acquired and the odometer scale becomes active. Then I begin to ride. I had not considered waiting a bit longer, yet.

I rode with the GPS unit again today. It registered 13.3 miles this time. I have written to the manufacturer about this problem.

My Garmin GPSMap 60Cx has been pretty much identical to properly configured bike odometers on most of the trips I've taken. Your observed error is way out of the expected normal. Compare on different routes.

You might want to set your GPS to update more often. I'm not familiar with your unit, but my Etrex has a setting that controls how often it takes a position.

In a straight line, a GPS should have the same accuracy as a bike computer, more or less. However, when you are going through a curve, and the GPS takes points, it calculates the distance between points and gives you the length of the chord of each arc rather than the length of the arc itself. This effect is magnified on a curvy course with frequent direction changes, and in a case like that, you would expect your GPS to register a shorter distance than a bike computer would.

Setting the GPS to update more frequently should help mitigate this effect, although I doubt if you can get rid of it entirely.

Rick

I want to express my sincere gratitude to all of you for your responses and helpful information. I sent an e-mail to the manufacturer of my GPS unit and gave them all of the information I could about the discrepancy in readings compared to actual mileage (and readings that actually vary from one another on different days), the conditions under which I use the unit, as well as on-board information about satellites acquired and accuracy figures. They responded promptly and asked me for the software version number. I sent it and thought I would hear from them again by this time, but have not. I will post something when I hear more.

Meanwhile, I am able to get latitude and longitude figures for any point by means of a version of a Google map and enter them into the unit as a way point. That means I can use the unit to locate a point on foot, in my car, or on my bike. It is still useful as an auxiliary to our GPS for our automobile, especially when my wife is traveling with that unit.

To add to Rick's post, I've found another significant source of GPS inaccuracy: the slight side-to-side motion of a bicycle during normal riding.

Look at the track of bike tires through light snow, for example. The slight back-and-forth snaking of the front wheel, and the reduced but still significant motion of the rear wheel, adds at least an additional 1% of actual travel distance for a bicycle as compared to the length of the straight line between two points on the journey. That's distance you have to provide the pedaling energy to traverse, but which isn't measured by most GPS units. That additional distance is not measured accurately by my Garmin E-trex, anyway.

The reported error of the original poster is large, though: about 8%. If the OP is sure about his wheel measurements for his odometers, then I'd agree that the GPS is just regularly losing contact with satellites along the way, because of trees, tunnels, large buildings, or whatever.

A couple of weeks ago I took this GPS unit with me in my car and tested it against my car's odometer over about seven miles. The GPS sat on the dash with a lot of glass around it and still gave a reading about 8 percent low. I sent e-mail to the company as mentioned above. They had always been good about responding, but have fallen silent. The unit is an Atech Professional Speedometer. You will see them on eBay. I think you can get a small Garmin for the same money. (Mine was a gift from my daughter who runs marathons and thought her dad would benefit from a GPS for his bicycle.) If you are tempted to buy one, you might resist the temptation. I think it would still be useful for a long ride with a series of waypoints, but would require outthinking it a bit. I do use it in the car for locating an address and it gets me within a couple hundred feet.

That sounds about right as car odometers are often a bit optimistic when displaying velocity/distance

If you really want to measure the distance, calibrate a bicycle computer and ride the distance or get a paper map with a ruler or use a computer map that measures distance.

Check it on a road with mileposts, or in an area where the roads follow section lines.

I'm more of a runner than biker, and I use the Garmin305 with its SirfStar chip, which is likely similar to the one your unit, if yours is a current-generation model.

I have run with my Garmin on offically marked beach courses, covered marked trails, and road races. I can say with certainty that on these courses, it has been 98-99% accurate to the mile, even after 20 miles, with trivial variations likely due to my own choice of running one one side of the road vs the other. I know this is accurate, as these marked courses have MILE signs posted along the way, and it's got to be more than coincidence for my watch to "beep" nearly synchronously with every single mile marker on 3 different courses (and more races) In fact, it's so reliable, that myself and other experinced runners value it much more than the "official mile markers" used in races, which can be surprisingly misplaced. (If you used these incorrectly placed markers and a regular watch, it would say you'd run a 5 minute mile for mile 1, then an 8 minute mile on mile 2 - while exerting the same energy. the Garmin gets it right every time, with even 6:30min/mile splits.)

I found that the only time it faltered a bit was when I ran on the inner lane of a school track. I think tight curves give it some problems, as it doesn't "sample" points frequently enough to get a perfect read. Still, it was over 95% accurate on the track. (5% is a big deal when you're doing sprints.)

I'd imagine on a bike, where there are much fewer of these turns, the Garmin GPS should be 95+% accurate, if not close to 99% accurate. If your unit is below 93% accurate, I'd suspect you have a malfunctioning unit, or you're not measuring your course correctly.

The only thing that I've noticed that it's completely horrendous with is the elevation map. Completely unreliable and useless. (Fortunately, you can download your workout data online and use the topo maps to get accurate elevation maps, so it doesn't matter.)

I have a Garmin Vista HCx that has a tendancy to drift while stationary according to the track log. Previously had a 60CSx but returned it because it was worse with the drifting. Both have high sensitivity GPS chipsets, but one is SiRF(CSx) & the other Mediatek(HCx). Not likely your problem, but something to be aware of with these units.

Can you do the reverse, ie. get the route coordinates you travelled out of the bike GPS and map them on Google or similar? That would tell you what route your GPS thinks you took. If the route seems to be correct, then you're back to square 1. But if your GPS misses waypoints, you'll see the route cutting corners on map (resulting to shorter distance). Missing waypoints can be due to lack of signal as suggested, or it can be a "feature" to save memory space on the GPS.

Regarding your reference devices, how do you calibrate your bike computer(s)? For a 700c wheel size, a 10mm change in wheel radius means 2-3% difference in circumference. Different tyre or tyre pressure can change radius that much. If you have used bike computer manufacturer's table to get your computer set up for your tyre size, your bike comp may be several % off. In normal use it's not a big deal, at least not for everyone, but if the total difference you see is 8%, it can be significant. If you want to make sure, verify the comp setting by measuring your tyre's true circumference with roll-out method.

As mentioned before, car and motorcycle speedometers are notoriously inaccurate. They should not be used as reference. Prior to GPS, I know a few motorcyclists who installed Sigma bike comps to get more accurate speed and distance readings.

--J