I have installed a new Sigma Sport BC500 computer on my bike. It is has a setting for my exact tire size, which is how it is configured: 26 x 1.9

However, I have carried my Magellan Meridian Gold GPS to compare and there is a discrepancy between the two.

Would you change the bike computer setting to match the GPS results?


Thanks

Simple answer - no. The GPS does not record every twist and turn in your trail, it just has a series of dots connected by straight lines.

Better to roll/ride your bike down a sidewalk at least two tire revolutions and measure the distance with a tape measure. Tires vary a lot in profile, 26x1.9 is not close to being accurate.

Simple answer - no. The GPS does not record every twist and turn in your trail, it just has a series of dots connected by straight lines.


Nope, a GPS is able to record velocity and distance as a function of doppler shift and time, "point to point" distance and velocity is the secondary mode that they operate in. This is one reason GPS is so accurate, it's not just using the signal to update it's position, but due to the doppler effect it which particular satallites signal is "compressed", it instantly knows direction, velocity, and distance changes.

I would be looking at the actual difference though, and if it's small I definitely wouldn't waste the time.

Simple answer - no. The GPS does not record every twist and turn in your trail, it just has a series of dots connected by straight lines.

The comparison I have done is straight line ride on level pavement for up to two miles at a time.

I'm not sure what you mean about the GPS recording in straight lines, dot to dot so to speak. That is not my understanding of it's operation at all. I know that this same GPS taken in my car is more accurate than the speedometer or odometer, based on everything that I have read and it would seem so in practice. It most certainly measures every little movement, even in a small circles. If I walk through my house it indicates both speed and a change of coordinates instantly and in fine increments. In fact, if you do not take a step but simply move the receiver it will indicate change, for example a matter of feet of elevation. I know there is a degree of error, but I would think a good GPS would be the reference when compared to a bike computer.

Nope, a GPS is able to record velocity and distance as a function of doppler shift and time, "point to point" distance and velocity is the secondary mode that they operate in. This is one reason GPS is so accurate, it's not just using the signal to update it's position, but due to the doppler effect it which particular satallites signal is "compressed", it instantly knows direction, velocity, and distance changes.

I would be looking at the actual difference though, and if it's small I definitely wouldn't waste the time.

So, you would use the GPS as the accurate reference, correct?

The difference is around 1/10 of a mile discrepancy, which really adds up.

Twice in the last year I had my GPS on my lap on commercial airline flights when the pilot came on and said our elevation and speed. The elevation was almost the same, but the speed indicated on my GPS was within 3 mph both times. And that's at speeds over 500 mph.

Yes GPS displays instant data (velocity) accurately but records data as a series of points (lat. and long., time). If you are uploading to a computer and comparing distances you will get the distance the software computes from point to point. The bike computer is potentially more accurate for getting the net distance of your ride, you just have to calibrate the tires. For distance I'd trust the bike computer it's "integrating" the data. Some GPS units allow you to change the resolution of the points, are you using the finest gradation?

Computer software for GPS does not account for the extra distance you traverse when climbing, it assumes a smooth earth.

Yes GPS displays instant data (velocity) accurately but records data as a series of points (lat. and long., time).

Then do you concur that calibrating the bike computer odometer to the GPS based on level straight line testing makes sense? I'm not uploading anything, just going by the readouts during the test.

Twice in the last year I had my GPS on my lap on commercial airline flights when the pilot came on and said our elevation and speed. The elevation was almost the same, but the speed indicated on my GPS was within 3 mph both times. And that's at speeds over 500 mph.


elevation will be different because at altitude the airliners fly at "pressure altitudes" to keep everyone on the same page, they all set their altimeters to 29.92" of mercury and this causes them to all be on the "same page". The GPS altitude, aka "true or absolute" altitude would not be the same as what the captian is going to say they are "flying at", often times it's not very far off, but your GPS was probably reading the correct altitude, within a couple feet. The pilot was stating what is on his altimeter, which is not the same.

Almost :D, you'd have to guarantee that the starting point was recorded in the GPS memory. I'd set the starting point as a "waypoint" and at the end of the run ask for a readout of the distance to the waypoint.

After that go to the local HS track (400m) pedal the oval for a while and see which device is closest to tthe true distance.

I have installed a new Sigma Sport BC500 computer on my bike. It is has a setting for my exact tire size, which is how it is configured: 26 x 1.9

However, I have carried my Magellan Meridian Gold GPS to compare and there is a discrepancy between the two.

Would you change the bike computer setting to match the GPS results?


ThanksYou need to do a roll out of your tire/wheel to use to calibrate your computer. I have not found any calibration chart that is accurate for every tire. Actual tire sizes vary too much (see my tire site). You will most likely still have a difference between the comp and GPS because of the different measuring methods.