Originally Posted by
pdlamb
Except if you’re riding through a city or suburb, and going around corners. Or you’re on a bicycle ghetto, aka MUP, which has lots of twists and turns. Or your road route has a lot of turns at intersections. Oh, like one of my typical road bike rides.
If you routinely ride on MUPs that twist and turn as much as a mountain bike trail, then you should do like mountain bikers and use a wheel sensor with your GPS.
Originally Posted by pdlamb
Originally Posted by tomato coupe
Multipath errors will not, in general, increase the distance.
Say what? You take a path, throw in a few points way off the true path, and it doesn’t add distance?? OK, perhaps your filtering/post-processing knocks that down – but that filtering is at the cost of accuracy going around corners or curves.
You clearly have no idea what multipath errors are.
Originally Posted by pdlamb
I’ll accept the correction if you can direct me to a specification for a consumer-grade GPS with that figure.
From Garmin:
"GPS drift, is the difference between your actual location and the location recorded by a GPS receiver. Consumer grade GPS receivers (such as the ones Garmin makes) are not 100% accurate, this will usually cause a difference between your actual location and your recorded location. The GPS location accuracy of Garmin fitness devices is around 3 meters (10 feet), 95% of the time. This means, at any given time, your device will save your location within 3 meters of your actual location."
I think they're a little optimistic. You missed the most important part anyway: "
Actually, the correct number is about half of that, at 5m. But, this is for absolute accuracy, which is not the important figure for a bike computer."
Originally Posted by pdlamb
Originally Posted by tomato coupe
Errors in the absolute position come from several sources, including ionospheric effects, timing errors, multipath signals, reference clock drift, ephemeris errors, etc. Luckily, however, the largest contributors to the error budget are slowly varying effects, which means they essentially become common mode errors for closely spaced measurements. In other words, two adjacent positions may be off by 5 m, but they're both off by the same amount in the same direction. That's why your GPS computer does not show you bouncing around ± 5 m every second when you're sitting at a stop light. It might put your position a few meters off the road, or partially into the intersection, but the point-to-point position is relatively stable. As a consequence, the distance calculated between two adjacent points will have an uncertainty much less (about 0.5 m) than the absolute uncertainty of the GPS readings.
As noted above, you can filter out those point to point variations, but that’s at the cost of filtering out accuracy on curves.
There's no filtering going on. The point-to-point scatter is much lower than the (3m or 5m) absolute accuracy of the system because the dominant sources of error in GPS are highly correlated for adjacent readings.
Originally Posted by pdlamb
After I get through calibrating the wheel sensor, I do know the magnitude and sign of the error.
This is the absolute silliest statement in this entire thread. I'll leave it at that ...