Originally Posted by
vikay
The Elemnt Bolt is able to get speed using GPS. The sensor is allegedly more accurate because during climbing, GPS doesn't account for elevation. Also, the sensor is useful for when riding indoors on a trainer.
But I honestly think the sensor is less accurate because you have to input the size of your tire circumference. Most people have it wrong. There is an option for "auto" though. I do not know what it does but I'd presume that it'd learn over time and calculates your tire circumference by comparing how many revolutions you've had with the GPS distance.
Well...yes/no, and no. There are a few things going on
A) GPS is quite
accurate but is rather
high-latency (delay from sample time to readout, as well as sample to sample), particularly on low-computing-power products. Which if you're driving steady-state in a car on the highway or interstate is no problem. But a bicycle is seldom in such a steady-state. A speed sensor takes far less compute power (for the same amount of data) so you get better battery life on mobile products, and is much lower latency beside. You need a very good rollout measurement to best GPS--which is why most-all GPS bike computers now will do a GPS-based rollout measurement for you over a mile or so; best of both worlds.
B) Altitude. GPS does, in fact, do altitude, but isn't
that accurate compared to a good/calibrated barometric-altimeter....at least so long as the weather is fairly constant over a ride. This is due to the problems of doing hyper-accurate trigonometry on the surface of a globe. GPS can be so accurate in Lat/Lon because of how far apart satellites can get (180 degrees practically), something they simply cannot do in the vertical because of the Earth under your feet. Hence the preference for GPS-calibrated barometric altimeters