Originally Posted by mac
Ah, but I have never had a bad point on my GPS. I upload my tracks to my PC and plot them in both National Geographic's TOPO! series and Garmin's Maps & Waypoints. I can see my speed increasing as I get further down the mountain, peaking, then slowing down as I bottom out. And I've done that route multiple times so I'm very confident I have an accurate number.
I use Garmin's GPSMap 60CS and have never had a problem with it, even in rain.
And, FWIW, I also reduced the circumference measurement on my cyclocomputer. I sat on my bike and rolled forward as my old man marked off a full rotation, but, according to my GPS, it was still a bit too large. So I dropped it about 10+mm and came out with the exact same top speed as the GPS.
No problem. Just noting that Garmin standard GPS accuracy is 15m on average. With WAAS (which the GPS60 has) it is 3m. If you are traveling at 50mph you cover 22.3m in 1 sec. A 1 mph error can be achieved with a .5m measurement error. So you may not have any bad (i.e. wrong) readings but if the distance between two readings is off by 3m, you could have a 6mph error if the samples are taken 1sec apart (which is the sample rate of the GPS50CS). Some GPS systems also may average readings, which reduces the error, but also smooths out any sudden speed changes you may really have. GPS with WAAS is very accurate for measuring longer distances or steady state speed, but not so good for instantaneous speed due to measurement accuracy for individual points that are only 1 sec. apart.
Al