Originally Posted by gpsblake
I've been using a GPS for years now. I have found they are very accurate most of the time and never need calibrating. I can switch the GPS to any bicycle I want without having to do a thing to it. Today's ride is an example. According to my GPS, I rode 23.36 miles. I then went to my Delorme mapping software, plotting out the exact route I rode today and used their software to determine the miles. 23.27 miles the software determined what I rode. That's pretty darn accurate.
Now in a downtown area with large skyscrapers or in a dense forest environment, the GPS probably isn't going to be as accurate as a probably calibrated cyclometer. But the key is PROPERLY calibrated cyclometer, meaning you take a ruler to measure your diameter of your tire rotation and use that figure, not the generic figure that the instructions tell you.
But there isn't a rule that says you can't use both? With a GPS you can download your rides to a map or use the GPS itself as a map. Cyclometers are very inexpensive and I used both on my tour in 2005.
Sure, in a flat area, with few and low hills, and no forests or tall buildings, the GPS is fairly accurate. Your close match was likely luck more than actual precision, though errors could average out over longer distances to some extent.
The cyclometer, on the other hand, has almost no sources of error if calibrated properly (which is easy to do, if the unit allows detailed settings). Just ride a known distance (straight street/road measured on a detailed map, or in a GIS) and adjust the distance per revolution setting to produce that distance.