How far did you bike today, Bob?
I, like many cyclists, like to have some idea of how far I'm biking. Reasons for this range from curiosity and pure vanity to more or less vague notions of physical fitness.
Although a cyclist can use a variety of tools to answer the question of distance biked, most boil down to variations on a few basic themes.
You can use maps and/or time spend biking to arrive at (usually crude) estimates of distance, or rely on technologies such as odometers and GPS receivers, which, when used properly, will tend to produce fairly accurate estimates.
Use of maps is limited by the fact that paths and the land beneath them change constantly. Maps are generally also two-dimensional, printed on paper, and therefore can not fully express contour details. Add in the curviness of paths biked and problems related to scale, and the challenges become even more pointed.
Estimating based on the amount of time spent riding the bike amounts to educated guesswork at best.
Quality GPS (Global Positioning System) receivers can yield quite accurate estimates by calculating position based on information received from military satellites orbiting the planet. However, a variety of compromising factors can throw everything off. They can be highly accurate, grossly inaccurate, or somewhere in between. The savvy cyclist can make some determination of accuracy by examining GPS receiver (GPSr for short, but that just doesn't look right to me) output and comparing it with known values. Anywhere near home, I can generally squeeze the needed info from them, but it can be hit & miss at times.
Odometers usually work by counting the number of times a bike tire rotates and calculating based on the diameter of the tire. Inaccuracy usually comes by way of tire diameter. Tire diameter isn't the same once a rider plops down on the bike. Also, tire diameter fluctuates along with the amount of air in the tube. There are ways to compensate for these problems, and quality odometers can be as accurate as one would reasonably hope under most normal circumstances.
My preferred approach is a combination of GPSr and odometer measurements. This reflects my geekery as much as anything else. When the new bike comes in, I have in mind to see just how much geeky stuff I can reasonably fit on it. And I'm prepared to budge a bit on the definition of reasonable.