12:47 p.m. on September 4, 2006 (EDT)
There are pedometers and then there are pedometers. Over the years, I have tried a variety of pedometers and talked with lots of people about their experiences with pedometers. The basic conclusion, in short, is that they are not very accurate. The last one I got is supposed to be "the best", a Sportline. But it, like most others, just counts steps and uses the step length you program into it to determine the distance. You can use an automated calibration, or you can set the step you measure by walking around a running track (running tracks are 440 yards in most of the US, 1/4 mile). It also has a "walking" and a "running" calibration.
But your actual pace varies with all sorts of things - slope, roughness of the trail, walking briskly or slowly, fatigue, you name it. I sometimes find a factor of two difference between the distance it give me going up a hill and returning down the same trail. This for $20 or $30 for a really top quality one. My advice? Don't waste your money, unless you just want a count of steps, in which case the ones that come in a cereal box will work just as well (yes, literally, some health food oriented cereal companies give them away.)
Recently, several of the heart rate monitor companies that cater to runners have added foot pods that use an accelerometer instead of a simple pendulum. These measure the acceleration and deceleration of your foot as you push off and land. So they are a lot more accurate, typically within 5 or 10 percent when calibrated to your walking/running style. But you have to buy the whole HRM setup, so count on a few hundred bucks. Unless you get really serious about training, it's probably not worth it (my wife and I used to race bicycles at National level, and the US Cycling Federation had essentially all Cat 1 and 2 racers using HRMs).
Another trend is GPS receivers. Some of the HRM companies have arm-band pods with GPSRs, and a couple companies have wrist-top GPSRs (one of the companies makes them especially tailored to golfers - pre-load the tee to green distance). You can also get basic GPSRs for $50 at WalMart that are adequate for very basic navigation and will measure the distance you travel. However, be aware that GPS receivers are subject to reception problems due to canyons (urban and natural) and canopy (vegetation and tunnels), especially the cheaper ones. You can sometimes lose reception for long distances in thick woods and hilly country.
Soooo, the best bet, as Dave S said, is measure it yourself off a good, up to date, topographic map. Trail signs are notorious for rounding off distances or not being changed when the trail is changed (last weekend's hike that Barb and I took in the Sierra said 4 miles to the meadow we camped, and the map, GPSR, and foot pod all said 5.1 miles - the trail was rerouted a couple years ago, with some of the survey stakes still in place alongside the trail). A lot of the distances on basic trail maps are also rounded off, and the roundoff error can add up quickly.
It is true that topo maps aren't revised all that often. But if you get a good computerized map program (I recommend National Geographic's Topo!), you can draw in the actual trail. NatGeo also has a "Live Map Update" feature with certain series of its software that will load the latest versions of the maps, trails, and roads via the internet into your computer.
None of these methods is perfect, but good topo maps are the most dependable. Even better, don't worry about it. Just get out there and hike as long as you feel like it, then camp.