In practice, how precise is a pedometer for estimating your progress on a trail? For example, if I'm to hike 10 miles, how far should I expect to be from the 10 miles marker?
Is there data comparing pedometers accuracy with GPS?
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Searching online I found lots of useful informations, like e.g. wiki says:
The accuracy of step counters varies widely between devices. Typically, step counters are reasonably accurate at a walking pace on a flat surface if the device is placed in its optimal position (usually vertically on the belt clip). Although traditional step counters get affected dramatically when placed at different angles and locations, recent advances have made them more robust to those non-ideal placements. Still, most step counters falsely count steps when a user is driving a car or makes other habitual motions that the device encounters throughout the day. This error accumulates for users with moderate commutes to work. Accuracy also depends on the step-length the user enters. Best pedometers are accurate to within ± 5% error.
On medicinenet one can find an overview on the topic. A useful quote on the topic here is this:
The accuracy of pedometers has been carefully studied because they are frequently used in studies and researchers demand to know if they are reliable and accurate. Research shows that pedometers tend to count steps more accurately at speeds greater than 3 miles per hour (mph) than at slower speeds. Accuracy can exceed 96% when speeds exceed 3 mph, whereas the accuracy drops to between 74% and 91% at speeds from 2 mph to 3 mph, and it drops even further to between 60% and 71% at speeds below 2 mph. The error has to do with the insensitivity of pedometers to detect steps when people shuffle or drag their feet at slow speeds. Detectable vertical movement of the hips is necessary for pedometers to work well.
The two types of pedometers differ in terms of accuracy depending on your speed and your habits:
Piezoelectric pedometers tend to be more sensitive than spring-levered at slower speeds and so they may be preferable for individuals who walk slowly. In addition, the tilt of the pedometer is critical for performance with spring-levered devices, but not so with piezoelectric devices (the piezoelectric mechanism is not position dependent). For example, a pedometer may tilt forward into the horizontal plane if an overweight individual with excess abdominal fat wears one on their waist. Tilt in a spring-levered pedometer can throw off the accuracy by as much as 20% at fast speeds and 60% at slow speeds. A piezoelectric pedometer does not have this tilt error except at the slowest speed (less than 2 mph), and the error is less than 10%.
More infos and tests on specific pedometers are given on walking.com.
I don't know if they compared to GPS data in the tests but I think GPS is always more accurate to measure your distance. These days you can find GPS watches so size/weight is not really a matter (but maybe the pricing).
Before you asked this question I also had no clue about the accuracy but reading a bit on the topic I think I am not going to try one on the trails ;)
It depends a lot on the terrain. I wear a pedometer throughout the week (health program for work), and I use a GPS when hiking.
From experience - on level terrain - I know that I get between 2100 and 2200 steps per mile. I walk between work and the coffee shop (a round trip of about 1.25 miles) each day, and this is fairly consistent.
If you calibrate your pedometer to your stride on level ground (such as a road or sidewalk), it will not be accurate when your environment changes. Likewise, your stride will be different when you are going uphill vs downhill.
When I go hiking, though, this changes. My stride tends to be longer on downhill sections of trail, shorter on uphill sections. Even more noticeable, when I am on rough, rocky trails, my steps can increase by as much as 50%. This variation is enough to make the pedometer useless for measuring distance while hiking.
You have to be careful with distance measured by GPS as well. In many cases, such as when you're moving consistently, it's quite accurate. Realize, though, that when you're sitting still, the jitter (noise resulting in inaccuracy) in the signal adds to the distance tracked. Just last weekend, I racked up 0.1 miles while sitting still for three or four minutes. Don't believe me? Open up your GPX track file on a computer and zoom in -- I guarantee you'll be able to spot the locations where you take breaks along the trail! ;)
I have found that I can more accurately measure hiking distance by time rather than paces. My pace tends to be about 2.8 mph with a medium-sized pack. 10 miles equates to just a little under four hours (3:45?) of walking.
Pedometers seem to be calibrated for use on flat ground. Rough or difficult terrain can cause you to take more steps and it is this mechanism which pedometers use to determine their output.
If you have a known pace length and can compensate for it using some measure for it to be diminished when walking a route that isn't flat then id say it could be mildly accurate.
Consider using Naismiths rule to determine a coefficient of terrain difficulty.