A (very limited) comparison of three iPhone sport monitors

I use quite often my iPhone as a sport tracker monitor, mainly when I bike but also when I walk.
A few days ago, I had activated two different monitors, one GPS based, the other using step counts. And I found a quite huge discrepancy between the two. I just could not know which one (if any) was right because I didn't knew the real distance I walked.

So, I repeated the experiment, with three apps activated, walking a known distance of 1200 m in 13 minutes, thus at a mean speed of 5.5 km/h which, for my weight, corresponds to a small 60 kcal effort.

Here are the counts of the three apps, together with their estimation of calories burned:

• Sports Tracker measured 1.36 km and 74 kcal
• Runtastic pedometer counted 1184 steps, translating them to 0.81 km and 124 kcal
• Endomondo measured 1.26 km and 227 kcal

Clearly, the GPS apps (Sports Tracker and Endomondo) are a little confused in the city environment and over estimate slightly the distance, while the step counter, looses steps, possibly because the phone was in my trouser's pocket.

What is stranger, is the discrepancy in kcal burned. Only Sports Tracker gave a reasonable number, slightly overestimated although. The other two apps, even if I indicated "walking" considered I was running, possibly because the GPS "jumped" between positions and the step counter counted some steps at a very dense pace.

Of course, that was too small a distance for the comparison to be reliable, I will try again with longer distances to see if the errors get relatively smaller. In the basis of this test it is impossible to choose between the three apps.

If you know (or have performed) similar tests, feel free to comment.