Accurate map readings

by Gilbert Keith

I decided to use my phone with a GPS-enabled running app today to check out whether the voice guidance features advertised are any helpful.

This post is not going to be about any of those apps, however. I’ll note that I tested the Runtastic App… it was ok. Just that most of the features on there can only be unlocked with the Pro version, so I’m not in a position to comment.

The problem I had was this:

I didn’t get my first mile-completion notification until 10:30 mins in. 10:30 is pretty slow. The half marathon I ran a few days ago was at a ~9:20 pace. I know I’m not going to be running blazing fast speeds so soon after the half… but still, a ~12% reduction in speed! Preposterous!

Human that I am, I started coming up with explanations. Maybe it was because I hadn’t slept well (I’ll leave you to follow the tweets below for the poor decision making that led to my sleeping at nearly 3 AM.) Maybe because it was hotter than I thought it was (I was expecting it to be in the low 70s; I never bothered checking what it actually was.) Maybe it was because I hadn’t eaten anything for a while.

ImageAnyway, the point is, I was feeling down. In my experience It’s not easy to significantly ramp up pace unless I’ve pysched myself up to run at the pace. I also had to climb up the hills on Pleasantview, so I was pretty sure that the next mile would be significantly slower.

And indeed, when I had finished mile 2, I was at nearly 21:40 (counting some 20 odd seconds it took for me to put my phone back in the pouch and get running.) So let’s say per the app, I had finished mile 2 at 21:20. That makes the average 10:40. The remainder of the way was somewhat downhill, so I figured I’d be able to pick up some pace… and I did indeed run the downhills harder than I would have usually. I had finished 3.25 miles in 34:12 per the app. Let’s say 33:30 to subtract all the time it took my clumsy self to stow away the phone and get going.

3.25 miles in 33:30 comes out to 10:18/mile.


All right, so I had picked up a lot of pace. Not bad.

This is what my map on Runtastic looked like:


These were the splits according to the app… to think that there was a half a mile stretch in which I was going at 4.8 mph! Quelle horreur!


I returned home, dejected. I was convinced I was running faster than 10+mins/mile averages, so I wanted to check my routes on gmap-pedometer… I’ve been using it for a while now, and I find it fairly trustworthy.

This is what I saw on gmap-pedometer: 3.52 miles! That’s nearly a 8% difference, which would account for most of the purported slowness!

ImageLook at the mile splits, too. Mile #1, per the below screenshot, happened at the southern edge of the park during the second circumambulation of said park. However, it happens much later per the GPS. Same with the remainder of splits, too. I know that gmap-pedometer pretty much assumes that you were sticking to the middle of the path you log, so I can expect that, say, at curves, it can over/underestimate the distance by a few feet. But still, this should, on average, cancel out.. and if you’re systematically running along the edge with the higher radius of curvature (which I always do,) gmap-pedometer should consistently underestimate by a few feet.

Why, then, is there such a discrepancy? It doesn’t seem like I can trust GPSs much… I know the gmap-pedometer estimates to be fairly accurate (based on reverse-engineering the path of actual 10Ks and half marathons.) It seems like the GPS, then, is a fairly useless tool unless you haven’t already traversed the path prior. The best option, this whole exercise would suggest, is to map out the run first; figure out the mile markers, and set your goals for those spots; and then mentally keep a track of whether or not you made those goals.