Question about running a virtual marathon with GPS

Only solution is to run 105.5 laps around a 400m track.

GPS accuracy varies by device and especially by environment. Weather, tree cover, large buildings and turns can throw off accuracy. There’s no one correction factor to assure an accurate distance.

I realize there is no way to run an accurate marathon distance with just GPS, but I'd like to run a great enough distance so that it is almost certain that the distance that I run is at least a full marathon. Wondering if 42.5km, so 300 meters extra is enough.

lposwhaqiddv1de6i3qu wrote:

I realize there is no way to run an accurate marathon distance with just GPS, but I'd like to run a great enough distance so that it is almost certain that the distance that I run is at least a full marathon. Wondering if 42.5km, so 300 meters extra is enough.

Why not use mapmyrun to measure the route beforehand?

It's really accurate (you can test it on a 400m track and see for yourself).

You can do an out and back course and you just have to memorise the starting/finishing point (easy, you pick it) and the return point.

hhhhh wrote:

lposwhaqiddv1de6i3qu wrote:

I realize there is no way to run an accurate marathon distance with just GPS, but I'd like to run a great enough distance so that it is almost certain that the distance that I run is at least a full marathon. Wondering if 42.5km, so 300 meters extra is enough.

Why not use mapmyrun to measure the route beforehand?

It's really accurate (you can test it on a 400m track and see for yourself).

You can do an out and back course and you just have to memorise the starting/finishing point (easy, you pick it) and the return point.

^^^This. Or wear a Stryd foot pod.

I've never ran a marathon but I'd hate to add any distance to any race, or to not really have a distinct finish point. I wouldn't be able to race it. My 10k pace is my 10k pace. No way I could run 10.5k at 10k pace.

My

^...My watch has never significantly overstated a race distance. It's generally either very accurate or slightly understates the difference. So when doing time trials I run to the distance my watch says.

Take a look at strava GPS track lengths for some marathons. The measured length of marathon are 42.195 km +42.154 meters (short course prevention factor), but the GPS lengths showing on runners' watches varies by individual device and course (tall buildings on some courses interfere with GPS signal).

NYC. I see a lot of 26.6x mile tracks, which works out to 42.8-42.9 km:

https://www.strava.com/running-races/2014-new-york-city-marathon

Honolulu marathon. I see mostly 26.3x (high) to 26.5(low), which is around 42.4 -42.7 km.

https://www.strava.com/running-races/2015-honolulu-marathon

LRCthreadObserver wrote:

^^^This. Or wear a Stryd foot pod.

Some (geeky) Stryd users have to use a different calibration for each shoe. And results seem to vary by angle on shoe or portion of laces on shoe. Doesn't sound like better accuracy than a GPS to me.

2020202 wrote:

The measured length of marathon are 42.195 km +42.154 meters (short course prevention factor)

Blotched that. Add *42.195 meters (0.1%) = 42.2372 km for measured course. Just about everyone always runs further on a real course.

And running a ‘virtual’ marathon will keep your streak alive of never having run a marathon.

2020202 wrote:

LRCthreadObserver wrote:

^^^This. Or wear a Stryd foot pod.

Some (geeky) Stryd users have to use a different calibration for each shoe. And results seem to vary by angle on shoe or portion of laces on shoe. Doesn't sound like better accuracy than a GPS to me.

https://www.letsrun.com/forum/flat_read.php?thread=9853287#9856633

Sounds like you've never actually used the footpod, then.

I think mapmyrun on a very visible course not taking the tangents is the best way. I run the same courses often with my garmin and it can sometimes be off by as much 10s/mile. So that’s over 4 minutes over a thon.

hhhhh wrote:

lposwhaqiddv1de6i3qu wrote:

I realize there is no way to run an accurate marathon distance with just GPS, but I'd like to run a great enough distance so that it is almost certain that the distance that I run is at least a full marathon. Wondering if 42.5km, so 300 meters extra is enough.

Why not use mapmyrun to measure the route beforehand?

It's really accurate (you can test it on a 400m track and see for yourself).

You can do an out and back course and you just have to memorise the starting/finishing point (easy, you pick it) and the return point.

Definitely do this. You can use MMR, onthegomap.com, or strava. It's super accurate and, depending on your watch, you can send the route from strava to your watch so you can follow along as you go. Otherwise, you can just do what they said and do an out and back (or a few laps, which could be nice if you want to set up some water/snacks or something and not have to carry it with you).

LRCthreadObserver wrote:

2020202 wrote:

Some (geeky) Stryd users have to use a different calibration for each shoe. And results seem to vary by angle on shoe or portion of laces on shoe. Doesn't sound like better accuracy than a GPS to me.

https://www.letsrun.com/forum/flat_read.php?thread=9853287#9856633

Sounds like you've never actually used the footpod, then.

No. You are saying it's perfectly accurate without calibration? A friend used it for a training cycle, assumed it was accurate because they were supposedly good out of the box, and found a discrepancy at a race, but assumed the race course was off. Then ran a marathon it was off by the same percentage. Moreover, the watts he thought he could do in the marathon would have been correct for the distance that the pod assumed, but that distance was short. Looking back, everything he did in that cycle was off by that fixed percentage that the pod was off in distance.

2020202 wrote:

LRCthreadObserver wrote:

Sounds like you've never actually used the footpod, then.

No. You are saying it's perfectly accurate without calibration? A friend used it for a training cycle, assumed it was accurate because they were supposedly good out of the box, and found a discrepancy at a race, but assumed the race course was off. Then ran a marathon it was off by the same percentage. Moreover, the watts he thought he could do in the marathon would have been correct for the distance that the pod assumed, but that distance was short. Looking back, everything he did in that cycle was off by that fixed percentage that the pod was off in distance.

You definitely need to calibrate it. But once calibrated it is very accurate. You do also have to calibrate it for different shoe models.

I can't wait for all the karens to submit their 6 hour marathons turned 4:30 prs when they run through the cities and have like 10k in 22 min from 30-40km in all their marathons.

MapMyRun, Google Earth, and similar apps are accurate, but ONLY if you use straight lines and follow all the tangents instead of just following the path of the road. If you follow road paths instead, the route will be short. To map out an entire 26.2 mile course with every turn is a series of small tangents is possible, but laborious. The reason that people's watches measure longer than 26.2 after a marathon is not because their watches are inaccurate, but primarily because people suck at running tangents, and it's very hard to do in a crowded race or if you don't know the course perfectly.

If you know how much your GPS is off simply add that in. I happen to know mine always tells me I ran about 1% farther than a measured course. Every marathon I run comes in almost exactly 0.25 miles over, at 26.45 miles, or 13.25 for a half marathon. And that's usually equates to the GPS telling me I'm going about 4-5 seconds per mile faster than I actually, am when my chip time comes in. So if you know your GPS, just add that error factor in. For me, that would be 26.45 miles, for me to know I'd have covered 26.2 miles on a measured course.