Discrepancies: Mapped versus GPS versus race (Read 853 times)

The short answer- Unless a race is a USATF certified course, do not expect it to be accurately measured. Congrats on your huge PR!

Has anyone else seen this kind of discrepancy before?

Yes.

Does anyone know what causes it.

I'd assume the GPS would be less accurate while you are downtown with all the tall buildings.

Info on course measurement here 1) congrats on the PR 2) don't expect any course to be exactly that distance 3) GPS will always measure long

Jim, Thanks for the link to the other thread (I too fell for the "gps error" link - I am an engineer...) I think I know what happened. The race was measured using shortest distance, which I of course didn't run. The mapping program measured center of the road and since we were in one lane of 4-6 lane roads over most of the race would put error into it as well. On top of the GPS reading errors, possibly due to tall buildings during a portion of the race, I ran the outside edges in order to avoid the cluster in the middle of the pack. Being a run/walk/run type I didn't want to run into anyone or have them run into me and this was the first time I ran with a pack. In the smaller 5k races I've run I was always left in the dust and ran close to the edge. So I ran even further than what the map measurement yielded for center of the roads. In fact in some cases I was running on the other side of the road in what would have been oncoming traffic. The mapping programs have agreed with my GPS during my training runs because I run in the middle of the road in a low height building neighborhood. I run at 5am in the morning and can get aways with middle of the road most days. This is the link to the workout http://www.runningahead.com/logs/e020354dbcc74c33b9c2011b4621fe57/workouts/171f9f9bb16d4ba7812609dd5d9bf0fd there are some strange entries in the GPS interval listing that point to inaccuracies, e.g. the 2:43 pace for the 1st .6mi, and the 3:34 pace for the second .1mi or walk interval. I did run thru some rest points but not at that pace! thanks! bob e v

I did some testing this morning and ran a 1.01 mi neighborhood loop route that used "follow road" and was centered for middle of the road running. I ran on the outside sidewalk for three laps. The GPS said I ran 3.15 mi instead of 3.03. That explains a good part of the difference. I ran this morning about 15-20 feet off the centerline and wound up running 4% further than the map. Now I've run the same route in the middle of the road before and the GPS matched the map distance. So, bottom line, as JimR said, GPS will always run long - unless you can run along the reasured route or shortest distance and avoid interference from tall buildings and trees. The lesson for me for my next race, especially a larger one, is to run on the inside as much as possible. More easily done on the smaller races since most of the runners leave me in the dust and I have the route to myself. Only have to worry about getting lost!

Only have to worry about getting lost!

That's where your GPS comes in. Oh wait. Yours doesn't do that?

Small earthquakes happen all of the time, and especially during races, which cause the earth's surface to expand and contract. Usually only myself and the sensitive mechanism of the 301 GPS notice this and measure the distance accurately. Complaints to race diectors are usually futile, so just endure the misery with a stiff upper one, and adjust your pace accordingly.

If geology is your thing , there's a new blog with the nifty title: "All of my faults are stress related" http://scienceblogs.com/stressrelated/2009/03/signs_of_spring.php

If your GPS said 13.2 on a half marathon there is a chance the course was slightly short. Was it certified? If yes then you did a hell of a job running tangents.

It was the 2008 Thunder Road Half in Charlotte. The full is a Boston Qualifier, and the half is 95% on the same course, other than that I am not sure if it's certified. 2009 is certified They added a little detour into a parking lot about 200 Meters from the finish, so I guess they did measure it fairly closely. My GPS was consistent at all the mile markers in that it ticked over the mile, within sight of the mile marker.