_Waffle_ wrote: »

Use the official chip time from the race result for a PR typically. If you're using a phone (Like my crappy Note 3. I hate the GPS on that) then you could be off by quite a bit. If you're using a dedicated GPS device like a Garmin watch you can still be off because you're probably not taking the most direct line on the course.

My December race shows me running 26.55 miles. On my watch I did 26.2 in 4:28:?? but the official chip time said 4:33:27 (something like that). The zig zagging is my fault. It took me a couple extra minutes but I can't say I crossed the finish line in 4:28 now can I? It was really close but no cigar if I would have stopped running at that second.

http://www.dcrainmaker.com/2009/03/racing-line-understanding-how-courses.html

Just like Waffle said, I go with chip time because I know that phone GPSs can be very inaccurate (mine lost half a mile on a run once and I compared my phone with Runtastic to my Garmin 410 with very different results...). I also don't wear my Garmin during races until recently. I'm really only doing it for the longer races so I can keep myself on pace.

My GPS trace from the London marathon last year shows me taking bizarre detour that looks like I popped into the shops to get a snack en route. The GPS can't handle the tall buildings, or the tunnels. I think I'm more inclined to trust an officially measured, certified route, measured by qualified course measurers, than I am to trust a GPS device that even its manufacturers describe as having quite a wide margin of error.

For official time I will use the race chip time because I will pause my garmin GPS watch at water stops or if I have to use the bathroom. So the time on my watch and my course time could be a little different. I also have to zig zag so that will increase my distance. I do load all of my time in my garmin dashboard and use that for comparison when training.

When looking back at the tracks on my phone it appears someone was shooting at me and I was running a zig zag evasive route. It always reads higher distance than the measured course. Trust the course.

My real Garmin GPS reads more accurately as to the distance.

The reason is that a phone app reads the location much less frequently than a dedicated GPS. As such the GPS follows the route and the phone zig zags and jumps all over.

First, your GPS device only needs to "see" three satellites to give your position fix to within about 30 meters. Where the satellites are in the sky and other interferences affect how accurately you are located. You need at least 4 satellites to obtain an estimate of altitude and that is the hardest one to get with any real accuracy

Second, even with the capability to "see" lots of satellites in get a relatively accurate fix (the most satellites I've noticed on my phone is 22 satellites with 17 max used for a fix using both US and Russian GPS satellites), still gives an accuracy estimate of about 3 meters or 10 feet in the x, y direction. In altitude, it tends to suck unless your GPS can fix on the WAAS geostationary satellites. Most phones and GPS chipsets/running software don't have that capability.

And while the programs and the data massaging is good enough to give your approximate location on a fixed road for driving purposes, it ends up being less optimal for running purposes Superimpose on that the fact that most phones are using an amalgam of GPS, cell tower pings and maybe even known Wi-fi locations to give a fix. other things like buildings, trees, and even atmospheric conditions can lower the accuracy.

I can download the GPS output as a .csv file from my Digifit program. The program stores the data every two seconds and gives the instantaneous distance, speed, pace calculations and the sum and averages. In addition to the zig-zag pattern mentioned above, what you don't see is that you might be "not moving" in the program and then suddendly you're in a different place at 20, 30, or even 100 mph for the span of that two second interval. Have you ever wondered why, when running at a perceived constant pace, your pacing graph also looks like you were running drunk? Averaged over a long enough time it does tend to smooth out and become a bit more accurate.

Finally, as was pointed out above, you may not be running the measurement line for the marathon distance. If you weave around alot, those steps can add up. Generally speaking, I've found that most of my programs over estimate the distance by around 2% (I will have run a measured mile and my phone app says I've run between 1.02 and 1.03 miles). Obviously, it results in saying my app pace is little faster than my 'real pace' (e.g., a 10:05/mile pace on my phone usually ends up being a real 10:20/mile pace. The percent error seems about constant so that a measured mile time of 6:56 showed up as a 6:45/mile pace on the app). In a race situation, the way I deal with this is by turning off the automatic mile notifications on my phone app and using the manual lap function to give me the time between miles as I run the course. When I take down the lap data recorded by the phone because I pushed the button, the real mile-to-mile paces becomes evident.

During the race, I can remember what the previous couple of miles looked like and evaluate accordingly. I also know that when I'm training, the same rules apply and a 10:00/mile real training pace will register at about 9:45-9:50/mile on the app.

One other thing that seems to have "helped" is turning off the lcation services so that my phone is relying only on the GPS during running. Seems like the less influenced by local cell towers the phone app is, the more accuarate it becomes. YMMV!

Use whatever the official results of the race are. I don't adjust my Garmin data - I can readily provide proof of my PRs when needed via www.athlinks.com