Forums >Gears and Wears>RA map vs. GPS accuracy
I'm wondering which is more accurate, the RA course mapping feature or my Garmin? Tonight there was a difference of nearly 2 miles-- RA said I did 15.8mi, but my watch only recorded 14.1 miles. That's quite a large difference!!
Sulphur Springs 50km-- Ancaster, ON-- May 28, 2022
Tally in the Valley 12 hours-- Dundas, ON -- July 30, 2022 (Support SickKids Toronto)
Stokely Creek-- 56km-- Sault Ste. Marie, ON-- Sept. 24, 2022
Did you look at the map that your Garmin made to see if it cut corners or turned off at some point? Depending on where you run, I've seen some odd routes (I.e. through buildings or across lakes). Also, if you premapped it on RA, did you actually run the exact same route that you planned?
I find a big difference in elevations between RA and the Garmin (i.e. RA will say a route has 1200' elev. gain while the garmin says 700'), but not in mileage. When there's a mileage difference, it's usually because I've screwed up when drawing the map on RA (i.e. added an extra in and out accidentally).
run, rest & read
I'm gonna say the mapping function, as long as you remember the real route you took. You can compare the RA course mapping with many others available online, and they'll come out nearly identical. My garmin doesn't usually hit the exact same numbers for the exact same route twice in a row, if ever.
The GPS capability of a small device you strap on your arm will always be limited. In order for GPS to work to its fullest extent you need an, x,y, and z axis. Easier stated, you need three satellites from equal angles around you. You can look up and get a lock in any direction you have line of sight visibility, but you'll never get a fix from the opposite side of the earth and that limits primarily GPS' ability to determine your height above earth. That's a dumbed down version cuz I'm no astroscientist guy.
I work with military GPS and north finding and we have the same problem. Unless and until we can figure out where north is, and where we are on the ground (with precision), our ability to geolocate, and then movement, will always be subject to the error budgets of the devices we use.
Overly complex way to say that mapping functions are probably a tad more accurate than GPS.
There is also error that builds up to extra mileage on your GPS measurements. It only knows where are you to within a couple of feet, so every time the GPS measures the distance between where it thought you were and where it thinks you are now, it calculates a little bit more than the distance between where you are and where you were.
Imagine this: if you jog in place, you have not gone anywhere (if you're doing it right). At the first time point, the GPS error is 1 foot to the north. After a second, the GPS error is 1 foot to the east; the GPS calculates that you have traveled 1.4 feet. At another second, the GPS says you are actually 2 feet to the west of where you are and says you have run 3.4 feet. You then post a thread on RunningAhead asking if you should email the jog-in-place race director asking for a refund because his/her course was long.
If you want to get a sense for how accurate the RA mapping is, try mapping on lap on a track. If you use an infinite number of points, it will be pretty darn accurate but the fewer the points you use, the worse off you'll be.
MTA: the error tends to build up more on elevation because that changes less between time points than your overall distance.
ffs, you guys.
Runners run