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Like many of you, I use Harry's Lap Timer at open lapping sessions. I also use a Garmin VIRB Ultra 30 camera linked to an OBDII over bluetooth to record video. The VIRB solution sometimes records faster lap times than Harry's - typically a few tenths. Harry's relies on the phone's GPS which updates about 1Hz. The VIRB has its own GPS and is rated at up to 10Hz. So on the surface it seems like the VIRB should be more accurate.
Below is a screenshot showing how they differ. I obviously prefer the VIRB time as it's typically faster. But I'm curious if any of you have used the VIRB and compared it to a more precise system such as Racelogic or equivalent.
BTW, they both record the same peak speed at the end of the straight so otherwise correlate pretty well.
The Garmin will be more accurate. I typically run both Harry’s and AIM Solo, and I always use the AIM Solo time (though in my case the AIM Solo is usually a tenth or so slower time).
Harrys is more accurate. I use a AIM SOLO and the VIRB. The VIRB is only more accurate when you have the exact start point adjusted reference a accurate lap timer like the AIM SOLO. Without that reference it will slightly incorrect times, either too fast or too slow. I've tested Harrys against my aim and it is accurate within about half a second on an iPhone (1hz gps). Harrys is usually .2 or so off on most laps. It is dead accurate with a 10hz receiver though.
They are likely using different maps, i.e. start/finish coordinates.
I noticed that it's an imperfect process to select the start/finish line with VIRB Edit software as I can only see the camera's perspective looking out the front window so can't know when the car is actually on the line. I noticed that when I chose the start/finish line again the lap time changed by a few thousandths. So that's probably the explanation why. Perhaps the cumulative error in the GPS location for both Harry's and VIRB results in their respective start/finish lines being farther apart than it appears (almost 4 tenths)?
In terms of which one to use, from the small sample size in this thread it seems that both correlate well to other more precise solutions. Probably makes sense to use Harry's as I would expect it to correlate to others who use it. VIRB isn't as popular and has more variation.
If you get through the corner before the start straight better at the beginning of the lap then moving the start/finish line toward that corner will shorten your lap time due to the higher speed on the straight.
If you get through the corner better at the end of your lap then moving the start/finish away from the corner will quicken the lap.
06-16-2018, 06:36 AM
Which is the correct lap time?
