GPS lap timing
How Outlap times laps from a GPS trace: gate crossings, interpolation, sector lines, pit-lap filtering and what limits accuracy.
Outlap times laps from position data the way a dedicated lap timer does. This page explains the method, so you can judge when to trust a number and how to get the best from your hardware.
From samples to lap times
- Clean the trace. Fixes without a timestamp or position, and
0,0fixes, are discarded. - Find the timing line. A gate is a short line, 40 metres wide by default, perpendicular to the direction of travel at the start/finish. See where it comes from.
- Detect crossings. For each pair of consecutive samples, check whether the segment between them crosses the gate. Crossings in the wrong direction (more than 90° from the gate’s heading) are ignored, as are crossings less than 15 seconds after the last.
- Interpolate. The crossing time is placed between the two samples in proportion to where the line was crossed. This is why a 10 Hz logger gives laps accurate to a few thousandths rather than the 100 ms the sample spacing suggests.
- Lap time = the difference between consecutive crossings. Laps shorter than 15 seconds or longer than 15 minutes are skipped.
- Drop pit laps. If at least three laps exist, any lap more than 1.6 times slower than the median is removed, and the rest renumbered.
Speeds are computed over roughly one-second windows, because deltas between raw 10 to 25 Hz samples are too noisy to be useful.
In the phone lap timer
The live engine adds to the above, in real time:
- Real sector times from split lines placed on your first lap (thirds by distance). The same lines are used every lap, so sectors compare.
- A live delta to your best lap by position. It compares where you are now with when you were here on your best lap, not elapsed distance.
- A predicted lap time (best lap plus the current delta), and Delta-V: your speed now versus your speed at this spot on your best lap.
- An ideal lap: the sum of your best sectors.
- Start/finish found automatically. From the circuit when its line is known, otherwise placed where you first reached speed and confirmed when you come back round. No button, and the first lap still counts.
What affects accuracy
| Factor | Effect |
|---|---|
| Sample rate | The main factor. 1 Hz phones are coarse; 10 to 25 Hz is excellent. |
| Antenna view | Tunnels, bridges, tall buildings and tree cover degrade the fix; GPS lap times get noisier. |
| Fix quality | RaceBox reports satellites and horizontal accuracy; low counts mean worse data. |
| Gate position | Any line across the track gives the same lap time; only sector positions depend on it. |
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