How is a line crossing detected?

For each consecutive pair of track points, the tool checks whether that segment intersects the gate line using a standard 2D segment-intersection test on longitude and latitude. The exact crossing time is interpolated between the two points based on where along the segment the intersection falls.

Why do I need timestamps in the GPX?

Lap durations come from the time tag on each track point. Without timestamps the tool can still detect crossings but cannot report lap times, so a GPX exported with timing is required for full results.

How are false double-crossings handled?

A short cooldown is applied after each crossing so that points clustered right on the line do not register multiple laps. You can adjust the cooldown if your laps are very short or very long.

Does the gate direction matter?

No. The intersection test is direction-agnostic, so the gate triggers whether you cross it one way or the other. For a real circuit, place the gate where the track only passes in one direction to avoid counting an out-lap.

Is my GPS data uploaded?

No. Parsing, crossing detection, and timing all run in your browser with the DOMParser API. Your track never leaves your device.

GPX Lap Timer — Gera Tools

GPX Lap Timer turns a recorded GPS track into lap times. Define a virtual start/finish line as two coordinates, and the tool finds every point where your track crosses that line, then measures the time between consecutive crossings. It works for any looping activity — track days, criteriums, running laps — and runs entirely in your browser.

How it works

The track is read into an ordered list of points, each with a longitude, latitude, and timestamp. The gate is a line segment defined by two coordinates. For every consecutive pair of track points, the tool tests whether the segment between them intersects the gate using the orientation-based segment intersection test: two segments cross when their endpoints straddle each other, detected via the sign of cross products.

When a crossing is found, the exact moment is interpolated. The intersection’s position along the track segment gives a fraction t, and the crossing time is time₁ + t × (time₂ − time₁). The duration of each lap is the difference between consecutive crossing times. A cooldown after each crossing suppresses duplicate triggers from points sitting right on the line.

Tips and example

Place the gate perpendicular to your direction of travel and wide enough to span the racing line, but short enough that the track only crosses it once per lap. If your fastest lap is, say, 90 seconds, a 10-second cooldown comfortably prevents double counts without missing real laps. Paste the sample, keep the suggested gate, and you will see the crossings and per-lap durations listed in order.