GPS converter

A GPS track is a list of exactly where you were and exactly when. A collection of them shows your home, your workplace, your routine and the times you are reliably out of the house — which is considerably more than most people intend to hand over when they just want a route in a different format.

Everything here runs in your browser and nothing is transmitted. Disconnect from the network and it still works, which is the demonstration rather than the promise.

GPX is the interchange format almost everything reads and writes, and is the safe default. KML is Google’s and is what you want for Google Earth or a map you intend to share visually. GeoJSON is the developer’s format and drops straight into web mapping libraries.

TCX is the outlier: it carries training data — heart rate, cadence, laps — that the others have no place for. Converting a TCX to GPX keeps the route and loses the workout metrics, which is fine if you want a map and not if you want your training history.

GPX distinguishes between a track, which is where a device recorded that you actually went, and a route, which is a set of points describing where somebody intends to go. They look similar in a file and behave completely differently in software.

A track carries a timestamp on every point, so speed, pace and elapsed time can be derived from it. A route usually carries none, because it has not happened yet. Converting a track to a route discards the timing permanently, and it is the most common way people lose the data they cared about.

GPS elevation is derived from satellite geometry and is markedly less accurate than horizontal position — errors of ten to twenty metres are ordinary, and a recording can show hundreds of metres of climb on a flat ride. Devices with a barometric altimeter are considerably better and drift with the weather instead.

Some formats and services correct elevation against a terrain model after the fact, which is why the same activity shows different climb totals in different applications. None of them is lying; they are using different sources. If elevation matters, note which figure you are comparing before concluding anything.

A recording started at home and stopped at home has your address at both ends of it, precisely and with a timestamp. That is the single most sensitive thing in these files and it is the easiest to overlook, because the interesting part of the activity is in the middle.

Most fitness platforms offer a privacy zone that hides a radius around a chosen point, and it only applies on that platform — the exported file still contains everything. If a track is going to someone directly, trimming the first and last few hundred metres is the reliable version.

GPX carries position, elevation and time as standard, with anything else — heart rate, cadence, power, temperature — living in extensions that not every reader understands. TCX was built by Garmin around training data and carries laps and heart rate natively. FIT is a compact binary format that carries the most and is the least readable.

Converting downward loses whatever the target has no place for, silently. A FIT file from a bike computer converted to plain GPX arrives as a path with timestamps and no power data at all, which is fine if you wanted a map and a loss if you wanted the ride.