GpxFix Blog

Why Your Elevation Gain Is Wrong And How to Fix It

By Roy Lachica on June 01, 2026. Last updated June 1, 2026.

You finish a steady road ride and Strava tells you that you climbed 1 800 metres — on a mostly flat course. Or you summit a well-known peak and your watch logs only half the ascent you expected. Elevation figures are among the least reliable numbers that GPS devices produce, yet they drive segment rankings, training load calculations, and the bragging rights attached to every activity. Understanding why the numbers drift and what you can do about it saves you from misleading data and frustrating comparisons.

Two very different ways devices measure altitude

Most modern sports watches combine two altitude sources, and the balance between them matters enormously.

• GPS-derived altitude: satellites triangulate your three-dimensional position. Vertical accuracy is inherently worse than horizontal accuracy — typical error margins range from 10 to 50 metres, and brief signal dips can create phantom climbs of hundreds of metres.
• Barometric altitude: a tiny pressure sensor estimates height by measuring air pressure. This approach is far more responsive to small changes in altitude, but it drifts with weather — a passing low-pressure system can shift the reading by 30–50 metres over the course of a long ride.

When both sources disagree — which is often — the firmware must decide which one to trust, and different manufacturers apply different blending algorithms. The result is that two athletes riding the same route side by side can record noticeably different elevation totals.

The most common causes of elevation errors

• Cold start indoors: if you begin recording while still inside a building and the pressure sensor calibrates to indoor air (which differs from outside), every subsequent reading inherits that offset.
• Weather change during a long outing: barometric pressure rarely stays constant over several hours. A front moving in adds a virtual climb or descent that has nothing to do with the terrain.
• Noisy GPS elevation data: satellites bounce signals off buildings and terrain — especially in urban areas and narrow valleys — introducing large altitude jumps that inflate cumulative gain figures.
• Firmware smoothing differences: some firmware versions apply aggressive smoothing that removes real rolling terrain; others are too sensitive and count tiny oscillations as genuine ascent.
• Altitude calibration not set: many watches allow you to enter a known elevation at the start of a session. Skipping this step means the barometer starts from whatever pressure it last measured.

Why Strava's elevation often differs from your watch

When you upload a GPX file, Strava does not always accept the altitude values embedded in the file at face value. For many activity types it substitutes its own elevation map — derived from digital elevation model (DEM) data — which can be more accurate on well-surveyed terrain but can also introduce its own errors on trails, indoors, or on routes that pass through tunnels and car parks. This means the elevation displayed on Strava may be completely independent of what your watch recorded.

How to tell whether your elevation data is faulty

• Plot the altitude profile in GpxFix and look for sudden vertical spikes — near-vertical lines on the chart are almost always sensor noise rather than genuine terrain.
• Compare the total ascent against a reliable source such as an OS map, a satellite DEM, or a known reference route that others have ridden or run.
• Look for a steady upward drift across an entire session that never descends to the starting altitude — a classic sign of barometric drift.
• Check the elevation at your known start and finish points; if they differ significantly from reality, the whole profile is offset.

Fixing elevation data with GpxFix

GpxFix gives you practical tools to clean altitude data before re-uploading to Strava or another platform.

1) Remove altitude spikes

2) Smooth a noisy barometric profile

What you cannot fix after the fact

If the pressure sensor was never calibrated correctly, or if the GPS had a complete outage for a long section, no post-processing tool can reconstruct the true terrain profile from that session alone. In those situations the best available option is to substitute DEM-derived altitude (which Strava may do automatically) or to accept that the numbers for that particular activity are approximate.

How to get better elevation data next time

• Start outdoors: give your watch at least 60 seconds outside before pressing start so the barometer calibrates to open-air pressure.
• Set a known altitude: on devices that support manual calibration, enter your starting elevation from a map or weather station before each session.
• Enable auto-calibration: many watches can pull DEM altitude from GPS satellites at regular intervals during a recording — check that this feature is switched on.
• Avoid tunnels and underground car parks: prolonged pressure changes indoors will throw the barometer off for the rest of the session.
• Update firmware: manufacturers regularly refine their sensor-fusion algorithms — keeping firmware current often reduces systematic elevation errors without any other changes.

Closing thoughts

Elevation gain is one of the hardest metrics to record accurately, yet it is one of the most searched-for numbers after every climb, hike or mountain ride. Knowing which errors are correctable — altitude spikes, sensor noise, offset profiles — and which are not helps you set realistic expectations and focus your editing effort where it matters most. GpxFix handles the correctable cases cleanly so the data you publish reflects the effort you actually put in. Upload your file and inspect the elevation chart — you may be surprised how much cleaner the profile becomes after a single pass.