Why elevation gain differs between apps

The main cause

Apps may be measuring different versions of the terrain.

A recorded activity can obtain height from a barometric altimeter, from satellite positioning, or from a combination of sensors. An imported route may contain elevation values in the file. A service can also replace or correct those values using a digital elevation model, which assigns a terrain height to positions along the route.

Each source has limits. Air pressure changes and blocked sensor ports affect barometers. GPS vertical measurements are generally noisier than horizontal position. Terrain models have a finite resolution and accuracy, and they represent the mapped ground rather than a bridge, building or every small feature beneath a tree canopy.

After measurement

Small calculation choices accumulate across a long route.

Elevation gain adds accepted upward changes. Raw height data often moves up and down by small amounts even on level ground. If every fluctuation counted, the total would be inflated. Services therefore smooth the profile, remove outliers or require a change to exceed a threshold before treating it as genuine climbing.

• Point spacingA dense series may capture short rises that a coarser series skips. It may also contain more noise for the software to handle.
• Smoothing strengthMore smoothing suppresses small fluctuations, including some real undulations. Less smoothing can count noise as ascent.
• Correction methodA service may preserve trusted barometric data but replace other elevation values with its own basemap.
• Route matchingSlightly different horizontal points can sample different terrain cells, especially beside steep slopes.

Two different estimates

A planned route and a completed activity need not agree.

The planned route is normally calculated from its drawn line and a terrain dataset. The activity reflects the positions and heights captured by the carried device, followed by any corrections made by the app or service. Minor deviations, pauses, poor fixes and a changed path can alter the activity total even when the outing broadly followed the plan.

Imports add another layer. The receiving service may trust the elevations inside the GPX file, recalculate them, or apply its own filtering. The number can therefore change without the horizontal route changing visibly.

A useful comparison

Choose one consistent reference for planning.

1. Match

   Check that the route geometry is the same.

   Compare distance, start, finish and obvious detours. A different line should produce a different elevation result.

2. Identify

   Find out whether the value is planned or recorded.

   Do not treat terrain-model ascent and device-recorded ascent as interchangeable measurements.

3. Inspect

   Look at the profile, not only the headline total.

   Sharp isolated teeth often suggest noise. Broad climbs appearing in both profiles are more likely to describe the actual terrain.

4. Repeat

   Use the same source for like-for-like decisions.

   Consistency makes it easier to compare the likely effort of several candidate routes, even when another service would calculate a different absolute total.

Investigate a large discrepancy, but avoid false precision. Elevation gain is an estimate derived from sampled data. What is elevation gain? explains the statistic itself.

The most useful number is one whose source and purpose you understand.