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Method: how we compute your TMB stages

You are entitled to know where the numbers in your plan come from. This page explains what is computed, how, and above all what those numbers cannot tell you. Distances, elevation and durations are computed deterministically from map data: they are not made up by an AI.

The data: OpenStreetMap and a terrain model

The route comes from OpenStreetMap: the main Tour du Mont Blanc relation (9678362), the alpine variants (relation 9678380) and local hiking ways. Elevations are read from the ASTER digital elevation model via OpenTopoData. From these two sources we derive each stage’s distance, total ascent and total descent. © OpenStreetMap contributors.

The walking-time formula

For each stage, the estimated walking time in hours is:

h = km ÷ 4 ; v = ascent ÷ 300 + descent ÷ 500 ; time = max(h, v) + min(h, v) ÷ 2

In other words: 4 km/h on the flat, 300 m/h ascent and 500 m/h descent, combined with the DIN 33466 rule (take the larger of horizontal and vertical time, then add half the smaller). The result is rounded to a tenth of an hour.

Hypothetical example: a 20 km stage with 1,000 m of ascent and 1,000 m of descent gives h = 5 h, v ≈ 5.3 h, then max + min/2 ≈ 7.8 hours of walking, breaks not included.

Durations stored in the database are not used as-is: they are recomputed from the stage metrics with this formula, so all figures stay consistent with each other.

The effort index

To spread walking days according to your level, the planner also uses a simple effort index: distance (km) + ascent (m) / 100. It is used to compare days with each other and avoid days that are too heavy for the chosen level. It is not a physiological measure.

What to trust, and what to verify

Reliable as an order of magnitude: distance and ascent/descent, for comparing splits and spotting hard stages.
Approximate: a terrain model smooths relief, and an OpenStreetMap track can differ slightly from the real trail or a temporary diversion.
Not accounted for: your fitness, pack weight, breaks, snow, weather, trail condition or closures.
Lodging: openings, prices, payment terms and closures change. Always check with each place, as in our guide how to book TMB huts for 2027.
GPX: it is the OSM track, the listed variants and waypoints. Do not navigate blindly; carry a map, compass or a trusted navigation app.

And what about AI?

Figures (distances, elevation, durations, critical nights) are computed by code, never by a language model. Accompanying text may be written from those facts or from fixed templates. When in doubt, the computed figure and the official source prevail.

Frequently asked questions

What walking-time formula do we use?

Horizontal time h = km ÷ 4; vertical time v = ascent ÷ 300 + descent ÷ 500; duration = max(h, v) + min(h, v) ÷ 2 (DIN 33466).

Where do distances and elevation figures come from?

From OpenStreetMap Tour du Mont Blanc geometry (relation 9678362, alpine variants 9678380 and local hiking ways) and the ASTER digital elevation model via OpenTopoData.

Can I navigate with the exported GPX alone?

No. The GPX exports the OpenStreetMap track, the listed variants and stage and lodging waypoints. Verify on the ground: do not navigate blindly.

Do the times account for my fitness or the weather?

No. It is a deterministic estimate from geometry. It does not account for your fitness, pack weight, breaks, weather or trail conditions.

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