Why does elevation gain change depending on which direction you hike a trail?

The short answer

Elevation gain is built by walking through a route in order and adding up every stretch where the ground goes up; elevation loss is the same sum for stretches where it goes down. Reverse the direction of travel and you walk through the exact same points in the opposite order, which flips every one of those stretches — what counted as a climb heading north counts as a descent heading south, and the other way around. So the gain for one direction is close to the loss for the other direction, and vice versa, purely because "up" and "down" swapped meaning while the ground underneath never moved at all.

That swap is exact for a route with only rolling terrain and no net change in elevation from one end to the other. Add a difference in elevation between the two trailheads — one end sitting higher than the other, which is normal for a point-to-point trail rather than a loop — and the two directions stop being simple mirror images of each other, because that difference gets added to one direction's gain and to the other direction's loss.

Every uphill in one direction is a downhill in the other

Picture a short stretch of trail that climbs 40 metres over a rise and then drops 25 metres into a dip on the far side, before the rest of the route continues. Hiked in that order, the 40 metres counts toward gain and the 25 metres counts toward loss. Hike the same stretch starting from the far end and you cover the same two changes in elevation in the opposite sequence: the 25-metre rise now comes first and counts toward gain, and the 40-metre drop comes second and counts toward loss. Nothing about the hill changed. Only which end you started from did, and that alone was enough to move 15 metres of net difference from one column to the other for this one stretch.

A real trail has this happening dozens or hundreds of times over its length, at every dip and rise the route crosses, not just once. None of those individual swaps is usually large, but a trail with a lot of undulation can accumulate a noticeably different total gain depending on which end you started from, even before the elevation of the trailheads themselves is part of the picture.

The elevation difference between the two ends

A loop hike always returns to its own starting point, so its net elevation change is zero no matter which way around it is walked, and reversing it mostly just redistributes the same rollers as described above. A point-to-point trail is different, because its two ends are not the same place and are rarely at the same elevation. A trailhead at 1,200 metres connected by trail to a trailhead at 1,800 metres has a net difference of 600 metres between them, and that 600 metres has to show up somewhere in the numbers regardless of which direction anyone walks it in.

Walked from the lower end to the higher one, that 600-metre difference adds itself into elevation gain on top of whatever rolling terrain is in between. Walked from the higher end down to the lower one, the same 600 metres adds itself into elevation loss instead, while gain drops to close to whatever climbing exists on the smaller ups along the way. This is the same net-change idea behind a trailhead sign quoting one climb figure while a GPS track reports something larger — except here the difference comes from picking a direction, not from the gap between net change and cumulative gain.

Why this is not the same as two apps disagreeing

Two recordings of the same hike in the same direction can already disagree by a few percent because of ordinary GPS and barometric noise, a different filtering threshold between apps, or the two passes over an out-and-back not landing on exactly the same points. Direction-dependent elevation gain is a different thing entirely: it shows up even with a perfect, noise-free record of the trail, because gain and loss are genuinely different quantities for the two directions, not two slightly different estimates of the same one. A guidebook or trailhead sign that only lists a single elevation gain figure for a point-to-point trail is implicitly describing one direction, and that figure will not simply flip sign if the trail is hiked the other way — it can come out higher, lower, or roughly the same, entirely depending on where the two trailheads sit relative to each other.

Where TrailCam fits in

TrailCam records elevation as part of the GPS track in whichever direction a hike was actually walked, and shows the resulting gain alongside distance, moving time and pace, with the climb drawn as a profile underneath the map when the route is replayed. Two recordings of the same trail from opposite ends are two separate hikes in your history, each with its own honest gain and loss rather than one figure standing in for both directions. The app is free to download.