#36 1911 · Roald Amundsen's South Pole expedition · Exploration
Amundsen marked his supply depots sideways to the route, not along it, so a lost party could still walk into one
the problem
A featureless landscape swallows single markers
background
On the Antarctic plateau there are no landmarks: no trees, hills or roads to triangulate against, only white in every direction, frequently erased further by fog and blowing snow. Amundsen's plan for reaching the South Pole depended on a chain of supply depots laid down in advance, each stocked with the food and fuel a five-man party would need on both the outward run and the return — miss one, on a return leg with dwindling rations, and the miss could be fatal.
The obvious way to mark a depot is to put a flag on it, or a line of flags along the route leading to it. But a single point is easy to walk past unseen in poor visibility, and a line of markers running parallel to the direction of travel fails exactly when it matters most: if a sledging party drifts even slightly off its compass bearing in a whiteout, it can pass within a few hundred metres of the depot's own line of flags and see nothing, with no way to know which side of the line it has missed on.
what everyone would do
The obvious way to mark a depot was to plant a flag on it, or a line of flags running along the direction of travel leading up to it, making the single correct point and the route toward it as visible as possible.
what they saw
Amundsen saw that a marker placed exactly where the depot was, or a line running parallel to the party's direction of travel, failed at exactly the moment it mattered most, since a party drifting even slightly off its compass bearing in a whiteout could pass within a few hundred metres of a marker line running the same direction it was traveling and see nothing. Rather than making the depot's own single point more visible, he laid a row of flagged poles perpendicular to the actual axis along which a lost party was most likely to drift, so the marker intersected the searcher's realistic error, not just the intended path.
the move
Amundsen marked each depot not with a flag on the spot, but with a row of numbered black-flagged bamboo poles running east and west of it — perpendicular to the north-south line of travel — spaced roughly 900 metres apart and extending about four to five miles out on either side. A party approaching from any point within that wide corridor, even badly off its intended line, would cross the marker row and could then read the numbered poles to tell which direction and how far to travel to reach the depot itself.
why it works
Placing numbered marker poles running east-west, perpendicular to the north-south line of travel, meant the marker row sat directly across the exact axis a party would drift along if it went off its intended bearing, so a party anywhere within the wide corridor the poles spanned would physically cross the marker line rather than needing to already be on the correct path to see it. Because the poles were numbered, a party that crossed the line even far from the depot itself could read its position and know precisely which direction and how far to travel to reach the actual depot, converting a chance encounter with the marker row into a reliable navigation fix rather than merely confirming the party was somewhere in the general area. This design against the searcher's likely error, not merely against the target's visibility, is why Amundsen's party located every depot on both legs of the roughly 1,860-mile round trip with zero resupply failures, in direct contrast to the fatal consequences of less redundant planning on the rival Scott expedition of the same season.
the payoff
Amundsen's party located every depot it laid on both the outward and return legs of the roughly 1,860-mile round trip to the Pole and back in 1911–1912, reaching Framheim without a single resupply failure. He recorded the method himself, explaining that markers running along the route "may easily be missed in fog" while a cross-line "cannot be missed" if the party is anywhere near its intended course.
where it breaks
The mechanism depends on correctly identifying the actual axis along which the searcher is likely to drift off course — Amundsen's markers worked because he understood that navigation error in a featureless whiteout tended to occur laterally relative to the direction of travel, and a marker system built around a different, incorrect assumption about how searchers actually get lost would fail to intersect their real error pattern. It also depends on being able to physically extend the marker corridor wide enough to catch a realistic range of drift, which requires real resources, poles, flags, time, spaced across several miles on either side, a cost that has to be weighed against the consequence of a missed resupply. And the redundant, perpendicular marking approach adds meaningful setup and maintenance overhead compared to a single marker at the target, meaning it's worth the investment specifically when the cost of the searcher missing the target entirely is severe, as it was for a polar expedition running on dwindling rations, rather than for lower-stakes situations where a missed marker is merely inconvenient rather than potentially fatal.
what came after
The depot-flagging system is one of the most frequently cited pieces of Amundsen's planning in polar histories, held up against Robert Falcon Scott's rival, fatal expedition of the same season as an example of designing for the failure mode (getting off course) rather than assuming perfect navigation; it remains a standard illustration in polar-exploration writing of engineering a system to survive the searcher's own error, not just to look for the target.
references
- [1]The South Pole: An Account of the Norwegian Antarctic Expedition in the 'Fram', 1910–1912Roald Amundsen, via Project Gutenberg, 1912gutenberg.org
- [2]Depot Journeys - Chapter 6, The South Pole by Roald AmundsenCool Antarctica, 2021coolantarctica.com