The Interval Decides What Counts as a Hill

Two Maps, One Ground

Take a ridge with a prominent shoulder — a long, rolling bench sitting perhaps eight metres below the crest. On a sheet drawn at a ten-metre contour interval, that shoulder earns its own line, maybe two. The walker sees a flattening, a place to stop. On a sheet drawn at a twenty-metre interval, the shoulder disappears between contours entirely, absorbed into the slope as if it were never there. The ground has not changed. The map's decision about what constitutes a measurable step in relief has changed everything.

This is what the contour interval commits you to. It is not merely a technical parameter tucked into the legend — it is an editorial choice about the minimum hill. Features whose vertical extent falls below the interval are systematically invisible, not because the survey missed them but because the interval excluded them before the first instrument was set up.

The Arithmetic of Disappearance

The relationship is direct. A forty-foot interval — standard for many mid-century American topographic series at the one-to-sixty-two-thousand-five-hundred scale — will not express any terrain feature whose total relief is less than roughly forty feet. Drumlin fields, coastal dunes, riverine terraces, the earthwork of a small fort: any of these might stand ten or fifteen feet above the surrounding land, register clearly to a walker, and yet leave no mark on the sheet. The surveyor measured them correctly. The interval erased them.

Smaller intervals carry their own costs. A five-metre interval on a mountainous sheet produces so many contour lines that the sheet becomes illegible — a dense brown thicket in which individual lines can no longer be traced. Cartographers resolve this tension in two ways. The first is the supplementary contour: a dashed or dotted line inserted at half the standard interval where the ground warrants it, a local concession to the rule. The second is the variable-interval series, in which the published interval changes with the mapped terrain, typically coarser in mountains, finer in lowlands. Both solutions make the sheet harder to read mechanically; the reader must check the legend not once but repeatedly.

Index contours — every fourth or fifth line drawn thicker and labelled — exist partly to manage the reading problem, but they introduce their own distortion. A reader skimming a sheet will unconsciously weight labelled lines as significant and unlabelled ones as lesser, even though the ground makes no such distinction. The interval shapes not just what is shown but what the eye treats as important once it is shown.

What the Choice Reveals About the Series

The interval a mapping agency selects tells you who the sheet was made for. A military series needs passable slopes and defensible ridges — a twenty-metre or fifty-foot interval is usually sufficient. A geological survey wants to correlate rock exposure with topographic form; it may need five metres to see the terracing that signals a lithological boundary. Engineering and cadastral work sometimes demands one-metre intervals. The Ordnance Survey's Landranger series uses ten-metre intervals across most of Britain, a choice calibrated to the walker and the planner rather than the climber or the civil engineer; the Explorer series, at a larger scale and a smaller interval, begins to show features the Landranger simply cannot.

None of this is a failing of the coarser-interval sheet. It is the nature of any single-interval product. The relief it renders is the relief its interval permits, and any use of the map that depends on features smaller than that interval is a use for which the sheet was not designed. Before you read a contour map, read the legend. The interval is stated there, quietly, as a number — and that number tells you what category of hill the sheet has agreed to acknowledge.