Benchmark symbol
BM on a topographic map marks a benchmark: a permanent survey mark, often a metal tablet, whose elevation has been measured. The number beside it is that elevation.
Easily confused with
How to read it
The USGS symbol sheet draws vertical control, a benchmark with a tablet, as the letters BM beside a small cross, with the elevation next to it, such as BM × 5280. A benchmark that is also a horizontal control point is drawn with a small triangle in place of the cross: BM △ with its elevation. NOAA’s National Geodetic Survey, which establishes the US vertical control network, defines bench marks as long-lasting points whose elevations have been determined, used to control other surveys and to watch for movement of the Earth’s crust.
Also searched as: bench mark, BM on a map, BM with an X, BM with a triangle, survey marker, vertical control.
These pages describe what each symbol on a map shows. They are not navigation instructions: the map’s own legend says what each of its symbols means, and symbols differ between map series and editions.
Differences by country
In Britain the Ordnance Survey’s benchmarks (BM) are marks cut into houses, churches, bridges and other structures. The OS describes hundreds of thousands of them across Great Britain and says it no longer uses them. Its maps mark triangulation pillars (trig points) instead, as a small blue triangle with a dot in the middle.
Look-alikes
A cross with a number and no BM is a Spot elevation: a height read from the map or the ground, with no survey mark there. A triangle without BM is a horizontal control point, a position fixed by survey.
Read next
- Next in Topographic map symbolsSpot elevationA small cross with a number beside it is a spot elevation: the height of that point, in the map’s units. There is no survey mark on the ground, which is what sets it apart from a benchmark.
- The same shape in Greek lettersQuantum circuit gate symbols (H, X, CNOT, measurement)The symbols of a quantum circuit diagram: each horizontal line is a qubit, a box with a letter is a gate (H for Hadamard, X, Y, Z, S, T), a dot joined by a line to ⊕ is a controlled-NOT, and a box with a meter is a measurement.
More topographic map symbols
The same shape elsewhere
Browse symbols drawn with crosses or letters and numbers.
Sources
- Topographic Map Symbols, USGS General Information Product (first posted 2005), U.S. Geological Survey, Control data and monuments
- Specifications to Support Classification, Standards of Accuracy, and General Specifications of Geodetic Control Surveys: Geodetic Bench Marks, NOAA Manual NOS NGS 1, Foreword, NOAA National Geodetic Survey, Foreword
- Benchmark or trig pillar: what’s in a name?, OS blog, 4 November 2022, Ordnance Survey, Benchmark or trig pillar
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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