Scattered across every USGS quad are small Xs, triangles, and the letters BM followed by a number. These are survey control points: places where surveyors measured an exact elevation or position, often leaving a bronze disk set in rock or concrete. They're the most precise elevations on the map, and finding the disk on a summit is a classic hiker's ritual.
What does BM mean on a topo map?
BM stands for bench mark: a surveyed point with a precisely measured elevation, usually marked on the ground with a metal disk called a tablet. The number next to it is the elevation, in feet on most USGS maps.
IN THIS GUIDE
Benchmarks and spot elevations · Triangulation and horizontal control stations · Boundary monuments and other markers · Finding and using benchmarks · FAQ
Benchmarks and spot elevations
These are vertical control points: places with a measured elevation. "Third-order or better" refers to the accuracy class of the survey. Third order is the minimum standard for these symbols.
Third-order or better elevation, with tablet
An X labeled BM with an elevation. A bench mark with a metal tablet set in the ground. Look for it on summits, along old roads, and at bridges.
Third-order or better elevation, recoverable mark, no tablet
An X with an elevation but no BM label. The point is marked by something durable, like a chiseled cross or a bolt, but has no tablet.
Bench mark coincident with found section corner
A bench mark at a red section corner. The same marker serves as both a land survey corner and an elevation point.
Spot elevation
A small X with an elevation. The elevation was measured, but nothing was left on the ground. Spot elevations are common on summits, saddles, lakes, and road junctions.
Triangulation and horizontal control stations
Triangles mark horizontal control points, whose exact positions were fixed by survey. Many sit on mountaintops, where surveyors could see neighboring stations.
Third-order or better, permanent mark
A triangle with the station's name, here Neace. A permanent survey mark, usually a disk, whose position is precisely known.
With third-order or better elevation
A triangle with BM and an elevation: a station whose position and elevation were both surveyed.
With checked spot elevation
A triangle with an elevation: a known position with a checked, but less precise, elevation.
Coincident with found section corner
A triangle sitting on red section lines: the station is also a found land survey corner.
Unmonumented Provisional maps
A plain cross on provisional-edition maps: a surveyed position with no permanent mark.
Boundary monuments and other markers
Small squares mark monuments set along boundaries. Some double as benchmarks.
Third-order or better elevation, with tablet
A small square with BM and an elevation: a boundary monument that's also a bench mark with a tablet.
Third-order or better elevation, recoverable mark, no tablet
A square with an elevation: a boundary monument with a surveyed elevation but no tablet.
With number and elevation
A square with two numbers: the monument's number (67) and its elevation (4567).
U.S. mineral or location monument
A black triangle labeled USMM and a number. A U.S. mineral monument, used as a reference point for mining claim surveys in the West.
River mileage marker
A cross labeled with a river mile, on large rivers. River miles are measured along the channel and used by boaters and river managers.
Principal point Provisional maps
A circle with a cross and a photo number, on provisional-edition maps. Marks the center point of an aerial photo used to make the map.
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Topo Map Reading, part 6: Map Symbols
Contour lines, benchmarks, rivers, streams, lakes, swamps, roads, trails, buildings, and campgrounds in under a minute.
Watch the full Topo Map Reading series →Finding and using benchmarks
- Look them up before you go. The National Geodetic Survey publishes descriptions of many survey marks, often with directions to find them. Search for the NGS Data Explorer.
- Leave them where they are. Survey markers are protected by law. Photograph the disk, but don't dig it up, pry it loose, or take it home.
- Use them to check your altimeter. A benchmark or spot elevation on your route is the best place to recalibrate a watch or GPS altimeter.
- Expect small differences. Older maps use an older vertical datum (NGVD 29), so elevations may differ by a few feet from newer maps and GPS readings.
PUT IT TO USE
See these symbols on a real map
Frequently asked questions
What's the difference between a benchmark and a spot elevation?
A benchmark (BM) is marked on the ground, usually with a metal tablet, and has a precisely surveyed elevation. A spot elevation is a measured height with no marker on the ground.
What does a triangle mean on a topo map?
A small triangle is a horizontal control station, also called a triangulation station: a point whose exact position was surveyed, often on a summit. It's usually labeled with the station's name.
Can you find benchmarks in the field?
Often, yes. Many bronze disks are still in place on summits, bridges, and rock outcrops. NGS descriptions can help you find them, but some have been destroyed or buried.
Are benchmark elevations in feet or meters?
Most traditional USGS quads give elevations in feet. Check the map margin, since some newer and metric maps use meters.
MORE SYMBOL GUIDES
Contour Lines on Topo Maps: Index, Intermediate & Depression Contours · Township, Range & Section Lines on Topo Maps · Map Grids on Topo Maps: Latitude, Longitude, UTM & State Plane
← Back to all USGS map symbols
Symbol artwork from the U.S. Geological Survey Topographic Map Symbols brochure (public domain).