MapServer Help: USGS Topographic Map Symbols

Control Data and Monuments

Control points are positions on a map (and/or on the Earth's surface) from which accurate measurements can be made. A horizontal control point is a location where the coordinates (latitude/longitude) are certain. Elevation may or may not be determined. Vertical control is the accuracy standard for elevation. For vertical (elevation) accuracy, no more than 10 percent of a map's contour invervals may be in error beyond half a contour interval. Maps that fail to comply are candidates for complete revision.

A benchmark, abbreviated "BM," is a location whose elevation and horizontal position has been surveyed as accurately as possible. Benchmarks are designed for use as reference points, and are usually marked by small brass plates.

In the definitions below, "third order or better" is an indication of position accuracy. Third order is the most generous standard acceptable on a USGS topo map. In the third order classification, surveys must be permanently marked and adequately described.

The Provisional Edition map series was established to expedite the completion of the large-scale topographic quadrangle coverage of the conterminous United States. They contain essentially the same level of information as the standard series maps. (One important difference is that roads on Provisional Edition maps are not classified as primary, secondary, or light duty. They are all symbolized as light duty roads.) This series is identified by the title "Provisional Edition" in the lower right hand corner of the maps.

Horizontal Control
Horizontal control with permanent benchmark
A horizontal control point, third order or better, with a permanent benchmark.
Horizontal control with permanent benchmark and elevation
A horizontal control point, third order or better, with a permanent benchmark that also includes elevation.
Spot elevation
Checked spot elevation. Typically indicated by an X on a USGS map, Spot Elevation indicates a point's elevation relative to sea level. Spot elevations are often used to designate the highest point on a mountain or hill.
Coincident with section corner.
Coincident with section corner.
Unmonumented (on Provisional Edition map)
Unmonumented (Provisional Edition maps only). Indicates that no permanent marker has been placed.


Vertical Control
Third order or better, with tablet
Vertical control point, third order or better, with tablet. A tablet is similar to a benchmark in that it is placed to permanently mark an elevation and horizontal position that has been surveyed as accurately as possible. Unlike a benchmark, which is marked by a brass plate, the control point shown here is marked by a tablet: a concrete marker with numbers etched onto the top (like a gravestone).
Third order or better, recoverable mark
Vertical control point, third order or better, recoverable mark. A recoverable mark is a naturally occurring landmark. The term "recoverable" means that it can be readily located on the terrain: for example, a property boundary monument, a railroad intersection, the corner of a large building, etc. When a landmark occurs naturally, no tablet or benchmark is necessary to mark the position.
Benchmark found at corner section
Benchmark found at corner section.
Spot elevation
Spot elevation.


Boundary Monuments
Boundary monument with tablet
Boundary monument with tablet.
Boundary monument without tablet
Boundary monument without tablet.
Boundary monument with number and elevation, without tablet or benchmark
Boundary with number and elevation, and without tablet or benchmark.


Other Control Symbols
Aerial photograph roll and frame number (Provisional Edition maps only)
Aerial photograph roll and frame number (Provisional Edition maps only).
U.S. location or mineral monument
U.S. location or mineral monument. Fixed reference points supplementing regular public land survey monuments. The survey or mineral claim is tied to the location monument, which is later tied to the public land surveys; most of them are marked stones with rock cairns nearby.