Survey marker

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Survey markers are objects placed on the Earth to mark a point with known coordinates. They are used in surveying and geodesy to identify exact locations. A marker can also be called a mark, monument, or station. A benchmark is a type of marker that indicates elevation. Horizontal-position markers used for triangulation are also known as triangulation stations. Markers that form a network are called a geodetic network. An active or reference station is a marker that stays in place with measurement instruments such as GPS or GNSS receivers.

Markers come in many forms, from brass disks to cairns or decorative designs. In the United States, the common markers are cast metal disks set into rock, embedded in concrete pillars, or attached to pipes sunk into the ground. They are intended to be permanent, and disturbing them is usually illegal. Some markers in other places have distinctive designs.

History and use: Markers were placed during triangulation surveys to map large areas and create accurate maps. A main station is usually surrounded by several reference marks to help future researchers find and recover the main marker if it gets disturbed.

Database and how they’re used: In the US, a national database kept by the National Geodetic Survey lists markers that meet accuracy standards. Each station has a Permanent Identifier (PID). Datasheets describe the marker’s exact coordinates (adjusted/precise or scaled/estimated), its reference marks, and notes about nearby features and past recoveries or changes. Modern markers typically use a punched-in metal rod with a grease-filled sleeve and a concrete cap for stability. These markers still help position GPS equipment for differential GPS surveys.

Other countries: Brazil also maintains a similar geodetic-marker database.

See also: Benchmark (elevation reference), boundary marker, milestone, triangulation station.