Survey markers, also called survey marks, survey monuments, survey benchmarks or geodetic marks, are objects placed to mark key survey points on the Earth's surface. They are used in geodetic and land surveying. Informally, such marks are referred to as benchmarks, although strictly speaking the term "benchmark" is reserved for marks that indicate elevation. Horizontal position markers used for triangulation are also known as triangulation stations.
Types
All sorts of different objects, ranging from the familiar brass disks to liquor bottles, clay pots, and rock cairns, have been used over the years as survey markers. Some truly monumental markers have been used to designate tripoints, or the meeting points of three or more countries. In the 19th century, these marks were often drill holes in rock ledges, crosses or triangles chiselled in rock, or copper or brass bolts sunk into bedrock. Today in the United States, the most common geodetic survey marks are cast metal disks with stamped legends on their face set in rock ledges, embedded in the tops of concrete pillars, or affixed to the tops of pipes that have been sunk into the ground. These marks are intended to be permanent, and disturbing them is generally prohibited by federal and state law. Survey markers in Nagoya, Japan, which bear stylized images of shachihoko, are noted for their elaborate design.
History
Survey markers were often placed as part of triangulation surveys, measurement efforts that moved systematically across states or regions, establishing the angles and distances between various points. Such surveys laid the basis for map-making across the world. Geodetic survey markers were often set in groups. For example, in triangulation surveys, the primary point identified was called the triangulation station, or the "main station". It was often marked by a "station disk", a brass disk with a triangle inscribed on its surface and an impressed mark that indicated the precise point over which a surveyor's plumb-bob should be dropped to assure a precise location over it. A triangulation station was often surrounded by several reference marks, each of which bore an arrow that pointed back towards the main station. These reference marks made it easier for later visitors to "recover" the primary mark. Reference marks also made it possible to replace a station mark that had been disturbed or destroyed. Some old station marks were buried several feet down. Occasionally, these buried marks had surface marks set directly above them.
Database
In the U.S., survey marks that meet certain standards for accuracy are part of a national database that is maintained by the National Geodetic Survey. Each station mark in the database has a PID, a unique 6-character code that can be used to call up a datasheet describing that station. The NGS has a web-based form that can be used to access any datasheet, if the station's PID is known. Alternatively, datasheets can be called up by station name. A typical datasheet has either the precise or the estimated coordinates. Precise coordinates are called "adjusted" and result from precise surveys. Estimated coordinates are termed "scaled" and have usually been set by locating the point on a map and reading off its latitude and longitude. Scaled coordinates can be as much as several thousand feet distant from the true positions of their marks. In the U.S., some survey markers have the latitude and longitude of the station mark, a listing of any reference marks, and a narrative describing other reference features and the distance and/or direction of these features from the marks, and giving a history of past efforts to recover these marks. Current best practice for stability of new survey markers is to use a punch mark stamped in the top of a metal rod driven deep into the ground, surrounded by a grease filled sleeve, and covered with a hinged cap set in concrete. Survey markers are now often used to set up a GPS receiver antenna in a known position for use in Differential GPSsurveying. In Brazil, a similar database is operated by the Brazilian Institute of Geography and Statistics.
