Survey markers, also called survey marks, and sometimes geodetic marks, are objects placed to mark key survey

Surveying

See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...

 points on the Earth's surface. They are used in geodetic

Geodesy

Geodesy , also named geodetics, a branch of earth sciences, is the scientific discipline that deals with the measurement and representation of the Earth, including its gravitational field, in a three-dimensional time-varying space. Geodesists also study geodynamical phenomena such as crustal...

 and land surveying

Surveying

See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...

. Informally, such marks are referred to as benchmarks

Benchmark (surveying)

The term bench mark, or benchmark, originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle-iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future...

, although strictly speaking the term "benchmark" is reserved for marks that indicate elevation. Horizontal position markers used for triangulation

Triangulation

In trigonometry and geometry, triangulation is the process of determining the location of a point by measuring angles to it from known points at either end of a fixed baseline, rather than measuring distances to the point directly...

 are also known as trig point

Trig point

A triangulation station, also known as a triangulation pillar, trigonometrical station, trigonometrical point, trig station, trig beacon or trig point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity...

s or triangulation stations.
All sorts of different objects, ranging from the familiar brass disks to liquor bottles, clay pots, and rock cairn

Cairn

Cairn is a term used mainly in the English-speaking world for a man-made pile of stones. It comes from the or . Cairns are found all over the world in uplands, on moorland, on mountaintops, near waterways and on sea cliffs, and also in barren desert and tundra areas...

s, have been used over the years as survey markers. Some truly monumental markers have been used to designate tripoint

Tripoint

A tripoint, or trijunction , is a geographical point at which the borders of three countries or subnational entities meet....

s, 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 chiseled 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, sunken into 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.

These marks 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 in the United States and 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" (see upper photo at left), 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 (usually three) reference marks(see second photo at left), each of which bore an arrow that pointed back towards the main station. These reference marks made it easier for later visitors to "recover" (or re-find) the primary ("station") mark. Reference marks also made it possible to replace (or reset) a station mark that had been disturbed or destroyed.

Some old station marks were buried several feet down (to protect them from being struck by plows). Occasionally, these buried marks had surface marks set directly above them.

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 (NGS). Each station mark in the database has a PID (Permanent IDentifier), 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 (with their distance and bearing from the station mark), and a narrative (which is updated over the years) describing other reference features (e.g., buildings, roadways, trees, or fire hydrants) and the distance and/or direction of these features from the marks, and giving a history of past efforts to recover (or re-find) these marks (including any resets of the marks, or evidence of their damage or destruction).

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 GPS

Differential GPS

Differential Global Positioning System is an enhancement to Global Positioning System that provides improved location accuracy, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations....

surveying.

External links

No website will allow for searching of USGS Marks. Going to this link will allow you to download *DSWorld to search for NGS PID's in a variety of methods, and imports the marks directly into google earth for fast and easy visual searching of NGS Marks.

• This website provides a state-by-state mapping of NGS survey marks on to Google Maps, enabling one to search for these marks visually. Links to the marks' listings on Geocaching.com are also provided.