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Survey markers, also called survey marks, survey monuments, or geodetic marks, are objects placed to mark key surveying points on the Earth's surface. They are used in geodesy and land surveying. A benchmark is a type of survey marker that indicates elevation (vertical position). Horizontal position markers used for triangulation are also known as triangulation stations. Benchmarking is the hobby of "hunting" for these marks.
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.
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),These marks were often set at roughly equal (120-degree) intervals from each other and were numbered in ascending order moving clockwise around the main station. 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 ploughs). Occasionally, these buried marks had surface marks set directly above them. Here is an account of recovering (re-finding) a buried station mark from 1890 for which the surface mark had been destroyed. This account also describes the use of reference marks to locate station marks.
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.ftp://ftp.ngs.noaa.gov/pub/uddf/info/Attach_10.pdf
Survey markers are now often used to set up a GPS receiver antenna in a known position for use in Differential GPS surveying.In the U.S., even though surveys carried out primarily by GPS are becoming more common, the network of physical survey markers continues to provide an important check on the accuracy of GPS-based methods, and one that is not dependent on satellites orbiting the Earth.
In Brazil, a similar database is operated by the Brazilian Institute of Geography and Statistics.
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