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. [1] Some 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 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, 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. [2]
Survey markers in Nagoya, Japan, which bear stylized images of shachihoko, are noted for their elaborate design. [3]
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" (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), [4] 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. [5]
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 (NGS), the successor agency to the United States Coast and Geodetic Survey (USC&GS). 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. [6] The NGS has a web-based form [7] that can be used to access any datasheet, if the station's PID is known. Alternatively, datasheets can be called up by station name. [8]
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. [9]
Survey markers are now often used to set up a GPS receiver antenna in a known position for use in Differential GPS surveying. [10]
In Brazil, a similar database is operated by the Brazilian Institute of Geography and Statistics. [11]
Geodesy is the science of measuring and representing the geometry, gravity, and spatial orientation of the Earth in temporally varying 3D. It is called planetary geodesy when studying other astronomical bodies, such as planets or circumplanetary systems.
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is called a land surveyor. These points are usually on the surface of the Earth, and they are often used to establish maps and boundaries for ownership, locations, such as the designed positions of structural components for construction or the surface location of subsurface features, or other purposes required by government or civil law, such as property sales.
The National Geodetic Survey (NGS) is a United States federal agency based in Washington, D.C. that defines and manages a national coordinate system, providing the foundation for transportation and communication, mapping and charting, and a large number of science and engineering applications. Since its founding in 1970, it has been part of the National Oceanic and Atmospheric Administration (NOAA), a division within the United States Department of Commerce.
A geodetic datum or geodetic system is a global datum reference or reference frame for precisely representing the position of locations on Earth or other planetary bodies by means of geodetic coordinates. Datums are crucial to any technology or technique based on spatial location, including geodesy, navigation, surveying, geographic information systems, remote sensing, and cartography. A horizontal datum is used to measure a location across the Earth's surface, in latitude and longitude or another coordinate system; a vertical datum is used to measure the elevation or depth relative to a standard origin, such as mean sea level (MSL). Since the rise of the global positioning system (GPS), the ellipsoid and datum WGS 84 it uses has supplanted most others in many applications. The WGS 84 is intended for global use, unlike most earlier datums.
The Meades Ranch Triangulation Station is a survey marker in Osborne County in the state of Kansas in the Midwestern United States. The marker was initially placed in 1891. From 1901, it was the reference location for establishing a system of horizontal measurement in the United States, known as geodetic datum. In 1913, the datum was adopted across all of North America, and the system revised and formalized as the North American Datum of 1927 (NAD27). A similar reference for vertical measurement was established in 1929 as the National Geodetic Vertical Datum of 1929. The NAD27 was later supplanted by the North American Datum of 1983 (NAD83), which was formally adopted by the United States in 1989 and Canada in 1990; the new system moved the reference point to a point in the Earth's core, and the Meades Ranch marker lost its special significance to the geodetic datum system.
Satellite geodesy is geodesy by means of artificial satellites—the measurement of the form and dimensions of Earth, the location of objects on its surface and the figure of the Earth's gravity field by means of artificial satellite techniques. It belongs to the broader field of space geodesy. Traditional astronomical geodesy is not commonly considered a part of satellite geodesy, although there is considerable overlap between the techniques.
A triangulation station, also known as a trigonometrical point, and sometimes informally as a trig, is a fixed surveying station, used in geodetic surveying and other surveying projects in its vicinity. The nomenclature varies regionally: they are generally known as trigonometrical stations or triangulation stations in North America, trig points in the United Kingdom, trig pillars in Ireland, trig stations or trig points in Australia and New Zealand, and trig beacons in South Africa.
The Zero Milestone is a zero mile marker monument in Washington, D.C., intended as the initial milestone from which all road distances in the United States should be measured when it was built. At present, only roads in the Washington, D.C., area have distances measured from it.
The term benchmark, bench mark, or survey 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. These marks were usually indicated with a chiseled arrow below the horizontal line. A benchmark is a type of survey marker.
A geodetic control network is a network, often of triangles, which are measured precisely by techniques of control surveying, such as terrestrial surveying or satellite geodesy.
Benchmarking, also known as benchmark hunting, is a hobby activity in which participants find benchmarks. The term "bench mark" is used only to refer to survey markers that designate a certain elevation, but hobbyists often use the term benchmarks to include triangulation stations or other reference marks. They typically then log their finds online. Like geocaching, the activity has become popular since 1995, propelled by the availability of online data on the location of survey marks and by the rise of hobbyist-oriented websites.
The North American Datum (NAD) is the horizontal datum now used to define the geodetic network in North America. A datum is a formal description of the shape of the Earth along with an "anchor" point for the coordinate system. In surveying, cartography, and land-use planning, two North American Datums are in use for making lateral or "horizontal" measurements: the North American Datum of 1927 (NAD 27) and the North American Datum of 1983 (NAD 83). Both are geodetic reference systems based on slightly different assumptions and measurements.
The Earth-centered, Earth-fixed coordinate system, also known as the geocentric coordinate system, is a cartesian spatial reference system that represents locations in the vicinity of the Earth as X, Y, and Z measurements from its center of mass. Its most common use is in tracking the orbits of satellites and in satellite navigation systems for measuring locations on the surface of the Earth, but it is also used in applications such as tracking crustal motion.
The North American Vertical Datum of 1988 is the vertical datum for orthometric heights established for vertical control surveying in the United States of America based upon the General Adjustment of the North American Datum of 1988.
Stateline is a former unincorporated community in El Dorado County, California, now incorporated into South Lake Tahoe, California. It lies at an elevation of 6,279 feet ASL. As its name suggests, it is at the state line with Nevada, and mirrors the adjacent community of the same name.
The National Spatial Reference System (NSRS), managed by the National Geodetic Survey (NGS), is a coordinate system that includes latitude, longitude, elevation, and other values. The NSRS consists of a National Shoreline, the NOAA CORS Network, a network of permanently marked points, and a set of models that describe dynamic geophysical processes affecting spatial measurements. The system is based on NAD 83 and NAVD 88.
The geographic center of the United States is a point approximately 20 mi (32 km) north of Belle Fourche, South Dakota at 44°58′2.07622″N103°46′17.60283″W. It has been regarded as such by the United States Coast and Geodetic Survey and the U.S. National Geodetic Survey (NGS) since the additions of Alaska and Hawaii to the United States in 1959.
