A person taking a photo of a located benchmark on a statue
Benchmarking, also known as benchmark hunting,[1] is a hobby activity in which participants find benchmarks (also known as survey markers or geodetic control points). The term "benchmark" 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.[2] Like geocaching, the activity has become popular since 1995, propelled by the availability of online data on the location of survey marks (with directions for finding them) and by the rise of hobbyist-oriented websites.[3]
Many survey markers in the U.S. were set over 100 years ago.[4] There was a surge in creating these marks in the U.S. from about 1930 to 1955, in conjunction with the expansion of map-making activities across the country.
Sources of data on U.S. marks
In the U.S., about 740,000 benchmarks with precise elevations or coordinates (but only a small fraction of the existing survey marks) are listed in a database maintained by the National Geodetic Survey (NGS) and accessible online. The majority of marks are set by the United States Geological Survey (USGS), the United States Forest Service, the United States Army Corps of Engineers, cities, states, and local authorities. Cadastral (land survey) marks are usually not measured for the geodetic database.
Each NGS-listed mark has a permanent identifier (PID), a six-character code that can be used to call up data about that mark on the National Geodetic Survey website.[5] Other websites offer maps of the locations (and PIDs) of marks in each individual state of the U.S.[6] Until 2023, Geocaching.com had a section of its site devoted to benchmarking.[7]
Some of these marks have precise coordinates, including latitude and longitude, that are accurate to millimeter precision.[8] Others, typically true elevation benchmarks, have only coordinates that have been scaled from a map. Scaled coordinates, unlike surveyed ones, were read from a topographic map. While many are accurate to within 100 feet, some are located as far as 3,000 or 4,000 feet away from the mark they refer to. This makes it difficult to locate them using a handheld GPS unit. Most marks have clear descriptions for how to reach them, but some lack complete descriptions. Additionally, changes in the surrounding buildings, roads, or terrain over decades may make the descriptions obsolete, as some marks have been removed due to construction or buried over time.
Useful tools
In addition to the survey data sheet for a mark being hunted, many hunters bring along a digital camera to take close-up and area pictures of the survey mark (a disk, a cross-cut in a rock, an old copper bolt, etc.). These photographs, as well as a current description of the mark by the person who finds ("recovers") the location of the mark, can then be uploaded to a website as proof of a find. Surveyors use the term "recovering" as a synonym for "finding" a mark. This does not mean that the found mark should be disturbed in any way. On the contrary—disturbing a survey mark in even a small way often destroys its usefulness to surveyors and others. In the U.S., benchmark hunters often reports on marks they find to the NGS database. It describes the mark's found/not found status, photos, current condition, and updated directions to reach it so that others (particularly surveyors) can easily re-find the mark.
Benchmark hunters also frequently carry a compass (to follow directions gleaned from the datasheet), a probe (like a long-bladed screwdriver) to search for buried marks, a trowel (or a small shovel) to uncover buried marks, a whisk broom (to clear away debris), and one or more tape measures of various lengths, used in taping out referenced distances found on the data sheets.
Handheld GPS receivers are often used to get within a few yards probable error of a mark that has adjusted coordinates. Metal detectors are useful for finding marks that have become deeply buried over time. The use of several of the tools in combination is sometimes required, but many marks are set on the surface of sidewalks, buildings, walls, boulders, or monuments and can often be found without the need for special tools.
Survey markers vary considerably from one country to another. In the U.S., they are usually bronze discs about 3.5inches in diameter. A typical disk is slightly inset into the top of a concrete pillar set vertically with its surface flush with the ground or projecting slightly. Others are metal rods driven into the ground and possibly surrounded by a capped metal or plastic pipe that can be engraved and stamped like disks. Disks can also be set in rock ledges or boulders and in the concrete of a large structure such as a building, bridge abutment, or the base of a tower. In the UK, the mark is usually carved into a wall, or on a metal plate set into a wall.
In the U.S., a survey disk is usually engraved with the name of the agency that placed it. The name of the mark (or the "station" it locates) is usually stamped into its surface, along with the date on which it was set (or re-set, since markers that have been destroyed, can be replaced). Disks marked with a triangle are known as "triangulation station disks" since they mark the position of the primary point used for triangulation (or map-making). Disks marked with an arrow are called reference marks (RMs) since they point (or "refer") to the principal station that may be located many feet away. A triangulation station often had two or three reference marks. Reference marks were set to enable the primary station to be re-established (or re-set) if needed.
Sometimes a survey mark is made much easier to find by the presence nearby of a witness post, a stake (or a small sign) driven into the ground, and used to draw attention to (and to warn against disturbing) the mark.
Care must be taken to ensure that a found disk is the one described on the data sheet, so as to avoid false reports. A reference mark may be mistakenly reported as the station mark. A disk set in 1945 may be confused with a similarly named disk that was set in 1946 by a different agency, and so on. A reset elevation benchmark should not be reported as the original. Sorting out these differences and reporting them correctly is an important part of the hobby.
Typical locations
In the U.S., markers are often found at the summits of mountains, along ridge lines, or on bare rock ledges with commanding views, because such sites provide good vantage points for triangulation lines to distant points.
U.S. elevation benchmarks were often placed along rail lines or roads that provided good sight lines for leveling.
A common location is the top level of the abutment of a highway bridge or its wing walls.
As part of triangulation networks, intersection stations were sighted. They are typically tall, prominent, and well-defined points like a smokestack, the peak of a water tower, or a church spire. However, many of these objects have been altered or replaced by similar nearby structures and no longer mark the original location, so careful identification is required.
U.S. benchmarks were commonly placed on public buildings, such as courthouses, post offices, city halls, and older schools.
Benchmarks are often located on private property, or access to them can only be achieved by crossing privately owned land, so permission is needed for access.
UK benchmarks tend to be on the corners of pubs, churches, and other public buildings, as well as farm buildings, railway bridges, and private houses especially those near a road junction. However, any building may be used, as well as natural features such as a rock outcrop.
In the United Kingdom, "trig-pointing" is a recreational activity similar to benchmarking. Searching for trig points is more popular than hunting benchmarks in the UK, but there is considerable overlap in participation. "Trig points" is the common name for "triangulation pillars". These are concrete pillars, about 4 feet tall, which were used by the Ordnance Survey in order to determine the exact shape of the country in a project known as the retriangulation of Great Britain, which was carried out from 1936 to 1962. They are generally located on the highest bit of ground in the area so that there is a direct line of sight from one to the next. By setting a theodolite on the top of the pillar, accurate bearings to nearby trig points could be taken. This process is called triangulation.
There are 6550 such pillars listed in the T:UK database. While most of them have fallen into disuse, about 184 of them are currently used in the Passive Station network. These are maintained so they can be used as accurately located anchor points for differential GPS comparisons and are re-surveyed every five years to calibrate for any geological movements of the ground.
In popular culture
The cartoon character Marvin Marker is based on a benchmark and was created to help preserve benchmarking in 1960.[9]
Related Research Articles
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.
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 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 - specifically a broad arrow - below the horizontal line. A benchmark is a type of survey marker.
Survey markers, also called survey marks, survey monuments, or geodetic marks, are objects placed to mark key survey points on the Earth's surface. They are used in geodetic and land surveying. A benchmark is a type of survey marker that indicates elevation. Horizontal position markers used for triangulation are also known as triangulation stations. Benchmarking is the hobby of "hunting" for these marks.
↑ "History of Survey Markers". Mountain Monkey Company.com. Archived from the original on January 24, 2022. Retrieved December 17, 2021. The original mandate of the Survey of the Coast in 1807 was to survey the coast of the United States in order to promote safe maritime commerce.
↑ "NGS Datasheet Sample PIDS Choice". National Geodetic Survey. mark with PID AH0147. Retrieved May 5, 2024. See the network accuracy estimates per FGDC Geospatial Positioning Accuracy Standards.
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