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Benchmarking (geolocating)
Encyclopedia
Benchmarking is an activity in which individuals or teams of participants go out and find benchmarks
(also known as survey marker
s or geodetic control points). They typically then log their finds online. Like geocaching
, the activity has become popular since about 1995, propelled by the availability of on-line data on the location of survey marks (with directions for finding them) and by the rise of hobbyist-oriented websites (for example, in the U.S., Geocaching.com).
Many survey markers in the U.S. were set over 100 years ago. There was also 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. In the U.S. some of these marks (triangulation stations and GPS points, loosely also referred to as "benchmarks") have precise "adjusted" coordinates (latitude and longitude), while others, typically true elevation Bench Marks, have only coordinates scaled from a map. Some marks were established with clear descriptions on how to reach them, while others were much more poorly documented. Over time, some marks have become buried beneath the ground, or surrounded by dense woodlands. Old-time "references" (e.g., the corner of a nearby building or a distinctive tree) have sometimes been destroyed altogether. Sometimes the mark being hunted is found to have been paved over by a parking lot or blown to bits as part of a construction project.
But this diversity is often what makes people enjoy the hobby of benchmark hunting. Some hunters stick to more well documented marks, and have an easier time of it. Others go looking for more poorly described marks, and may spend hours and hours in their searches (bushwacking through difficult back country) and return to a site again and again, often without finding their quarry.
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.
Benchmark hunters also frequently carry a compass (to follow directions gleaned from the data sheet), 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 find the general location of a mark that has adjusted coordinates, since they can often locate a precise latitude and longitude within a 10-foot circle of probable error
. Some searchers also use metal detectors, which are sometimes essential in locating older marks which have become deeply buried over time (or were set underground in the first place).
None of these tools, however, can be relied upon to find every mark every time, and their use in combination is often required to recover many marks. On the other hand, many marks are set right on the surface of sidewalks, walls, boulders, or monuments, and these can often be found without any special tools at all.
Survey marker
s vary considerably from one country to another. In the U.S they are usually brass discs about 3.5 inches in diameter. A typical disk is often slightly inset into the top of a concrete pillar several feet tall which is buried vertically in the ground, with its surface flush with the ground (or projecting slightly). Disks can also be set in rock ledges or in large boulders, and are sometimes set directly in the concrete of a large structure such as a bridge abutment, the base of a high-tension electric tower, a road, or a sidewalk. 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.
Sometimes (in the U.S.) a metal stake or rod is driven in the ground and used as a mark. The stake may be in the ground by itself, or may be surrounded with a small plastic or metal tube covered with a lid, known as a "cap." The cap may be stamped just like a survey disk. And 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.
Given all the different types of survey marks and the various stampings they can carry, benchmark hunters sometimes get confused into making 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.
lines to distant points. Today, it means that benchmark hunting can often take the hunter to scenic vistas of breathtaking beauty.
U.S. benchmarks are also often found at railroad stations, or along railroad rights of way, often within 25 feet of a rail. Sometimes they have been buried by the crushed rock used as ballast for the tracks. BE VERY CAREFUL, HOWEVER! In the U.S., active and abandoned railroad rights of way are often private property, actively patrolled by railroad police. And for good reason—railroad rights of way can be very dangerous places to do benchmark hunting!
Benchmarks can be in a variety of places in and around old bridges. One common location is the top level of a bridge abutment (the concrete beam carrying the weight of the bridge) or in the trestle of the bridge. The benchmark may also be in the bridge structure itself, or be set in the actual roadbed or sidewalk of the bridge.
In the U.S., a special type of benchmark is called an intersection station. This is a tall, prominent and well-defined point (like a smokestack, the peak of a water tower, or a church spire) that is visible for miles around. In this case, the tall structure itself is defined as the benchmark. In these situations, if the structure can can be seen, it counts as a find. One does have to be careful that the water tower found, for example, is the same one that existed at the time the mark was set (which is often hard to determine).
Another common place to find U.S. benchmarks is at county court houses, either embedded on the building itself or nearby on the lawn. Old post offices, city halls, and older schools are also commonplace for benchmarks. In the U.K., 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 (and sometimes natural features, such as a rock outcrop).
A final point should also be kept in mind. Benchmarks are often located on private property, or access to them can only be achieved by crossing privately owned land. It is very important that property owners' permission be obtained before entering their property. Hobbyists should also be aware that many owners are leery of "surveyors" or of people they believe might be involved in redefining their property.
s is more popular than hunting benchmarks
in the UK, but there is a considerable overlap in participation. Trig points are 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.
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...
(also known as survey marker
Survey marker
Survey markers, also called survey marks, and sometimes 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...
s or geodetic control points). They typically then log their finds online. Like geocaching
Geocaching
Geocaching is an outdoor sporting activity in which the participants use a Global Positioning System receiver or mobile device and other navigational techniques to hide and seek containers, called "geocaches" or "caches", anywhere in the world....
, the activity has become popular since about 1995, propelled by the availability of on-line data on the location of survey marks (with directions for finding them) and by the rise of hobbyist-oriented websites (for example, in the U.S., Geocaching.com).
Many survey markers in the U.S. were set over 100 years ago. There was also 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. In the U.S. some of these marks (triangulation stations and GPS points, loosely also referred to as "benchmarks") have precise "adjusted" coordinates (latitude and longitude), while others, typically true elevation Bench Marks, have only coordinates scaled from a map. Some marks were established with clear descriptions on how to reach them, while others were much more poorly documented. Over time, some marks have become buried beneath the ground, or surrounded by dense woodlands. Old-time "references" (e.g., the corner of a nearby building or a distinctive tree) have sometimes been destroyed altogether. Sometimes the mark being hunted is found to have been paved over by a parking lot or blown to bits as part of a construction project.
But this diversity is often what makes people enjoy the hobby of benchmark hunting. Some hunters stick to more well documented marks, and have an easier time of it. Others go looking for more poorly described marks, and may spend hours and hours in their searches (bushwacking through difficult back country) and return to a site again and again, often without finding their quarry.
How to find data on the (U.S.) marks
In the U.S., about 740,000 "benchmarks" with the most 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 on-line. Each NGS-listed mark has a permanent identifier (PID), a six-character code that can be used to call up data about that mark. Using a form for an internet query like this, the PID for the mark can be entered and a data sheet for the mark can be viewed on-line or printed off. A data sheet obtained through such a query looks like this. There is also a website which uses Google Maps to show the locations (and PIDs) of marks in each individual state of the U.S. These tools are often enough to provide basic data to support the benchmark hunter, but specialized hobbyist websites (like Geocaching.com) FAQ and its Benchmark Hunting forum can provide even more information.Useful Tools
Benchmark hunters also frequently carry a compass (to follow directions gleaned from the data sheet), 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 find the general location of a mark that has adjusted coordinates, since they can often locate a precise latitude and longitude within a 10-foot circle of probable error
Probable error
-Statistics:In statistics, the probable error of a quantity is a value describing the probability distribution of that quantity. It defines the half-range of an interval about a cental point for the distribution, such that half of the values from the distribution will lie within the interval and...
. Some searchers also use metal detectors, which are sometimes essential in locating older marks which have become deeply buried over time (or were set underground in the first place).
None of these tools, however, can be relied upon to find every mark every time, and their use in combination is often required to recover many marks. On the other hand, many marks are set right on the surface of sidewalks, walls, boulders, or monuments, and these can often be found without any special tools at all.
What To Look For
Survey marker
Survey markers, also called survey marks, and sometimes 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...
s vary considerably from one country to another. In the U.S they are usually brass discs about 3.5 inches in diameter. A typical disk is often slightly inset into the top of a concrete pillar several feet tall which is buried vertically in the ground, with its surface flush with the ground (or projecting slightly). Disks can also be set in rock ledges or in large boulders, and are sometimes set directly in the concrete of a large structure such as a bridge abutment, the base of a high-tension electric tower, a road, or a sidewalk. 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.
Sometimes (in the U.S.) a metal stake or rod is driven in the ground and used as a mark. The stake may be in the ground by itself, or may be surrounded with a small plastic or metal tube covered with a lid, known as a "cap." The cap may be stamped just like a survey disk. And 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.
Given all the different types of survey marks and the various stampings they can carry, benchmark hunters sometimes get confused into making 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.
Where the Marks Are Often Found
In the U.S., benchmarks are often found at the summits of mountains, along ridge lines, or on bare rock ledges with commanding views of their surroundings. The reason for this is straight forward: such sites provided good vantage points for surveyors who were seeking to run triangulationTriangulation
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...
lines to distant points. Today, it means that benchmark hunting can often take the hunter to scenic vistas of breathtaking beauty.
U.S. benchmarks are also often found at railroad stations, or along railroad rights of way, often within 25 feet of a rail. Sometimes they have been buried by the crushed rock used as ballast for the tracks. BE VERY CAREFUL, HOWEVER! In the U.S., active and abandoned railroad rights of way are often private property, actively patrolled by railroad police. And for good reason—railroad rights of way can be very dangerous places to do benchmark hunting!
Benchmarks can be in a variety of places in and around old bridges. One common location is the top level of a bridge abutment (the concrete beam carrying the weight of the bridge) or in the trestle of the bridge. The benchmark may also be in the bridge structure itself, or be set in the actual roadbed or sidewalk of the bridge.
In the U.S., a special type of benchmark is called an intersection station. This is a tall, prominent and well-defined point (like a smokestack, the peak of a water tower, or a church spire) that is visible for miles around. In this case, the tall structure itself is defined as the benchmark. In these situations, if the structure can can be seen, it counts as a find. One does have to be careful that the water tower found, for example, is the same one that existed at the time the mark was set (which is often hard to determine).
Another common place to find U.S. benchmarks is at county court houses, either embedded on the building itself or nearby on the lawn. Old post offices, city halls, and older schools are also commonplace for benchmarks. In the U.K., 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 (and sometimes natural features, such as a rock outcrop).
A final point should also be kept in mind. Benchmarks are often located on private property, or access to them can only be achieved by crossing privately owned land. It is very important that property owners' permission be obtained before entering their property. Hobbyists should also be aware that many owners are leery of "surveyors" or of people they believe might be involved in redefining their property.
In the UK
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.Trigpointing
In the United Kingdom, trigpointing is a recreational activity similar to benchmarking. Searching for trig pointTrig 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 is more popular than hunting 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...
in the UK, but there is a considerable overlap in participation. Trig points are the common name for "triangulation pillars". These are concrete pillars, about 4 feet tall, which were used by the Ordnance Survey
Ordnance Survey
Ordnance Survey , an executive agency and non-ministerial government department of the Government of the United Kingdom, is the national mapping agency for Great Britain, producing maps of Great Britain , and one of the world's largest producers of maps.The name reflects its creation together with...
in order to determine the exact shape of the country in a project known as the retriangulation of Great Britain
Retriangulation of Great Britain
The retriangulation of Great Britain was a triangulation project which involved erecting concrete pillars on prominent hilltops throughout 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
Theodolite
A theodolite is a precision instrument for measuring angles in the horizontal and vertical planes. Theodolites are mainly used for surveying applications, and have been adapted for specialized purposes in fields like metrology and rocket launch technology...
on the top of the pillar, accurate bearings to nearby trig points could be taken. This process is called 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...
.
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
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....
comparisons, and are re-surveyed every five years to calibrate for any geological movements of the ground.
External links
- Yahoo! Group devoted to the study of the Ordnance Survey triangulation network.
- The benchmark hunting portion of the Geocaching.com website.
- Live UK Benchmark Database, primarily focusing on flush bracket benchmarks
- UK Ordnance Survey benchmark source database, not updated since 1975
- UK Trigpoint Database
- Waymarking - Benchmarks
- Benchmarks overlaid on a Google Map
- http://www.newenglandbenchmarks.com