Benchmarking (geolocating)

Benchmarking is an activity in which individuals or teams of participants go out and find benchmarks (also known as "geodetic control points"). They typically then log their finds online. Like geocaching, the activity has mainly become popular in the early years of the 21st century.

Benchmarks are no longer created or maintained, and most of them have been lost to redevelopment, so many users use a GPS receiver to help them locate the supposed position of a benchmark, but it is also possible to find them without using GPS technology.

Required Materials

Nothing is required to start benchmarking except an internet connection to look up the survey data (location, coordinates, directions) of each benchmark. The location of markers is generally found either online or through survey documents, stored at nearby libraries or courthouses. Many people also carry a camera to record a particular find. The photographs, as well as a description of how the mark can be found, can then be uploaded to a website as proof of a find. Most people (or those wishing for a slightly easier find) will sometimes use GPS receivers or even metal detectors in order to locate the marks. In addition, range or distance finders are sometimes used to determine distances prescribed in the original directions to each benchmark.

What to Look For

Survey markers vary considerably from one country to another. In the USA they are usually metal discs about four inches in diameter. A typical disk may be slightly domed, but it is normally set flush in a concrete monument. These monuments can be more than a foot tall, but most are a simple mound of concrete in the ground that resembles a large anthill. In the UK, the mark is usually carved into a wall, or on a metal plate set into a wall. A marker can also be set directly in the concrete of a large structure such as a bridge abutment, road, or sidewalk. Sometimes a marker will be set in a hole in the ground, a result of a change in the surrounding landscape from when the mark was established. The markers themselves are engraved with the name of the survey group that placed the mark, the reference name of the mark for documentation, and a small cross in the exact center of the mark for surveying purposes. Sometimes the disc will include information about the altitude of the benchmark, as well as a warning against removal of the mark. Sometimes 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 and covered with a lid, known as a "cap." The cap is usually, but not always, stamped with the name of the surveying group that set the benchmark.

Finding the Marks

Every benchmark is unique, but there are some places where benchmarks are often found.

Railroads

In the US, benchmarks will often be located at railroad depot stations, or along and slightly below the level of the track bed. In cases where the track has been removed, check to see that the track bed in the area near the suspected location of the benchmark has not been overly disturbed or dug-up. If it has, it is possible that the benchmark was buried when the rails were taken out and a metal detector may be required to find the mark. In many cases, the mark will have been destroyed.

Bridges

Benchmarks can be in a variety of places in and around old bridges. One common location is the top level of a bridge abutment, or bridge support. The benchmark may also be in the bridge structure itself, or be set in the actual roadbed of the bridge.

mokestacks, Antennas, Steeples, and Towers

In the US, a benchmark will sometimes be described as a smokestack or tower. The tall structure itself will be considered the benchmark, and survey reference is made from the highest point of the structure, even though there is not a marker there. In these situations, if an object can be seen, it counts as a find.

County Court Houses & other public places

Another common place to find 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 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, and sometimes natural features such as a rock outcrop.

Trigpointing

In the United Kingdom, trigpointing is an analogous recreational activity to benchmarking. Participants search for trig points rather than benchmarks. Trig points are the common name for "triangulation pillars". These are concrete pillars, about 4' 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, being re-surveyed every five years to calibrate for any geological movements of the ground.

GPS Receiver Issues

One problem with using a GPS receiver to assist the search is that vertical control benchmarks typically have scaled latitude and longitude coordinates, while horizontal control benchmarks have adjusted latitude and longitude coordinates.

If the stated coordinates are described as SCALED, that means the coordinates were scaled from a grid-system map and are often not exact. If the stated coordinates are described as ADJUSTED, it means that very precise equipment such as a commercial grade GPS receiver was used in figuring the coordinates and therefore the given position is extremely accurate, usually within a centimeter.

Therefore, if one feeds the given coordinates into a GPS receiver, the result could be anywhere from totally accurate to more than one hundred yards inaccurate, and one must remain aware whether a particular benchmark's coordinates are scaled or adjusted.

The elevation of a vertical control benchmark is usually extremely accurate as they use very precise equipment such as gravimeters to determine the elevation.

When in doubt, the best course of action is to follow the written directions to the benchmark as given by the benchmark's official data sheet.

ee also

Benchmark (surveying)

External links

* [http://www.geocaching.com/mark/ Benchmarking using a GPS in the US]
* [http://www.bench-marks.org.uk Live UK Benchmark Database, primarily focusing on flush bracket benchmarks]
* [http://benchmarks.ordnancesurvey.co.uk/pls/htmldb/f?p=111:3:15816778491873164759::NO:3:: UK Ordnance Survey benchmark source database, not updated since 1975]
* [http://www.trigpointinguk.com UK Trigpoint Database]
* [http://www.waymarking.com/cat/details.aspx?f=1&guid=905a74da-86b5-430d-94fb-d8be675283f7 Waymarking - Benchmarks]