Benchmarking (geolocating)
From Wikipedia, the free encyclopedia
Benchmarking is a sport 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.
Many users use a GPS receiver to locate the known position of a benchmark, but it is also possible to find them without using GPS technology.
It is similar to geocaching and trigpointing.
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[edit] Required Materials
Nothing is required to start benchmarking except an internet connection to look up the survey data (location, coordinates, directions) of each benchmark. One popular website for this is the Geocaching.com benchmark site. In some cases, a local library or courthouse will also be able to provide you with information on nearby benchmarks. Most persons also carry a camera of some sort to record proof of a particular find. These pictures, as well as a description of how you found the mark, can later be uploaded to the Geocaching website. More experienced benchmarkers (or those wishing for a slightly easier find) will sometimes use GPS receivers and/or metal detectors in order to locate the marks. In addition, range or distance finders are sometimes employed to help with measuring distances as prescribed in the written directions to each benchmark.
[edit] What to Look For
Survey markers 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. 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.
Not all benchmarks are discs, however. 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.
[edit] Finding the Marks
Each individual benchmark is unique in its location, however there are some common places where benchmarks are often found.
[edit] Railroads
Often, benchmarks will 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 some cases, the mark will have been destroyed.
[edit] Bridges
Benchmarks can be in a variety of places in and around 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. Sometimes, the bridge itself may be the benchmark. Know the pattern of traffic crossing a particular bridge before you attempt to locate the benchmark, and be on the lookout for cars or trains that may be approaching your location.
[edit] Smokestacks, Antennas, Steeples, and Towers
Many times a benchmark will be described as a smokestack or tower. This is usually accurate. The tall structure itself will be considered the benchmark, and survey reference is made from the highest point of the structure. Do not attempt to climb these structures in search of a benchmark! In these situations, if you can see the object, it counts as a find.
[edit] 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.
[edit] GPS Receiver Issues
One problem with using a GPS receiver to assist the search is that old latitude and longitude points were translated from grid-system maps, and so the coordinates provided by the United States Geodetic Survey (USGS) and the United States Coast and Geodetic Survey are sometimes quite different from the actual GPS coordinates of a benchmark. 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.
If the stated coordinates are SCALED, that means the coordinates were scaled from a map & are often not exact. If the stated coordinates says ADJUSTED, it means that a very precise commercial grade GPS was used in figuring the coordinates and therefore the given position is extremely accurate, usually within a centimeter.
The elevation of a benchmark is usually extremely accurate as they use equipment like 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 survey group.
