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Benchmarking (geolocating)
Benchmarking (also known as benchmark hunting) is a hobby activity in which participants find benchmarks (also known as survey markers or ''geodetic control points''). Technically, the term "bench mark" is only used to refer to survey markers that designate a certain elevation, but hobbyists often use the term benchmarks to include triangulation stations or reference marks. 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.
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). The "ADJUSTED" coordinates are precise to sub-centimeter accuracy, while others, typically true elevation Bench Marks, have only coordinates scaled from a map. "SCALED" coordinates were read from a topographic map, rather than being surveyed. Many are accurate to 100 feet but some few are as much as 3,000 or 4,000 feet distant from the mark to which they refer, rendering a handheld GPS unit of little use in locating them. Most marks have clear "to-reach" descriptions, but some lack complete descriptions, or changes in the surrounding buildings, roads, or terrain over decades make the descriptions obsolete. Marks may have been removed by construction or buried.
==Sources of data on 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. The majority of marks set by the U.S. Geological Survey (USGS), the Forest Service, the Corps of Engineers, or cities, and states, and local authorities. Cadastral (land survey) marks are usually not measured for the geodetic data base. The database used by Geocaching.com, the hobbyist website for U.S. benchmark hunters, is only a "snapshot" of the marks that the NGS had documented by the year 2000, and has not been updated since then.
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 viewed. 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. Specialized hobbyist websites (like (Geocaching.com )) FAQ and its Benchmark Hunting forum can provide more information.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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