Benchmarking, also known as benchmark hunting, is a hobby activity in which participants find benchmarks (also known as survey markers or geodetic control points). The term "bench mark" 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. They typically then log their finds online. Like geocaching, the activity has become popular since 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. History Many survey markers in the U.S. were set over 100 years ago. 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 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 online. The majority of marks set by the U.S. Geological Survey (USGS), the Forest Service, the Corps of Engineers, cities, and states, and local authorities. Cadastral (land survey) marks are usually not measured for the geodetic database. 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 datasheet 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. Some of these marks 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, by contrast, were read from a topographic map, rather than being surveyed. Many are accurate to 100