Cartography Word of the Day: Benchmark

Little brass discs with a lot of legal weight

In computing, the benchmark is a test run to determine the baseline performance of a component or system. It is a stable point of reference to which all other performance or sets of attributes can be referenced to.

A “station mark”, a metallic disc representing a survey point–in this case, the “Willamette Stone”, the point of origin for all public surveying of Oregon and Washington, USA. (click for a better look)

To the geographer and map-user, the term “benchmark” means more or less the same thing, but to these people a benchmark can deliver such information as latitude, longitude and altitude. Surveyed by the government (typically the US Geological Survey) they provide stable, known points whose locational attributes are established, and by providing such reference points give accurate bases for surveying, legally describing, and mapping other things.

Benchmarks are found on government-produced relief maps by a small cross with a notation such as “B.M. 345″. Seeking out this point typically reveals a metallic disk set into the ground in such a way that it’s not likely to move. This is called a “station mark”, and is the actual physical representation of the benchmark that surveyors can use to dependably site from.

Station mark monuments are made usually of brass or a metal which changes little with respect to conditions such as temperature and are inscribed with the agency planting the mark, a symbol for aligning at the center, and either altitude information or a terse instruction to contact the agency for information on the mark.

Another look at the Willamette Stone station mark, with clarification. The circumferential notations should be self-explanatory. The date in the top is the date of original survey; the one at bottom is the date this particular monument was placed. The numbers preceded by “S” note which sections of each surrounding township meet there, and the initial point itself is physically located at the intersection of the two lines (click for a closer look).

A particularly notable example of a station mark can be found in the western hills of the city of Portland, Oregon. In a small, pocket-sized state park off a road locally known as NW Skyline Blvd, a silver disk is set into a stone monument. This monument, known as the Willamette Stone, is the center, or initial point, of a system of surveying that covers both American states of Oregon and Washington, providing coordination for all surveys of both public and private lands.

The public lands survey system of the United States is beyond the scope of this Word of the Day, so it will hopefully suffice to say that a network of surveyed points referencing this one real point (represented physically by the intersection of the crossed lines at the center of the disk) support mapping and legal surveying of over 180,000 square miles of the earth’s surface.

Station marks make for interesting finding, add additional spice to hikes, and are just plain neat things to know about.