Bench Mark (BM) 1
Bench Mark (BM) 1 : A fixed physical object or mark used as reference for a horizontal or vertical datum. A tidal bench mark is one near a tide station to which the tide staff and tidal datums are referred. A primary bench mark is the principal mark of a group of tidal bench marks to which the tide staff and tidal datums are referred. The standard tidal3 bench mark of the National Ocean Service is a brass, bronze, or aluminum alloy disk 3-? inches in diameter containing the inscription NATIONAL OCEAN SERVICE together with other individual identifying information. A geodetic bench mark identifies a surveyed point in the National Spatial Reference System. Bench mark disks of either type may, on occasion, serve simultaneously to reference both tidal and geodetic datums. Numerous bench marks of predecessor organizations to NOS, or parts of other organizations absorbed into NOS, still bear the inscriptions: U.S. COAST & GEODETIC SURVEY, NATIONAL GEODETIC SURVEY, NATIONAL OCEAN SURVEY, U.S. LAKE SURVEY, CORPS of ENGINEERS, and U.S. ENGINEER OFFICE
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