bench mark—A reference point, usually permanent in nature, which may be natural or artificial, and whose elevation is known with respect to a reference datum (level surface) from which other elevations can be established.

bench mark, permanent—A bench mark of as nearly permanent character as it is practical to establish; intended to maintain its elevation with reference to an adopted datum without change over a long period of time. Usually designated as BM; sometimes as PBM (permanent bench mark).

bench mark, supplementary—Established to hold the end of a completed section of a line of levels and serve as an initial mark from which the next section is run. Spikes and screws in poles, bolts on bridges, and chisel marks on masonry are used for this purpose. Supplementary bench marks, also referred to as temporary bench marks (T.B.M.), can be kept in place for a number of years but usually for a shorter period of time than are permanent bench marks.

bench mark, tidal—A bench mark set to reference a tide staff at a tidal station whose elevation is determined with relation to the local tidal datum.

bench mark junction—A bench mark selected as the common meeting point for lines of levels or links of levels.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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