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Bench mark

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Bench mark

a fixed, more or less permanent reference point or object of known elevation, the U.S. Geological survey (USGS) installs brass caps in bridge abutments or otherwise permanently sets bench marks at convenient locations nationwide, the elevations on these marks are referenced to the National Geodetic vertical datum (NGVD), also commonly known as mean sea level (MSL), locations of these bench marks on USGS topographic maps are shown as small triangles, since the marks are sometimes destroyed by construction or vandalism, the existence of any bench mark should be field verified before planning work which relies on a particular reference point, the USGS or local state surveyors office can provide information on the existence, exact location and exact elevation of bench marks.