A surveyor's mark on a permanent object of predetermined position and elevation used as a reference point WordNet 3.0 & Open English WordNet

The term benchmark, bench mark, or survey benchmark originates from the chiseled horizontal marks that surveyors made in stone structures, into which an angle iron could be placed to form a "bench" for a leveling rod, thus ensuring that a leveling rod could be accurately repositioned in the same place in the future. Wikipedia

A point of known elevation marked for the purpose of surveying Wikipedia Disambiguation

Point with known height used in surveying when levelling Wikidata

A surveyor's mark made on some stationary object and shown on a map; used as a reference point. Wiktionary

Surveyor's mark. Wiktionary (translation)