What is a Benchmark?
The term 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. These marks were usually indicated with a chiseled arrow below the horizontal line.
The height of a benchmark is calculated relative to the heights of nearby benchmarks in a network extending from a fundamental benchmark, a point with a precisely known relationship to the level datum of the area, typically mean sea level. The position and height of each benchmark is shown on large-scale maps.
(from Wikipedia about the use of benchmarks in surveying)
Wherever 'datum' is used in the City of Manitowoc, is shall mean "United States Coastal and Geodetic Survey". Manitowoc Municipal Code 7.040 has more information.