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A spot height is an exact point on a map with an elevation recorded beside it that represents its height above a given datum. [1] In the UK this is the Ordnance Datum . Unlike a bench-mark , which is marked by a disc or plate, there is no official indication of a spot height on the ground although, in open country, spot heights may sometimes be ...
The term is generally applied to any item used to mark a point as an elevation reference. Frequently, bronze or aluminum disks are set in stone or concrete, or on rods driven deeply into the earth to provide a stable elevation point. If an elevation is marked on a map, but there is no physical mark on the ground, it is a spot height.
It took some time for the changed definition — Liverpool to Newlyn — to work through the system: by 1950, some 40% of the lower secondary and tertiary levellings were still using the Liverpool datum. But following the Third Geodetic Levelling, Ordnance Survey maps published since March 1956 give spot heights above the Newlyn datum. [4] [5]
Reduced level (RL) – equating elevations of survey points with reference to a common assumed datum. The elevation is positive or negative according as point lies above or below datum. Spirit level – instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). While used by surveyors, different types of spirit ...
This is the most common way of visualizing elevation quantitatively, and is familiar from topographic maps. Most 18th- and early 19th-century national surveys did not record relief across the entire area of coverage, calculating only spot elevations at survey points.
The zero-elevation baseline or vertical datum to which a measurement of elevation or altitude is relative, e.g. the mean sea level calculated for a given location over a given period of time. [4] See also geodetic datum. de facto segregation The spatial and social separation of populations that occurs without legal sanction. [2] de jure segregation
In aviation, the term elevation or aerodrome elevation is defined by the ICAO as the highest point of the landing area. It is often measured in feet and can be found in approach charts of the aerodrome. It is not to be confused with terms such as the altitude or height. [1] Part of a topographic map of Haleakala , showing elevation.
A benchmark is a type of survey marker that indicates elevation (vertical position). Horizontal position markers used for triangulation are also known as triangulation stations. Benchmarking is the hobby of "hunting" for these marks.