Reference height
In geosciences and technology, the reference altitude is understood to be the altitude above sea level to which precise altitude measurements from a state or state survey refer. Landlocked states define derived geodetic datum points (fixed points "zero order") on their national territory with their exact sea level. They mostly also serve as points of the gravity network .
In coastal states, the original reference point is usually a mareographer (or a system of gauges ) on the nearest sea coast. From this reference level, the respective national survey - mostly for the first time in the 18th or 19th century - carried out a network-like precision leveling over the national territory. The results of such height networks were / are marketed along the measuring lines as a large number of derived height control points (HFP) in stable buildings ( tower or leveling bolts) or in natural rock and secured by further control points in the vicinity, sometimes also as assigned height of measurement points in their data file specified. They are available to technicians at a distance of a few 100 meters to kilometers. Because of their precise reference to a national system, these altitude points can also be viewed as indirect or local reference altitude.
In addition to geodesy, the term "reference height" is mainly used in navigation :
- as a calculation variable for the barometric altitude formula , which serves as the basis for comparing the measured air pressure - see QNH
- for the pressure altimeter in aviation during cruise, either the true or the (fictitious) sea level related to normal pressure
- Before landing, the altitude reference to be set by the pilot for the altimeter , which generally corresponds to the terrain altitude of the airfield - see touchdown point and QFE
In meteorology
The Meteorology refers calculated or measured data often to a certain height - u. a. in the humidity and in models of the wind field, especially the high-altitude winds . In view of the strong upward increase in wind speeds (see logarithmic wind profile ), the current wind at a certain altitude is related to that at the "reference altitude" in order to put the wind formula for the vertical increase on a suitable empirical basis.