Vertical position or vertical location, also known as vertical level or simply level, is a position along a vertical direction above or below a given vertical datum (reference level). Vertical distance or vertical separation is the distance between two vertical positions. Many vertical coordinates exist for expressing vertical position: depth, height, altitude, elevation, etc.
- depth: "distance of a point from a chosen reference surface measured downward along a line perpendicular to that surface."
- height: "distance of a point from a chosen reference surface measured upward along a line perpendicular to that surface";
ISO 6709 (2008 version) makes the following additional definition:
- altitude: "height where the chosen reference surface is mean sea level"
The International Civil Aviation Organization (ICAO) offers similar definitions:
- altitude: "the vertical distance of a level, a point or an object considered as a point, measured from the mean sea level (MSL);"
- height: "the vertical distance of a level, a point or an object considered as a point, measured from a specific datum."
ICAO further defines:
- elevation': "the vertical distance of a point or a level, on or affixed to the surface of the earth, measured from mean sea level."
I.e., elevation would be the altitude of the ground or a building.
Several physical quantities may be defined based on the definitions above:
- Depth below seafloor
- Depth in a well
- Drying height
- Dynamic height
- Ellipsoidal height
- Geocentric altitude
- Heights in geodesy
- Height above mean sea level
- Height above average terrain
- Height above ground level
- Measured depth
- Normal height
- Orthometric height
- Thickness (geology)
- True vertical depth
Certain vertical coordinates are not based on length, for example, geopotential numbers have units of m2/s2. Normalization by a constant nominal gravity value (units of m/s2) yields units of metre, as in geopotential height (based on standard gravity) or dynamic height (based on normal gravity at 45 degrees latitude). Despite the dimension, the vertical coordinate still does not represent distance, as would be measured with a ruler. Sometimes a standard geopotential metre (symbol gpm or m') is introduced for emphasis. However, this practice is not acceptable with the International System of Units (SI): the Guide for the Use of the International System of Units, section 7.5 (Unacceptability of mixing information with units), states that "When one gives the value of a quantity, any information concerning the quantity or its conditions of measurement must be presented in such a way as not to be associated with the unit.".
Another non-SI unit is the vertical metre, introduced when there may be confusion between vertical, horizontal, or slant distances. It is used for distance climbed during sports such as mountaineering, skiing, hiking, running or cycling In German-speaking countries the abbreviation 'Hm' for Höhenmeter ("height metre") is used; if it is preceded by a '±' it refers to the cumulative elevation gain.
Various instruments and techniques may be used for measuring vertical position:
- Benchmark (surveying)
- Depth gauge
- Depth sounding
- Tide gauge
- Water level (device)
Many physical phenomena are related to vertical position, as driven by gravity:
- Hydraulic head
- Mean sea level
- Temperature lapse rate
- Vertical displacement
- Vertical pressure variation
- "ISO 6709:2008(en) preview". www.iso.org. ISO. Retrieved 8 June 2016.
- United Nations - International Civil Aviation Organization. "Annex 4: Aeronautical Charts - Chapter 1 (Definitions, Applicability and Availability)" (PDF). Archived from the original (PDF) on 2016-09-17. Note: Annex 4 is one of the (currently) 19 annexes to the Chicago Convention on International Civil Aviation (ICAO Doc. 7300).
- World Meteorological Organization - Guide to Meteorological Instruments and Methods of Observation - Preliminary seventh edition - WMO-No. 8 - Secretariat of the World Meteorological Organization – Geneva – Switzerland - 2006
- Thompson, Ambler; Taylor, Barry N. (2018-11-10). "Guide for the Use of the International System of Units (SI)". NIST. Retrieved 2020-12-09.
- Nash, Mike, Exploring Prince George: A Guide to North Central B. C. Outdoors, Vancouver: Rocky Mountain Books, 2004, p. 105.
- IOGP (2018) Geomatics Guidance Note 24: Vertical data in oil and gas applications, International Association of Oil & Gas Producers (IOGP), Geomatics Committee, Geodesy Subcommittee. Report 373–24, April 2018.