GGOS Focus Area
Unified Height System
Establishment of a Global Unified Height Reference System: A GGOS priority
Heights referring to the mean sea level
A height (also called altitude or elevation) is the vertical distance between a given object on (or close to) the Earth’s surface and a reference surface. Traditionally, the average (or mean) sea level is used as the reference level, because the sea surface is available worldwide. However, the sea surface has slight hills and valleys in it, similar to the topographic surface although much smoother. Depending on the location, the sea level may be closer to or farther from the centre of the Earth. Thus, the zero elevation as defined by one country differs up to ±2 metres from the zero elevation as defined by other country.
Physical and geometric heights: how the water flows
In practice, a variety of techniques and equipment are used to determine heights. However, there is no guarantee that each technique will produce the same height. For instance, heights determined with GNSS (like the US Global Positioning System, the European Galileo, the Russian GLONASS, or the Chinese BeiDou) do not refer to the mean sea surface, but to a geometry model of the Earth called ellipsoid.
Heights referring to the mean sea level describe flow of water and are called physical heights (they depend on the Earth’s gravity field). Heights determined by GNSS do not describe flow of water (the ellipsoidal height of two points located at the same level may be different) and are called geometric heights (they do not depend on the Earth’s gravity field). Differences between GNSS heights and those referring to the mean sea surface may reach up to ±80 m. These differences are represented by the geoid heights. The geoid is a level surface of the Earth’s gravity field acting as reference surface for the determination of physical heights.
Why to make heights consistent?
The state-of-the-art demands an accurate knowledge of the inconsistencies existing between the different heights. For instance, airplanes are equipped with barometric and GNSS navigation systems. Heights provided by the barometric instruments refer to the mean sea level. Heights provided by GNSS refer to the ellipsoid. An airplane approaching for landing, having the wrong height information can have terrible consequences.
With no conventional global height system definition, most countries today use regional or local height systems, realization of which has been implemented individually, applying in general non-standardized procedures. At present, there are some hundred local and regional physical height systems in use.
They refer to local sea surface levels, are usually stationary (do not consider variations in time), realize different physical height types, and their combination in a global frame presents uncertainties at the metre level. An accurate, consistent and well-defined global vertical reference system that is in accordance with the increased precision of modern observational techniques and is capable of supporting the present needs of science and society regarding geo-referenced data of high resolution has yet to be established as an international standard.
A global unified height system: a priority of GGOS
A global unified height system is needed for all studies or applications requiring a global view of the Earth with measurements not only on land, but over the oceans and ice caps; for instance,
- combination of vertical monitoring in land and ocean areas;
- determination of the correlation between sea level changes and polar ice-cap volume variations;
- combined analysis of satellite-based sea surface monitoring, GNSS positioning, geodetic levelling, and oceanographic procedures for determining sea surface changes over long distances; and
- in general, the monitoring of the System Earth phenomena governed by the gravity field.
From the practical point of view, a global unified physical height system is needed, among others,
- to provide a reliable reference for the accurate connection of national/regional height systems;
- to remove inconsistencies in height-dependent quantities resulting from the use of different local height systems; and
- to support the precise combination of physical and geometric heights in order to exploit at a maximum the advantages of satellite geodesy (e.g. combination of satellite positioning and gravity field models for worldwide unified precise height determination).
The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG), taking care of providing a precise geodetic infrastructure for monitoring the System Earth, promotes the standardization of height systems worldwide by establishing a global unified height system. The corresponding activities are coordinated by the GGOS Focus Area Unified Height System.