International Gravity Standardization Net 1971

International Gravity Standardization Net 1971
Name	International Gravity Standardization Net 1971
Abbreviation	IGSN71
Established	1971
Type	Geodetic gravity datum
Jurisdiction	International
Related	International Association of Geodesy, International Union of Geodesy and Geophysics

International Gravity Standardization Net 1971 The International Gravity Standardization Net 1971 was a globally coordinated gravity reference system produced to provide consistent absolute and relative gravity values across continents for geodesy, oceanography, and geophysics. It synthesized measurements from national agencies and observatories to create a unified set of gravity stations used for calibration, crustal studies, and datum definition. The network underpinned many regional height systems and satellite missions during the late 20th century.

Background and Purpose

The project originated from proposals at meetings of the International Union of Geodesy and Geophysics and the International Association of Geodesy, driven by the need to reconcile disparate gravity surveys conducted by agencies such as the United States Coast and Geodetic Survey, Ordnance Survey, Institut Géographique National, and national observatories including USNO and Royal Greenwich Observatory. Objectives included harmonizing absolute gravity values from laboratories like Wettzell Observatory and BIPM-linked metrology institutes, improving geoid determination for projects such as Geodetic Reference System 1980 and supporting satellite missions like LAGEOS and TOPEX/Poseidon.

Development and Computation

Computation of the network combined absolute gravimeter ties, relative spring-based gravimeter traverses, and gravity reduction models from contributors including NASA, European Space Agency, National Oceanic and Atmospheric Administration, and national mapping agencies. Methods referenced theoretical frameworks from researchers associated with Carl Friedrich Gauss-inspired least-squares adjustments and analytical techniques promoted by the International Geodetic Student Organization and commissions of the IUGG. Adjustments accounted for tidal corrections derived from formulations used in International Earth Rotation and Reference Systems Service products, atmospheric loading treated in studies by Alexander von Humboldt-influenced climate observatories, and ellipsoidal reduction consistent with World Geodetic System 1972 conventions.

Network Stations and Coverage

Stations in the net included established absolute sites such as Wettzell, Metsähovi Observatory, Sutherland Observatory, and national fundamental stations in Paris, Washington D.C., Tokyo, Canberra, and Moscow. Coverage spanned continents via coordinated campaigns by organizations like the Federal Office of Topography (Switzerland), Geoscience Australia, and the Geological Survey of Japan. Island and oceanic control came from measurements tied to coastal observatories and shipborne surveys linked to institutes including Scripps Institution of Oceanography and Institut Français de Recherche pour l'Exploitation de la Mer.

Instruments and Measurement Procedures

Instrumentation combined absolute ballistic gravimeters (following designs influenced by work at BIPM laboratories), spring relative gravimeters from manufacturers associated with Leica Geosystems and legacy firms, and calibration suites maintained by national metrology institutes such as Physikalisch-Technische Bundesanstalt and NPL. Procedures mandated intercomparisons, drift monitoring, and baseline ties using methods standardized in technical notes circulated by the IAG commissions and field campaigns coordinated with observatories like Gravimetric Observatory Walferdange and Tarawa Observatory.

Datum and Reference Values

IGSN71 defined reference gravity values and a conventional datum for normal gravity and gravity anomalies, anchored to adopted absolute values at selected reference stations. The datum facilitated conversion between observed gravity and derived geoid undulations consistent with contemporary formulations like Normal Gravity on the Ellipsoid and comparisons to later systems such as EGM96 and EGM2008. The network specified standard reductions for latitude, elevation, and Free-Air and Bouguer corrections used across national leveling and gravity projects by agencies including USGS and Canadian Geodetic Survey.

Impact and Applications

The standardized gravity values supported improved geoid modeling, informing projects in oceanography (supporting mean sea surface studies), crustal deformation research tied to seismic monitoring networks, and resource exploration workflows used by the United Nations and regional geological surveys. IGSN71 underpinned calibration of satellite altimetry missions such as ERS-1 and provided baseline ties for regional geoid models used in infrastructure projects overseen by bodies like World Bank and national ministries of transport.

Revisions and Successor Networks

Subsequent initiatives superseded or updated IGSN71 with refined reference systems, including later global gravity models and networks developed by the International Gravity Field Service and datasets incorporated into ICGEM archives. Regional densification campaigns and absolute gravimetry advances led to successor frameworks aligned with IGRF-referenced satellite missions and newer realizations of geodetic datums such as ITRF updates and the adoption of GRS 80-consistent standards in global gravity modeling.

Category:Geodetic datums