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Information on Geodesy
Sea Level Science and Geodesy (the science of the shape of the Earth) are closely related subjects - indeed the sea surface defines the shape of the Earth over two-thirds of the globe. In determining the shape of the Earth precisely, geodesists have developed techniques for measuring small changes in position. Sea level research has come to depend upon a number of these new techniques for the measurement of sea level changes worldwide. They are fundamental to fulfilling our objectives of understanding how fast and why global sea level rise is occurring. This leaflet provides several examples of the importance of geodetic techniques to Sea Level Science. For a standard text book on geodesy, see:
- Torge, W. 1991. Geodesy (2nd. Edition). Berlin: Walter de Gruyter. 264pp.
In the UK, the Ordnance Survey produced the following 'A Guide to Coordinate Systems in Great Britain' by Dr. P. Davies. Although obviously UK-biased it provides an excellent introduction to the geoid, heights 'above sea level', map coordinates etc.
Other information, mostly for geodesy professionals, may be obtained via the web page of the International Association of Geodesy.
Chris Hughes together with Rory Bingham authored the following paper “An Oceanographer's Guide to GOCE and the Geoid”. Amongst other issues they give a review of the geodetic concepts necessary for oceanographers to make use of satellite gravity data to define the geoid, and to interpret the resulting product. The geoid is defined, with particular attention to subtleties related to the representation of the permanent tide, and the way in which the geoid is represented in ocean models. The article is open access.
In 2013, Mark Tamisiea gave a talk at the Challenger Society for Marine Science meeting on "Measuring the boundaries of sea level". The talk and the subsequent paper focuses on the contributions from geodesy to sea level science and the role of the ocean's bounding surfaces: the sea surface and the Earth's Crust. The article is open access.
Information on GNSS systems at Tide Gauges
Global Navigation Satellite Systems (GNSS with the Global Positioning System, GPS, being one component) is used in conjunction with tide gauges in order to separate vertical land movements from the 'real' sea level variations contained within the records of 'relative' sea level change recorded by the gauges. Land movements can result from tectonic motion, glacial isostatic adjustment or serious subsidence. If GNSS (or another space geodetic technique or absolute gravity) are used to measure the vertical land movements at points near to the tide gauges, then this signal can be removed from the tide gauge measurements, improving estimates of 'real' or 'absolute' sea level changes. GNSS at tide gauges is also used to refer the tide gauge data to a global, external reference system (such as the International Terrestrial Reference Frame) for comparison to data from satellite radar altimetry.
Background History
In 1988 a meeting of tide gauge experts and geodesists was held at the Woods Hole Oceanographic Institution under the auspices of the IAPSO Commission on Mean Sea Level and Tides. The conclusions of the meeting were published as Carter et al. (1989):
A key recommendation of the meeting was that geocentric coordinates of tide gauge benchmarks, derived primarily from differential Global Positioning System (GPS) measurements relative to International Earth Rotation Service (IERS) 'fundamental points' (but not exclusively e.g. DORIS), should be stored at the PSMSL alongside the sea level data.
A follow-up meeting was held in December 1993 at the Institute of Oceanographic Sciences Deacon Laboratory, Wormley, Surrey to review progress, with the conclusions and recommendations published as Carter (1994):
Since the Woods Hole meeting, considerable developments had taken place with the GPS technique in particular and with the organisation of centres to analyse such data, in particular with the development of the International GNSS Service for Geodynamics (IGS). By now the concept of 'campaign mode' differential measurements to 'fundamental points' was beginning to change to the use of Continuous GPS (CGPS) receivers located within the global GPS network itself.
In 1997 a workshop on "the methods for monitoring sea level: GPS and tide gauge benchmark monitoring and GPS altimeter calibration" was held at the Jet Propulsion Laboratory and organised by the IGS and PSMSL.
The Workshop report:
Neilan, R., Van Scoy, P.A. and Woodworth, P.L. (eds). 1998. Proceedings of the workshop on methods for monitoring sea level: GPS and tide gauge benchmark monitoring and GPS altimeter calibration. Workshop organised by the IGS and PSMSL, Jet Propulsion Laboratory, 17-18 March 1997. 202pp.
It provides important background information on the use of GPS at tide gauge sites for measuring changes in the heights of benchmarks for studies of long term trends and for altimeter calibration. The report is available in both paper and electronic forms. For paper versions, email psmsl@noc.ac.uk. An electronic version is contained in two Adobe Acrobat files:
See also the reports of the IAPSO/IAG/PSMSL/IGS/GLOSS Working Group on
A survey by Guy Wöppelmann of the University of La Rochelle, France on the availability of permanent GPS receivers near to tide gauges has recently (July 2007) been updated on behalf of GLOSS, PSMSL, EUREF and other bodies. This is now mainly superseded by the latest report from Guy and others to the GLOSS Group of Experts meeting in 2012. You can find the report here
Basic Information on GPS
There is a lot of basic information on GNSS available on the web from both universities and GNSS manufacturers etc. A rather old but still relevant introduction is given in the notes by Peter H. Dana at the University of Colorado (formerly presented by the University of Texas).
To calculate a position on (or near) the Earth an unobstructed view to at least four GNSS satellites is required (three for position and one to correct for any error in the receiver clock). Over time the position changes of the GNSS antenna can then be measured.
For sea-level applications we require a GNSS receiver that is capable of millimetre-level precision. This precision is obtained by tracking multiple GNSS satellite signals simultaneously and then post-processing the data together with data from many globally distributed stations to provide a robust position estimate. This post-processing of GPS data from a network of sites can eliminate or reduce various errors such as satellite orbit and clock errors in order to achieve the desired millimetre level precision.
A good site for information on appropriate GNSS receivers and antenna for high precision Earth Science (including sea-level) studies is the UNAVCO website.
For more detailed, specialist information, see the web pages of the International GPS Service.
Other Useful Materials
The following (in Word 97 format and Adobe Acrobat) consist of a short set of notes on the basics of levelling. The file is based on lecture notes by Professor Charles Merry of the University of Cape Town at the 1998 GLOSS Training Course at UCT.
