Height reference systems and fixing tide gauge benchmarks

Work package leader : Bernd Richter,
Bundesamt für Kartographie und Geodäsie,
Richard Strauss Allee 11,
D-60598 Frankfurt,
Germany.

email Bernd Richter, richter@ifag.de
URL http://www.ifag.de/

Work package description:

New technologies have become available over the last decades for establishing global and continental height reference systems in which land subsidence can be monitored and to which tide gauge benchmarks can be connected. Most of these have their origin in space geodesy (e.g. GPS, SLR, VLBI) but also other techniques provide us with essential information. Absolute height measurements are important for the discrimination between eustatic sea level rise and land subsidence.

In many countries, GPS is already being applied, or planned, for the fixing of tide gauge benchmarks. International standards and guidelines are still lacking however. The main objectives in this work package concern the design of an integrated monitoring system, combining different techniques and information. In addition, attention is focused on the definition of international standards of measurement and data assembly, and the coordination of international surveying campaigns. Important related work in this field has already been undertaken, or is being planned/discussed (EUVN/EPTN), under the auspices of the EUREF (EUropean REference Frame) commission of the IAG (International Association of Geodesy). Also the IGS (International GPS Service for Geodynamics) should be mentioned, which plays a crucial role in GPS by providing users with the essential precise orbit information.

The first meeting of this work package was organised by Bernd Richter, of the Bundesamt für Kartographie und Geodäsie, and took place in Potsdam over September 18-19, 1997. An outcome of that meeting was the following possible specification for linking tide gauges to international levelling reference frames:

Distance between tidegauge and permanent GPS station should not exceed 20 km
This GPS station should be linked to a primary reference point less than 75 km away
In addition to the reference height at the GPS station, there should two or more control points within 2 km of the reference height. The local network defined in this way should be levelled at least annually to the 1 mm level
To connect the reference height to the tide gauge benchmark (TGBM), a second GPS receiver should be established at the TGBM.
Both receivers should be dual frequency
The two permanent stations should be operated as if part of the national/international (eg EUREF) network, i.e. with daily remotely controlled data transmission by phone lines to internet nodes, handling by a regional data centre, and continuous analysis within a regional network.
A letter of intent should be submitted to investigate the interest of other European groups in including the GPS tide gauge observations in their programs

With this specification, it was expected that one of the following scenarios would be experienced:

Either the GPS data from both the reference height and the tide gauge benchmark are of good quality, and transmitted regularly
Or, experiments in the harbour vicinities show strong radio interference in the GPS signal, predominantly on the L2 frequency. This would require investigation into matching the partly disturbed GPS signal with the signal at the reference height, to give a handle on the magnitude of the expected slow height variations apparent in the signal.
If the above investigation fails, then the tide gauge + GPS should be moved to a more suitable spot in the same harbour, or a different tide gauge should be selected

In addition to these GPS measurements, it was proposed that gravity observations should be performed to underline vertical movements detected by GPS and changes in the gravity field caused by changes in sea level. To set up a global standard reference, one (or better, two) reference gravity stations should be established as close as possible to the tide gauge. The reference gravity measurements should be carried out with absolute gravimeters, and relative gravimetric ties should be measured to the reference height and the tide gauge benchmark. To determine the trend versus local disturbances, this levelling should be repeated annually.

The management meeting identified the following challenges for this work package as a result of the working group's meeting:

Need specification of requirements for the GPS stations
Arrange the processing of GPS data as a subnetwork of the EUREF permanent GPS network
Need a schedule for installing GPS

Work package contributors:

Hans-Peter Plag	Susanna Zerbini	Astrid Sudau
Geodetic Institute Norwegian Mapping Authority Kartverksveien 21 N-3500 Hønefoss Phone: +47-32118100 Fax: +47-32118101 E-mail: plag@gdiv.statkart.no	Dipartimento di Fisica - Settore di Geofisica Universita' degli studi di Bologna Viale Berti Pichat, 8 40127 Bologna Italy Phone: +39-051-2095019 Mobile phone: +39-0335-5939354 Fax: +39-051-2095058 e-mail: zerbini@astbo1.bo.cnr.it	Bundesanstalt für Gewässerkunde Referat Geodäsie Federal Institute of Hydrology Department Geodesy Kaiserin-Augusta-Anlagen 15 - 17 D-56068 Koblenz Phone: +49- 261-1306-5287 Fax: +49-261-1306-5280 e-mail: sudau@bafg.de Web: http://www.bafg.de
Trevor Baker	Igor Strojan	Richard Bingley
Proudman Oceanographic Laboratory Bidston Observatory Prenton Wirral CH43 7RA U. K. Phone: +44-151-6538633 Fax: +44-151-6536269 e-mail: tfb@pol.ac.uk	Hidrometeoroloski Zavod Republike Slovenije Ministrstvo za Okolje in Prostor Republic of Slovenia Phone Fax: 386 61 1331396 e-mail: igor.strojan@rzs-hm.si	Institute of Engineering Surveying & Space University of Nottingham University park Nottingham NG7 2RD Great Britain Phone: +44-115-951-3880 Fax: +44-115-951-3881 E-mail: richard.bingley@nottingham.ac.uk
Carine Bruyninx	Claude Boucher	Luisa Bastos
Observatoire Royal de Belgique Avenue circulaire 3 B-1180 Bruxelles Belgium c.bruyninx@oma.be	Institut Geographique National ENSG/LAREG 6-8 Avenue Blaise Pascal F-77455 Marne-la-Vallee France Phone: +33-1-64153250 Fax: +33-1-64153253 E-mail: boucher@ensg.ign.fr	Observatorio Astronomico Universidade do Porto Monte da Virgem 4400 Vila Nova de Gaia Portugal Phone: +352-2-782-0404 Fax: +352-2-782-7253 e-mail: lcbastos@oa.fc.up.pt

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