The EUROGAUGE Project

Introduction

Records from a number of tide gauges in Europe indicate a rise in Mean Sea Level (MSL) of between 15 and 20 cm over the last century. However, tide gauge records alone cannot distinguish between absolute (eustatic) sea level variations and vertical (eperiogenic) movements of the land at the tide gauge site. Hence, in order to evaluate the magnitude, and rate, of the change in absolute MSL, the land at tide gauge sites has to be monitored with respect to a global geocentric reference framework.

In 1988, an international meeting of oceanographers and geodesists took place at the Woods Hole Oceanographic Institute in Massachussetts which recommended the use of GPS to connect tide gauge benchmarks (TGBMs) to the International Terrestrial Reference Frame (Carter et al, 1989).

Following on from these recommendations, in 1992, the European Commission funded the EUROGAUGE project (EC Contract SC1*-CT92-0821). The project involved two independent measurement campaigns, with a four month separation, in order to prove zero vertical land movement, while providing first epoch measurements for longer term studies of MSL variations at sixteen tide gauge sites on the Atlantic Coast of Europe.

The EUROGAUGE Measurement Campaigns

The measurements involved in determining the height of a TGBM in the ITRF are illustrated in Figure 1: The Use of GPS to Monitor Vertical Land Movementand Absolute Mean Sea Level at a Tide Gauge Site. Firstly, a tide gauge GPS station has to be established, as close to the tide gauge as possible, but in a location suitable for GPS measurements. The height of this tide gauge GPS station is then determined in the ITRF using the fiducial GPS technique. Following on from this, the height of the TGBM can be determined by making a local precise spirit levelling connection between the tide gauge GPS station and the TGBM.

The selection of the sixteen tide gauge sites for EUROGAUGE was based on the criteria that they should have at least 20 years of PSMSL (Permanent Service for Mean Sea Level) data or be a GLOSS (Global Sea Level Observing System) tide gauge. The tide gauges selected are shown in Figure 2: EUROGAUGE GPS Stations, along with the primary ITRF stations in Europe which form part of the global GPS network of the International GPS Service for Geodynamics (IGS).

Two independent measurement campaigns were carried out, in November 1993 and March 1994, at the full network of stations shown in Figure 2 (Ashkenazi et al, 1994). Each of the measurement campaigns consisted of:

- Network GPS Observations, ie five consecutive days of simultaneous, 24 hour, GPS observations at the sixteen tide gauge GPS stations and the ten fiducial stations.

- Local Stability Surveys, between a tide gauge GPS station and five auxiliary GPS stations, located within 5 km of each TGBM.

- Levelling Link Surveys, between the tide gauge GPS station and its associated TGBM.

The EUROGAUGE measurement campaigns were carried out by the four national survey organisations, namely the Ordnance Survey of Great Britain (UK), the Institut Géographique National (France), the Instituto Geográfico Nacional (Spain) and the Instituto Portugues de Cartografia e Cadastro (Portugal).

The Processing of EUROGAUGE

The network GPS observations from both measurement campaigns have been processed independently at three processing centres, and using three different software packages, namely GAS at the University of Nottingham (UK), BERNESE at the University of Newcastle-upon-Tyne (UK) and GIPSY/OASIS at the Consejo Superior de Investigaciones Cientificas (Spain). The Proudman Oceanographic Laboratory (POL) were responsible for the development of models for the computation of vertical station displacements due to ocean tide loading, which were implemented during the processing.

Over the four month time interval between the two measurement campaigns, it has been assumed that the vertical land movement at the tide gauge sites was less than 1 or 2 mm. Hence, they have been used to assess accuracy, through campaign-to-campaign coordinate agreements. With this in mind, a single reference frame was defined for both measurement campaigns, based on the ITRF93 GPS coordinates for Onsala, Kootwijk, Wettzell, Matera and Madrid, motioned from the ITRF reference epoch of 1993.00 to the mid-epoch of the two measurement campaigns (1994.05) using the ITRF93 velocity field.

The accuracy of the final coordinate sets, established from a combination of the independent solutions produced by the three processing centres, is illustrated in Figure 3: Campaign-to-Campaign Coordinate Agreements for the Final Coordinate Sets. The RMS campaign-to-campaign coordinate agreements obtained for the 16 tide gauge GPS stations were6 mm in East, 8 mm in North and 8 mm in height.

The three different software packages produced very consistent results, in terms of campaign-to-campaign height agreements, as shown in Figure 4: Campaign-to-Campaign Height Agreements for the Three Different Software Packages. The RMS campaign-to-campaign height agreements obtained for the 16 tide gauge GPS stations were 9 mm for all three processing centres. More importantly, however, similar magnitudes and directions for the height agreements at an individual tide gauge GPS station were obtained by the three processing centres.

Conclusions

The results from the two independent measurement campaigns, involving 16 tide gauges along the Atlantic Coast of Europe, indicate that accuracies better than 10 mm in plan and height have been achieved for the EUROGAUGE project. These high accuracies have been confirmed by three independent processing centres, using three different software packages. It is intended that periodic re-measurements will be made at these tide gauge GPS stations in order to monitor vertical land movements. Over the next 10 to 20 years, such measurements will be used to determine the vertical motion of the tide gauge GPS stations, which can then be removed from the tide gauge records, to determine absolute changes in global mean sea level (Baker, 1993).

References

Ashkenazi, V, Bingley, R M, Chang, C C, Dodson, A H, Torres, J A, Boucher, C, Fagard, H,Caturla, J L, Quiros, R, Capdevila, J, Calvert, C, Baker, T F, Rius, A, and Cross, P A, 1994. "EUROGAUGE: The West European Tide Gauge Monitoring Project". Proceedings of the International Symposium on Marine Positioning (INSMAP 94), pp 224 - 234.

Baker, T F, 1993. "Absolute Sea Level Measurements, Climate Change and Vertical Crustal Movements". Global and Planetary Change, 8(1993), pp 149 - 159, Elsevier Science Publishers B V.

Carter, W E, Aubrey, D G, Baker, T F, Boucher, C, Le Provost, C, Pugh, D T, Peltier, W R, Zumberge, M, Rapp, R H, Schutz, R E, Emery, K O, and Enfield, D B, 1989. "Geodetic Fixing of Tide Gauge Benchmarks". Woods Hole Oceanographic Institute Technical Report, WHOI-89-31/CRC-89-5.