Funcionamiento de mareógrafos modernos: hacia la precisión milimétrica

Considerable efforts are being made worldwide to upgrade tide gauge networks using new technologies. Because of the unique location of the Kerguelen Islands, the measurement of sea level there has received particular attention, with up to four systems equipped with modern sensors functioning simultaneously (two pressure tide gauges, a radar tide gauge, and a GPS-equipped buoy). We analysed and compared the sea level data obtained with these systems from 2003 to 2010, together with a time series of tide pole observations. This is the first time that a multi-comparison study with tide gauges has been undertaken over such a long time span and that the stability of modern radar tide gauges has been examined. The multi-comparison enabled us to evaluate the performance of the tide gauges in several frequency ranges, identify errors and estimate their magnitude. The drift of the pressure sensors (up to 8.0 mm/yr) was found to be one of the most relevant sources of systematic error. Other sources of difference such as clock drift, scale error and different locations of the instruments were also detected. After correcting the time series of sea level for these errors we estimated an upper bound for the radar instrumental error in field condition at ~0.3 cm.Actualmente se están realizando muchos esfuerzos para renovar las redes maregráficas utilizando nuevas tecnologías. En este contexto, la monitorización del nivel del mar en las islas Kerguelen ha recibido una atención particular debido a su localización única. Hasta cuatro equipos han realizado medidas simultáneamente: dos mareógrafos de presión, un mareógrafo radar y una boya GPS. En este trabajo se analizan y comparan los datos obtenidos con dichos equipos desde 2003 hasta 2010, complementándolos con observaciones realizadas con una escala de marea. Es la primera vez que se plantea una comparación de estas características durante un periodo de tiempo tan largo, y que se aborda el estudio de la estabilidad de los mareógrafos radar a largo plazo. La comparación permitió evaluar el comportamiento de los mareógrafos en distintos rangos de frecuencia, identificar errores y estimar su magnitud. La deriva del sensor de presión apareció como la fuente de error más relevante (hasta 8 mm/año). También se detectaron otras fuentes de diferencias como derivas en el reloj, el error de escala o la diferente localización de los instrumentos. Tras corregir esos errores fue posible estimar un límite superior de ~0.3 cm para el error instrumental del radar en condiciones de campo

Measuring Sea Level with GPS-Equipped Buoys: A Multi-Instruments Experiment at Aix Island

Measuring sea-level in a global reference frame with sub-centimeter accuracy is a relevant challenge in the context of current global warming and associated sea-level rise. Global Navigation Satellite Systems (GNSS) can provide sea-level measurements directly referenced in an absolute geocentric frame. We present here the results of a multi-instruments experiment with three buoys equipped with Global Positioning System (GPS), a radar tide gauge and a tide pole. This experiment was carried out at Aix Island (West coast of France) on the 27-28 March 2012. The GPS buoys were evaluated against conventional tide gauge measurements through a Van de Casteele test. The Root Mean Square Error (RMSE) computed from the difference between the GPS-buoys and radar tide gauge data ranges from 1 cm to 2.2 cm, which is suitable for tidal applications and offers interesting perspectives for future sea-level variations studies.La medición del nivel del mar en un marco de referencias globales con una precisión subcentimétrica es un desafío importante en el contexto del calentamiento mundial actual y del aumento del nivel del mar asociado al mismo. Los Sistemas Mundiales de Navegación por Satélite (GNSS) pueden proporcionar medidas del nivel del mar directamente referenciadas en una estructura geocéntrica absoluta. Presentamos aquí los resultados de un experimento multi-instrumentos con tres boyas equipadas de un Sistema de Posiciona-miento Global (GPS), un mareógrafo con sistema de radar y una escala de mareas. Este experimento fue llevado a cabo en la Isla de Aix (Costa Occidental de Francia), los días 27 y 28 de Marzo del 2012. Las boyas GPS fueron evaluadas comparándolas con las medidas de los mareógrafos convencionales mediante un test Van de Casteele. El Error Cuadrático Medio (RMSE) calculado a partir de la diferencia entre los datos de las boyas GPS y los datos de mareógrafo, oscila de 1 a 2,2 cm, lo que es apropiado para las aplicaciones de mareas y ofrece perspectivas interesantes para futuros estudios de variaciones del nivel del mar.La mesure du niveau de la mer dans un référentiel mondial avec une précision sub-centimétrique est un défi pertinent dans le contexte actuel du réchauffement climatique et de l’élévation du niveau des mers qui en résulte. Les systèmes mondiaux de navigation par satellite (GNSS) peuvent fournir des mesures du niveau de la mer directement rapportées à un référentiel géocentrique absolu. Nous présentons ici les résultats d’une expérience multi-instruments avec trois bouées équipées d’un système de positionnement par satellite (GPS), un marégraphe à radar et une échelle de marée. Cette expérience a été effectuée à l’île d’Aix (côte ouest de la France) les 27 et 28 mars 2012. Les bouées GPS ont été éva-luées par rapport aux mesures du marégraphe conventionnel au moyen d’un test de Van de Casteele. L’erreur quadratique moyenne (RMSE) calculée à partir de la différence entre les données des bouées GPS et celles du marégraphe radar est comprise entre 1 cm et 2,2 cm, ce qui convient pour les applications marégraphique et offre d’intéressantes pers-pectives pour les futures études des variations du niveau de la mer

The status of measurement of the Mediterranean mean dynamic topography by geodetic techniques

We review the measurement of the mean dynamic topography (MDT) of the Mediterranean using ellipsoidal heights of sea level at discrete tide gauge locations, and across the entire basin using satellite altimetry, subtracting estimates of the geoid obtained from recent models. This ‘geodetic approach’ to the determination of the MDT can be compared to the independent ‘ocean approach’ that involves the use of in situ oceanographic measurements and ocean modelling. We demonstrate that with modern geoid and ocean models there is an encouraging level of consistency between the two sets of MDTs. In addition, we show how important geodetic MDT information can be in judging between existing global ocean circulation models, and in providing insight for the development of new ones. The review makes clear the major limitations in Mediterranean data sets that prevent a more complete validation, including the need for improved geoid models of high spatial resolution and accuracy. Suggestions are made on how a greater amount of reliable geo-located tide gauge information can be obtained in the future

Extension of high temporal resolution sea level time series at Socoa (Saint Jean-de-Luz, France) back to 1875

In this data paper sea level time series at Socoa (Saint Jean-de-Luz, Southwestern France) is extended in a data archaeology exercise. We have catalogued water level records stored in ledgers and charts, as well as other associated documents (metadata) in thorough research of national and local archives. An extensive effort was made to rescue these documents by archiving them in digital formats. Based on this large set of rescued documents, the Socoa time series is further extended back in time by about 40 years, at hourly (for ledgers) to 5-minutes (for charts) sampling. Analysis of the precise levelling information reveals that the datum of the tide gauge site has been stable. We assessed the consistency of this new century-long time series based on nearby tide gauge data. Although the overall timeseries is generally consistent, siltation is found to be a recurrent problem of the stilling well which impacted some part of the extended data. However, being a high temporal resolution sea level time series spanning more than 100 years, this new dataset will be useful for advancing climate research, particularly the decadal scale variations in the North Atlantic, as well as the storminess and extreme events along the French Basque coastal region.</p

Is land subsidence increasing the exposure to sea level rise in Alexandria, Egypt?

Delta margins are subject to relatively high rates of land subsidence and have the potential to significantly exacerbate future changes in sea levels predicted by global warming models used in impact studies. Through a combined analysis of GPS and persistent scatterer interferometry data, we determine that most of the coastline of Alexandria has been subject to moderate land subsidence over the past decade (0.4 mm/yr on average and up to 2 mm/yr locally). This contrasts to previous studies that suggested subsidence in excess of 3 mm/yr. Based on our findings, we infer that on multi-century to millennia timescales, land subsidence in the area of Alexandria is dominated by tectonic setting and earthquakes or gravitational collapse episodes of a growth fault, whereas on shorter interseismic decadal to century timescales, subsidence rates are likely steady and moderate, in agreement with natural compaction and dewatering of the observed Holocene sediment layer. Key Points Use of a combined analysis of GPS and PSI data Alexandria coastal are has been subject to moderate land subsidence Important considerations for local planning and policy development ©2013. American Geophysical Union. All Rights Reserved.The work presented in this article was supported by the French Research National Agency (ANR) through the CEP-2009 program under the grant number ANR-09-CEP-001-01 (Project CECILE or “Coastal Environmental Changes: Impact of sea LEvel rise”). Universitat de les Illes Balears provided a visiting professor grant for G. Wöppelmann, whereas M. Marcos acknowledges a “Ramon y Cajal” contract funded by the Spanish Ministry of SciencePeer Reviewe

Spécial Environnement

Bulletin d'information de l'IGN N°66Ce rapport rassemble les contributions de I'IGN dans le domaine de l'environnement et de l'information géographique. Il dresse un bilan pluriannuel et aborde de nombreux thèmes de la géographie numérique : cartographie, images spatiales, système d'information géographique (SIG}, géodésie . . . De façon transversale, les sujets environnementaux sont tout aussi variés : protection des paysages, hydrologie, risques technologiques majeurs, effet de serr

Forcing factors affecting sea level changes at the coast

We review the characteristics of sea level variability at the coast focussing on how it differs from the variability in the nearby deep ocean. Sea level variability occurs on all timescales, with processes at higher frequencies tending to have a larger magnitude at the coast due to resonance and other dynamics. In the case of some processes, such as the tides, the presence of the coast and the shallow waters of the shelves results in the processes being considerably more complex than offshore. However, ‘coastal variability’ should not always be considered as ‘short spatial scale variability’ but can be the result of signals transmitted along the coast from 1000s km away. Fortunately, thanks to tide gauges being necessarily located at the coast, many aspects of coastal sea level variability can be claimed to be better understood than those in the deep ocean. Nevertheless, certain aspects of coastal variability remain under-researched, including how changes in some processes (e.g., wave setup, river runoff) may have contributed to the historical mean sea level records obtained from tide gauges which are now used routinely in large-scale climate research

On the Scientific Applications of IGS Products: An Assessment of the Reprocessed TIGA Solutions and Combined Products

Global sea levels have risen since the early 19th century and this rise is likely to accelerate through the 21st century and beyond. Much of the past information on sea level rise stems from the instrumental records of tide gauges, which measure changes in sea level relative to a tide gauge benchmark (TGBM) situated on land. In order to assess regional or global sea level changes the vertical land movements (VLM) at the tide gauge and its TGBM need to be monitored. GNSS, in particular GPS, has been recognized as one space-geodetic technique to provide highly accurate estimates of VLM in a geocentric reference frame for tide gauges and their TGBMs. As it turned out, this scientific application of GNSS poses the most stringent requirements on the consistency and homogeneity on the data, processing strategies, satellite products, bias models and reference frames used in the analysis of GNSS measurements. Under the umbrella of the International GNSS Service (IGS), the Tide Gauge Benchmark Monitoring (TIGA) Working Group (WG) has the objective to provide highly-accurate positions and VLM estimates for a global network of tide gauges contributing to the Global Sea Level Observing System (GLOSS) and the Permanent Service for Mean Sea Level (PSMSL). As such TIGA forms an important contribution of the IGS to the goals of the Global Geodetic Observing System (GGOS), the Global Climate Observing System (GCOS) and the World Climate Research Programme (WCRP). To achieve the TIGA-WG objectives, five TIGA Analysis Centers (TACs) contributed re-processed global GPS network solutions to TIGA, employing the latest bias models and processing strategies in accordance with the second IGS re-processing campaign (repro2). These individual TAC solutions were then used to compute the combined products by the TIGA Combination Centre (TCC) at the University of Luxembourg using an in-house modified version of the CATREF software package. In this study, we present and internally evaluate the individual TAC and TIGA combined products. We investigate station positions, scale and origin biases, including their frequency content. We also externally evaluate the combined products, particularly the VLM estimates, using solutions from the ITRF2008, ITRF2014 and the glacial isostatic adjustment model ICE-6G (VM5a). Finally, we draw some conclusions on the recent advances and remaining limitations of the various IGS products required for the challenging application to sea level studies