UK Hydrographic Office Research Articles

Improved bathymetry leads to >4000 new seamount predictions in the global ocean - but beware of phantom seamounts!

Seamounts are important marine habitats that are hotspots of species diversity. Relatively shallow peaks, increased productivity and offshore locations make seamounts vulnerable to human impact and difficult to protect. Present estimates of seamount numbers vary from anywhere between 10,000 to more than 60,000. Seamount locations can be estimated by extracting large, cone-like features from bathymetry grids (based on criteria of size and shape). These predicted seamounts are a useful reference for marine researchers and can help direct exploratory surveys. However, these predictions are dependent on the quality of the surveys underpinning the bathymetry. Historically, quality has been patchy, but is improving as mapping efforts step up towards the target of complete seabed coverage by 2030. This study presents an update of seamount predictions based on SRTM30 PLUS global bathymetry version 11 and examines a potential source of error in these predictions. This update was prompted by a seamount survey in the British Indian Ocean Territory in 2016, where locations of two putative seamounts were visited. These 'seamounts' were targeted based on previous predictions, but these features were not detected during echosounder surveys. An examination of UK hydrographic office navigational (Admiralty) charts for the area showed that the summits of these putative features had soundings reporting 'no bottom detected at this depth' where 'this depth' was similar to the seabed reported from the bathymetry grids: we suspect that these features likely resulted from an initial misreading of the charts. We show that 15 'phantom seamount' features, derived from a misinterpretation of no bottom sounding data, persist in current global bathymetry grids and updated seamount predictions. Overall, we predict 37,889 seamounts, an increase of 4437 from the previous predictions derived from an older global bathymetry grid (SRTM30 PLUS v6). This increase is due to greater detail in newer bathymetry grids as acoustic mapping of the seabed expands. The new seamount predictions are available at https://doi.pangaea.de/10.1594/PANGAEA.921688.

How the Liverpool Tidal Institute was established: industry, navy and academia

Abstract. This paper outlines the establishment of the Liverpool Tidal Institute in 1919. There is a particular focus on early patrons and supporters in the context of both previous tidal research on the accuracy of predictions and debates about the involvement of state actors in science at the end of the First World War. It discusses how, and to what extent, various actors – Liverpool University, the British Association for the Advancement of Science, the UK Hydrographic Office, and the shipping industry – became involved with the institute and what their roles were in its creation. It shows that industrial support was crucial in the establishment of this academic institute which later became a key contractor to the Navy.

Interpreting monitoring data for shoreline and geohazard mapping

The demand for marine-related spatial information has become increasingly apparent in recent years at a European and national scale, due to the increased pressures on the sea-floor environments and marine resources of UK territorial waters. The advent of economically viable swath bathymetry data acquisition in the coastal zone and effective collaborative partnerships between the Channel Coastal Observatory, Maritime and Coastguard Agency, UK Hydrographic Office, British Geological Survey and academic institutions, have opened up new opportunities to produce a robust scientific evidence base to inform integrated coastal zone management objectives and contribute to wider scientific initiatives. Interpretation of high-quality bathymetric data, acoustic backscatter and ground-truthing data allows zones of exposed bedrock, rock outcrops and pinnacles to be identified, along with areas of mobility or stability of surficial sediments. Temporal and spatial analyses of coastal and marine monitoring datasets also contribute to improved understanding of interactions between natural coastal process and coastal-defence and beach-management operations. Furthermore, developments in three-dimensional mapping techniques and visualisation technologies have enabled seamless high-resolution coastal geology maps to be re-interpreted and extended offshore, providing a more complete picture of the baseline geology, physical properties, structure and geohazards in the coastal and nearshore zone. The full paper details the methodology developed to produce a range of indicative marine mapping layers, and presents examples from eastern and southern England where marine-related spatial data has contributed to the multi-disciplinary scientific evidence base to inform development of UK marine policy and planning, coastal management and coastal zone geological mapping.

Global Ocean Tide Models: Assessment and Use within a Surface Model of Lowest Astronomical Tide

The UK Hydrographic Office (UKHO)-sponsored Vertical Offshore Reference Frames (VORF) project aims to develop tidal level transformation models that are referenced to the GRS80 ellipsoid and thus compatible with GNSS positioning; in particular, heighting. Benefits include increasing the efficiency of hydrographic surveying, providing a stable consistent reference frame and enabling integration with land data in the coastal zone. Seven contemporary global ocean tide models are used to derive Lowest Astronomical Tide (LAT) surfaces which are each assessed by comparison with LAT values from the 7,389-strong UKHO tide gauge database, with the results correlated with distance from land. The proportion of truly offshore and pelagic gauges is relatively limited; however, the transition zone whereby the global ocean tide models commence to deteriorate in accuracy is evident at approximately 30km from the coast. The DTU10 model was selected as the strongest candidate overall. Subsequently, a thin plate spline method is used with the tide gauge dataset to enhance the DTU10 LAT surface in the coastal zone, creating a high resolution global LAT surface with respect to mean sea level. It is seen by cross-validation that the method may be used to predict LAT in near-shore locations with a standard error of 0.23 m.

Interpolation of Tidal Levels in the Coastal Zone for the Creation of a Hydrographic Datum

Abstract As part of the U.K. Hydrographic Office (UKHO)-sponsored Vertical Offshore Reference Frames (VORF) project, a high-resolution model of lowest astronomical tide (LAT) with respect to mean sea level has been developed for U.K.–Irish waters. In offshore areas the model relies on data from satellite altimetry, while in coastal areas data from a 3.5-km-resolution hydrodynamic tide-surge model and tide gauges have been included. To provide for a smooth surface and predict tidal levels in unobserved areas, the data have been merged and interpolated using the thin plate spline method, which has been appropriately tuned by an empirical prediction test whereby observed values at tide gauges were removed from the solution space and surrounding data used to predict its behavior. To allow for the complex coastal morphology, a sea distance function has been implemented within the data weighting, which is shown to significantly enhance the solution. The tuning process allows for independent validation giving a standard error of the resulting surface of 0.2 m for areas with no tidal observations.

Digital charting: a Royal Navy navigator's view

The IMO have given their approval to the concept of an electronic chart display and information system (ECDIS) (IMO Assembly resolution A.817(19)) but the detailed technical specification required for Type Approval is still to be completed. Meanwhile, the approved 'vector data' required for ECDIS is only just beginning to be available and its form is still evolving. The Royal Navy is moving towards adoption of this new technology and has mandated its use for a future class of frigate. It is already involved in trials of prototype equipment, albeit using a (raster) data format which has yet to receive international approval. Trials supported by UK manufacturers and the UK Hydrographic Office are proving beneficial to all involved and vector data was incorporated during 1996. The need to strike a balance between adapting current navigation techniques to the electronic era and devising new ones has already been recognised, as is the inevitable impact on training. The ability to measure positions, bearings and distances far more accurately than ever before is putting the hydrographers on their mettle, while the need to understand and correctly apply datum shifts and match projections when displaying different forms of data is posing a new challenge for the navigator. The end of the paper chart in the RN may be forecastable but is not yet in sight and there are a number of questions that need to be resolved before ECDIS becomes accepted as the prime method of navigation.