R&D and Education
Conor Cahalane, Aidan Magee, Xavier Monteys, Gema Casal, Jennifer A. Hanafin, P. Harris (2019) A comparison of Landsat 8, RapidEye and Pleiades products for improving empirical predictions of satellite-derived bathymetry, Remote Sensing of Environment, DOI: https://doi.org/10.1016/j.rse.2019.111414
Satellite derived bathymetry (SDB) enables rapid mapping of large coastal areas through measurement of optical penetration of the water column. The resolution of bathymetric mapping and achievable horizontal and vertical accuracies vary but generally, all SDB outputs are constrained by sensor type, water quality and other environmental conditions. Efforts to improve accuracy include physics-based methods (similar to radiative transfer models e.g. for atmospheric/vegetation studies) or detailed in-situ sampling of the seabed and water column, but the spatial component of SDB measurements is often under-utilised in SDB workflows despite promising results suggesting potential to improve accuracy significantly. In this study, a selection of satellite datasets (Landsat 8, RapidEye and Pleiades) at different spatial and spectral resolutions were tested using a log ratio transform to derive bathymetry in an Atlantic coastal embayment. A series of non-spatial and spatial linear analyses were then conducted and their influence on SDB prediction accuracy was assessed in addition to the significance of each model's parameters. Landsat 8 (30 m pixel size) performed relatively weak with the non-spatial model, but showed the best results with the spatial model. However, the highest spatial resolution imagery used – Pleiades (2 m pixel size) showed good results across both non-spatial and spatial models which suggests a suitability for SDB prediction at a higher spatial resolution than the others. In all cases, the spatial models were able to constrain the prediction differences at increased water depths.
Kieran Westley, Ruth Plets, Rory Quinn, Fabio Sacchetti, Mekayla Dale, Rory McNeary and Annika Clements
Abstract
Conservation of historic shipwrecks is prohibitively expensive and in situ preservation and recording are the preferred archaeological
approaches. Non-destructive high-definition 3D imaging is therefore essential for recording and managing submerged historic shipwrecks. Multibeam echosounders (MBES), the standard tool for hydrographic survey, can produce point clouds to image complex 3D structures. However, wreck imaging is often done using MBES in traditional survey mode optimised for morphological characterisation of the seafloor. This does not necessarily provide high-definition imagery required by archaeologists. This study demonstrates key factors influencing high-definition MBES imaging of wrecks through a controlled field experiment. Results show that optimal high-definition 3D imaging is achieved through maximising the pulse rate, narrowing the angular sector, using the highest frequency and shortest pulse lengths, applied to at least 3 to 5 overlapping centreline-parallel and offset passes with additional perpendicular/oblique lines. Variations in survey design are demonstrated to exert strong controls on sounding density and distribution, with high-density on horizontal and vertical wreck surfaces enabled by a combination of overlapping passes and offset lines. Adoption of this method would result in more widespread high-definition 3D imaging of wrecks to benefit archaeological research and develop effective mitigation strategies to minimise loss of the fragile underwater resource.
Cite this article as:
Westley, K., Plets, R., Quinn, R. et al. Archaeol Anthropol Sci (2019).
https://link.springer.com/article/10.1007/s12520-019-00831-6
Gema Casal, Xavier Monteys, John Hedley, Paul Harris, Conor Cahalane and Tim McCarthy (2018) Assessment of empirical algorithms for bathymetry extraction using Sentinel-2 data, International Journal of Remote Sensing, DOI: 10.1080/01431161.2018.1533660
Bathymetry estimated from optical satellite imagery has been increasingly implemented as an alternative to traditional bathymetric survey techniques. The availability of new sensors such as Sentinel-2 with improved spatial and temporal resolution, in comparison with previous optical sensors, offers innovative capabilities for bathymetry derivation.
This study presents an assessment of the fit between satellite data and the underlying models in the most widely used empirical algorithms: the linear band model and the log-transformed band ratio model using Sentinel-2A data. Both models were tested in two study areas of the Irish coast with different morphological and environmental conditions. Atmospheric correction, bottom type influence, and water column conditions proved to be key factors in the bathymetric derivation using these satellite datasets.
Download the publication here: https://www.tandfonline.com/doi/abs/10.1080/01431161.2018.1533660
Anu Marii Kaskela, Aarno Tapio Kotilainen, Ulla Alanen, Rhys Cooper, Sophie Green, Janine Guinan, Sytze van Heteren, Susanna Kihlman, Vera Van Kancker, Alan Stevenson and the EMODnet Geology Partners.
Geosciences 2019, 9(2), 84; https://doi.org/10.3390/geosciences9020084
The importance of an integrated knowledge on seafloor sediment distribution for European seas is highlighted in work just published: "Picking up the pieces – harmonising and collating seabed substrate data for European maritime areas" in the Geosciences Special Issue on "Geological Seafloor Mapping".
The publication describes how the European Marine Observation and Data Network (EMODnet) initiated by the European Commission, has assembled and provided free access to marine geological data through a web data portal.
EMODnet Geology has collated all available seabed substrate maps for European seas to create multiple maps at different scales and Folk granularities, where Folk refers to the internationally recognised Folk classification scheme. INFOMAR contributes data on Ireland’s seabed substrate distribution from all surveyed coastal areas extending to the deeper water depths ~4500 m.
A confidence assessment of the data is provided to ensure users understand the ‘confidence’ in the map reflecting the amount of information available from acoustic data and seabed samples to create the map. INFOMAR data has an associated high level of confidence for much of the Irish offshore highlighting the benefits of full coverage survey data in Ireland’s Exclusive Economic Zone.
Information on seabed substrates is particularly valuable for marine spatial planning where it facilitates investment in sustainable coastal and offshore activities through better access to standardised marine data.
Download the publication Picking Up the Pieces—Harmonising and Collating Seabed Substrate Data for European Maritime Areas
