Mapping seabed habitats in the Firth of Lorn off the west coast of Scotland: evaluation and comparison of habitat maps produced using the acoustic ground-discrimination system, RoxAnn, and sidescan sonar

In September 2003 a national workshop took place in the United Kingdom with the aim of assessing the accuracy of the acoustic ground-discrimination system (AGDS), RoxAnn ,a s a tool for mapping seabed habitats in Special Areas of Conservation (SACs). A heterogeneous area of seabed, approximately 1 km 2 in size, was selected for the study. The area was first surveyed using a sidescan-sonar system and a mosaic of the output was produced covering 100% of the survey area. Interpretation of the mosaic identified three acoustically distinct seabed types, the spatial distributions of which were mapped. Four RoxAnn data sets were then collected over the same area of seabed applying different survey parameters (e.g. different survey grids, track spacing, survey vessels, survey speeds, and RoxAnn systems). Extensive ground-truthing was carried out involving 26 drop-down video stations, and from these data six benthic classes (seabed habitats) were identified. Following interpolation of the RoxAnn track-point data to produce full spatial-coverage data, these sixseabed habitat categories were used to conduct supervised classification of the RoxAnn data to produce full-coverage habitat maps of the area for each of the four RoxAnn data sets. Comparisons were then made between the four RoxAnn maps and the sidescan-sonar interpreted map. The accuracy of each map was assessed and the application of this mapping approach for mapping seabed habitats in SACs is discussed.