English Channel towed sledge seabed images. Phase 1:udscoping study and example analysis

摘要

During the 1970’s and 1980’s, the late Dr Norman Holme undertook extensive towedudsledge surveys in the English Channel and some in the Irish Sea. Only a minority of theudresulting images were analysed and reported before his death in 1989 but logbooks, videoudand film material has been archived in the National Marine Biological Library (NMBL) inudPlymouth. A scoping study was therefore commissioned by the Joint Nature ConservationudCommittee and as a part of the Mapping European Seabed Habitats (MESH) project toudidentify the value of the material archived and the procedure and cost to undertake furtherudwork.udThe results of the scoping study are:ud1. NMBL archives hold 106 videotapes (reel-to-reel Sony HD format) and 59 videoudcassettes (including 15 from the Irish Sea) in VHS format together with 90 rolls of 35 mmudcolour transparency film (various lengths up to about 240 frames per film). These areudstored in the Archive Room, either in a storage cabinet or in original film canisters.ud2. Reel-to-reel material is extensive and had already been selectively copied to VHSudcassettes. The cost of transferring it to an accepted ‘long-life’ medium (Betamax) would beudapproximately £15,000. It was not possible to view the tapes as a suitable machine wasudnot located. The value of the tapes is uncertain but they are likely to become beyondudsalvation within one to two years.ud3. Video cassette material is in good condition and is expected to remain so for severaludmore years at least. Images viewed were generally of poor quality and the speed of towudoften makes pictures blurred. No immediate action is required.ud4. Colour transparency films are in good condition and the images are very clear. Theyudprovide the best source of information for mapping seabed biotopes. They should beudscanned to digital format but inexpensive fast copying is problematic as there are noudbetween-frame breaks between images and machines need to centre the image based onudbetween-frame breaks. The minimum cost to scan all of the images commercially isudapproximately £6,000 and could be as much as £40,000 on some quotations. There is audfurther cost in coding and databasing each image and, all-in-all it would seem mostudeconomic to purchase a ‘continuous film’ scanner and undertake the work in-house.ud5. Positional information in ships logs has been matched to films and to video tapes.udDecca Chain co-ordinates recorded in the logbooks have been converted to latitude andudlongitude (degrees, minutes and seconds) and a further routine developed to convert touddegrees and decimal degrees required for GIS mapping. However, it is unclear whetherudcorrections to Decca positions were applied at the time the position was noted. Tow tracksudhave been mapped onto an electronic copy of a Hydrographic Office chart.ud6. The positions of start and end of each tow were entered to a spread sheet so that theyudcan be displayed on GIS or on a Hydrographic Office Chart backdrop. The cost of theudHydrographic Office chart backdrop at a scale of 1:75,000 for the whole area was £458udincl. VAT.ud7. Viewing all of the video cassettes to note habitats and biological communities, even byudan experienced marine biologist, would take at least in the order of 200 hours and is notudrecommended.udEnglish Channel towed sledge seabed images. Phase 1: scoping studyudand example analysis.ud6ud8. Once colour transparencies are scanned and indexed, viewing to identify seabedudhabitats and biological communities would probably take about 100 hours for anudexperienced marine biologist and is recommended.ud9. It is expected that identifying biotopes along approximately 1 km lengths of each towudwould be feasible although uncertainties about Decca co-ordinate corrections and exactudpositions of images most likely gives a ±250 m position error. More work to locate eachudimage accurately and solve the Decca correction question would improve accuracy ofudimage location.ud10. Using codings (produced by Holme to identify different seabed types), and someudviewing of video and transparency material, 10 biotopes have been identified, althoughudmore would be added as a result of full analysis.ud11. Using the data available from the Holme archive, it is possible to populate variousudfields within the Marine Recorder database. The overall ‘survey’ will be ‘English Channeludtowed video sled survey’. The ‘events’ become the 104 tows. Each tow could be describedudas four samples, i.e. the start and end of the tow and two areas in the middle to giveudexamples along the length of the tow. These samples would have their ownudlatitude/longitude co-ordinates. The four samples would link to a GIS map.ud12. Stills and video clips together with text information could be incorporated into audmultimedia presentation, to demonstrate the range of level seabed types found along audpart of the northern English Channel. More recent images taken during SCUBA diving ofudreef habitats in the same area as the towed sledge surveys could be added to the Holmeudimages.