S-44 evolution Multibeam sonar workshop, 3-4 April 2005 St. Andrews, NB Rob Hare, P. Eng., C.L.S. Canadian Hydrographic Service Chris Howlett UK Hydrographic Office
Overview Some History Problems identified with 4th edition Suggested improvements Organisation of 5th edition Working Group Some thoughts for discussion Other TORs
The past decade • S-44 3rd Edition, 1987 in force • Standards for Hydrographic Surveys • Adequate for systematic single beam surveys • Scale-based, line spacing, plotting interval, check-line spacing • Used 90% confidence level for depth (95% for positions) • Separated depth measurement from depth reduction error • Classification Criteria for Deep Sea Soundings • Procedures for Elimination of Doubtful Data • Review and update every 5 years?
The 4th Edition, April 1998 • Introduced 4 Orders of surveys • Requirements for total bottom coverage and target detection in higher orders • Reduced dependency on survey scale • Considered fixed and variable error contributions, TPE • All estimates at 95% • Combined reduced depth error • Made accuracy attribution mandatory • Proposed complete replacement of doubtful data • Classification of Deep Sea Soundings unchanged (as Annex A)
What good came from it? • Promoted accuracy estimation and attribution in the hydrographic community (HOs and manufacturers) • Lead to RTQA and CUBE approaches • Increased MBES resolution • Improvements in attitude sensors • More frequent sound speed sampling • Adaptations: • SMA (Sweden) Exclusive Order • USACE shallow water standards • LINZ provisional swath sonar survey specifications • IMCA offshore construction standards • CHS (in press) standards and survey management guidelines • Added Exclusive and Imprecise Orders
Problems with 4th Edition • Lack of clarity • Target detection depth, shape of target • Overlap versus coverage • Confusion over fixed and variable error bounds (a and b) versus TPE • No discussion of cross-checks, post-mission assessment techniques • On along-track sounding spacing • Failed to consider: • Non-navigation users – e.g. UNCLOS, Habitat mapping • How HOs and manufacturers would go about proving detection of targets or complete bottom coverage
Suggested improvements • Clarify target detection depths, types of targets, coverage and overlap • Enough details to allow manufacturers to prove their systems can meet the standard • Clarify fixed and variable error bounds vs TPE • Consider non-navigation users • Uses of multibeam backscatter • UNCLOS requirements • Provide more guidance and specificity • E.g. along-track sounding spacing, cross checklines, beamwidths, post-mission assessment • Ensure that standards are achievable
Organisation for 5th Edition • S-44 is the minimum standard for hydrographic surveys to provide data for navigational products • 14 specific items covered in TORs • Each item to be the subject for a sub group • Sub groups to complete their task and report to main WG by May 2006 • No formal meetings planned (maybe) • Whenever interested people get together they can discuss an aspect. Report to WG or sub group via a WG member
Organisation for 5th Edition • IHO web site will hold details of WG members and sub group composition • E-mail ‘news group’ to be set up for WG correspondence. Only WG members can write to this but messages open to all to read • Possible meeting in Monaco May 2006 to finalise sub group workings
Colour Postscript Plotter Workstation Printer Some Thoughts for Discussion • S-44 should concentrate on how well the terrain model agrees with the actual sea bed. Sounding / depth accuracy, 100% bathy and object detection criteria only go to define this accuracy! Seabed mapping process Sounding process (on ship) Data processing Motion compensation Sound vel modelling Positioning Echosounder = ? = Seabed Terrain Terrain model Thanks to Kongsberg Maritime
Some Thoughts continued • I suggest a new form of survey classification that defines the quality of a survey. The classification needs to be ‘future proof’ in that it caters for accuracies that are not achievable as yet while also linking with ZOC. • A 3 character code XYZ such that: • X = overall assessment of quality (uncertainty?) • Y = horizontal accuracy (minimum horizontal TPE?) • Z = vertical accuracy (minimum vertical TPE?)
Some Thoughts continued • X roughly equates to ZOC in that: • A = fully surveyed, no objects exist that are larger than the accuracy defined by Y and/or Z above • B = not fully surveyed, but no objects that are larger than the accuracy defined by Y and/or Z above are thought to exist • C = not fully surveyed, objects that are larger than the accuracy defined by Y and/or Z above are thought to exist • D = open line spacing only
Some Thoughts continued • Y and Z define various accuracies in the horizontal and vertical planes. These may be minimum TPE values or some other measure.
Some Thoughts continued • S-44 defines the standard that is required for each given area: e.g. harbour approaches could require surveys of ADD standard or waters off shipping lanes as BGG???
Some Thoughts continued • Object detection requirements (currently 2m down to 40m and 10% thereafter – IHO order 1) should change to enable the survey to meet the relevant ZOC. Hence if we are looking for a survey of AAA standard, the object detection must equate to that size. • Whether we need 100% bathymetry is also governed by the required order and possibility of achieving the designated accuracy. In flat featureless mud single beam / side scan (or skunk striped multi beam) will suffice as the interpolated model between lines will be accurate providing that any features identified on side scan are covered by additional lines.
Some Thoughts continued • S-44 has to define how HOs can confirm that the required accuracies have been achieved. Maybe density of soundings (UKHO uses 9 valid soundings in a target sized bin). This is HO and equipment specific and may need constant updating. • Maybe need to retain an active set of pages on IHO web site to include new ideas in this area??
TOR • To help with maintenance, maybe split S-44 into standards and guidelines? • Metadata requirements for hydrographic surveys – compatibility with other standards • Processing and archiving data • Annex A: retain or update? • Draft for approval by end 2006