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Uncertainty Workshop: Sounding attributes

This workshop, led by Rob Hare from the Canadian Hydrographic Service, explores the critical attributes of soundings in hydrographic surveys, focusing on sources of error and methods for error prediction. Attendees will learn about key data quality elements, including completeness and positional accuracy, and the importance of various statistical estimators for quality assessment. The workshop will cover vertical absolute accuracy, common errors in measurements, and techniques to estimate and minimize uncertainty, providing insights for real-time QA and post-mission assessment.

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Uncertainty Workshop: Sounding attributes

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  1. Uncertainty Workshop:Sounding attributes Rob Hare Manager, Hydrographic Surveys Canadian Hydrographic Service, Pacific Region CHC2004 May 24, 2004

  2. Abstract • A discussion of attributes on soundings, sources of error and computation of sounding error. • Plus a whole lot more • An error prediction tool along with the characteristics and capabilities of (a few) sounding systems will be reviewed.

  3. Data Quality Elements • Completeness • Logical consistency • Positional accuracy • Has horizontal and vertical components • Sub-elements: absolute accuracy, attribute completeness, shape fidelity, time accuracy, topologic consistency • Temporal accuracy • Thematic accuracy

  4. Estimation of Quality metrics • Direct and indirect methods • Require absolute coordinate reference frame (e.g. WGS-84) • Most direct methods impractical • source inter-comparison is an exception • Indirect methods require validation • e.g. deductive error estimation (forward error prediction)

  5. Statistical estimators • Measures of central tendency • Sample mean • Median, Mode • Measures of dispersion (1-D and 2-D) • Standard deviation (or variance), RMS • CEP, MSEP,drms • Total Propagated Error (TPE) • CAUTION - many hydrographic measurements are correlated (e.g. H&V components of soundings)

  6. Why estimate error/uncertainty? • Preanalysis • Will my system meet specifications? • Do I purchase a new … C/B analysis? • Real-time QA • Am I collecting enough data to meet specifications? • Do I modify my sampling/processing strategy, discard outer beams, increase overlap, take more sound speed profiles, etc.? • Post-mission assessment • Did I meet specifications? Classification - what Order did I achieve? • Provide metadata for informed decision making/risk assessment • Data attribution for integration/validation/comparison of different data sets • Assessment of historic sources • Initialization of CUBE estimator? • Create Source Classification or Reliability Diagrams or ZOCs

  7. Part 1 - The VERTICAL

  8. Absolute Vertical Accuracy • Estimates of vertical precision • Errors common to all vertical measurements • Errors common to GPS vertical measurements • Depth soundings • Shoal examinations • Drying heights • Elevations and clearances

  9. Depth soundings • Leadline, sounding pole, rod • Single-beam echosounder - SBES • analogue sounders • digital sounders • Sweep (multi-transducer) • Lidar • Swath (multibeam echosounder - MBES) • Other – e.g. TIBS, wire or bar sweep, diver

  10. Sources of error - water levels • gauge measurement precision • method of filtering sea surface waves • timing synchronisation of gauge and measurement • vertical datum precision • spatial extrapolation to the location of the vertical measurement, or in the case of predictions, • quality of constituent set (length/quality of observations) • correction for environmental effects

  11. Other sources of vertical error • Draft and squat or settlement • Antenna to transducer offset (GPS) • Datum separation model (GPS) • Sounding measurement • Heave • Sound speed

  12. Dynamic draft Tide, WL θ r Chart datum Measured Depth, d Charted Depth, D Traditional sounding reduction • D = d + draft – WL • d = r cos (θ+R) cos P • r = range • Θ = beam angle • R = Roll angle • P = Pitch angle

  13. GPS RTK, Z Ellipsoid Antenna Height, K Separation Model, M Dynamic draft Tide, WL θ r Chart datum Measured Depth, d Charted Depth, D RTK GPS sounding reduction • D = d + K – Z – M • K = Δx sinP • + Δy cosP sinR • + Δz cosP cosR

  14. Vertical error propagation

  15. SBES error sources • depth measurement • algorithm, frequency, beamwidth, pulse length • sound speed correction method • draft (and squat if applied) • heave (measured) • tides • manual trace reading, resolution, recording and reduction method

  16. Lidar error sources • Depth measurement • Refraction correction (calibration) • Sea-surface modelling • Tides • Footprint spreading

  17. MBES error sources • Range and beam angle measurement • detection method (amplitude or phase) • Refraction correction • Dynamic draft • includes squat, settlement, change of trim • Heave (measured and induced) • Tides or water levels • Roll, pitch, heading • Calibration (patch test) offsets • Roll, pitch, heading, sensor latency • Positioning system

  18. Alignment errors

  19. MBES coordinate systems

  20. Mapping depth errors

  21. Confidence levels • For a normal distribution, the probabilities of univariate random errors of a single measurement falling within a certain level of error (number of standard deviations, ) are given in the following table.

  22. Depth error estimates

  23. Additional vertical corrections • sounding datum adjustments • metric conversion • may include generation of metric contours • sound speed corrections

  24. Potential limitations • heave estimation (manual) • phase lag or latency • sound speed changes • manual trace reading • vertical display resolution • stepped vertical datum zones • shoal biasing • datum separation models

  25. Part 2 - The HORIZONTAL

  26. Absolute Horizontal Accuracy • Estimates of horizontal precision • Field sheet processes • Soundings • Shoal examinations • Other data types • Heights, elevations, clearances • Navigational Aids • Shoreline • Bathymetric contours • Cartographic processes (including digitizing)

  27. Sounding Position Equations

  28. Estimates of precision • Rigorous - error ellipses • HDOP • MSEP • CEP • CSE • drms

  29. Errors in Multibeam soundings • GPS or POS/MV position error • transducer - antenna offsets • sounder measurement error • range, beam angle, detection method • refraction correction error • transducer shape, orientation, roll-modulation • roll, pitch and heading error • latency between systems • GPS, sounder, VRU

  30. Mapping position errors

  31. Confidence levels

  32. Position error estimates

  33. Field Sheet processes • Horizontal datum • Manual processes • Materials and construction • Horizontal control • Station plotting • Data types • Soundings • Shoal examinations, drying heights • Contours • Navigational aids • Shoreline (natural and man-made) • Seafloor samples • Topography, elevations, clearances These are scale-dependent errors

  34. Soundings - sources of error • Positioning system/method • Sounding system • including offsets in space and time • Plotting • Inking • Digitizing

  35. Positioning systems/methods • Manual/Optical • Sextant (eccentric circle LOP) • Subtense (eccentric circle LOP) • Range poles (straight-line LOP) • Azimuth (straight-line LOP) • Electronic positioning systems (EPS) • Range-bearing (hybrid - circle and line) • Two-range or range-range (later multi-range) - concentric circle LOP • Hyperbolic (2 and later multi-hyperbola) LOP • Transit satellite (Doppler) - spherical LOP • GPS (spherical LOP) • DGPS • OTF

  36. Plotting, Inking, Digitizing errors • Plotting • 3-arm protractor • lattices • collector registration • fixes and ‘tweeners’ • Inking • on collector • on field sheet • registration • Digitizing • digitizer resolution • registration (rms of fit on HCP) - transformation used • datum shift • projection These are scale-dependent errors

  37. Systematic error sources (safety biasing) • Generalization • Line smoothing • Symbolization • Feature displacement • soundings • bottom samples (more for clarity than safety)

  38. Drafting (from final compilation manuscript) • Control points • Linework (shoreline) • Soundings • Symbols • Contours • Bottom samples

  39. Digitizing • Registration/rectification method • Digitizer precision, resolution • Optical centre recognition • Line-following ability • QC tolerances (chart specifications)

  40. Sources, products and points of digitization

  41. Data inter-comparisons:validating error prediction assumptions • Methods • compare same data set at different process stages • difference DTM (soundings) • inter-comparison of point features (shoal examinations, drying heights) • linework comparisons (HWL, LWL, contours) • validate inference methods (expected vs. actual) • Limitations • small statistical sample • is it the same feature? • separation of depth from position error

  42. Quality Implementations • Data collection - ASCII, Simrad, Hypack, NMEA • Data processing - HIPS (Quality flag, coverage, standard deviation), CUBE (stochastic surface, etc.) • Data storage - Caris ASCII, GSF, SDS, … • Paper Charts - Source Classification/Reliability Diagrams, Explanations in Sailing Directions/Pilots • ENC - S-57 objects and meta-objects • (M_QUAL, M_ACCY, CATZOC, CATQUA, QUASOU, TECSOU, POSACC) • Raster Charts?

  43. Summary • Depth and position errors can be estimated using forward error prediction • These estimates can be validated by inter-comparison of data sets • Estimates can be used: • To make decisions regarding equipment selection and purchase • To determine if specifications can be/have been met • To adapt your sampling strategy • As an input to statistical processing algorithms • These are but two of many Quality/Uncertainty Measures

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