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Uncertainty Workshop: Sounding attributes

Uncertainty Workshop: Sounding attributes. Rob Hare Manager, Hydrographic Surveys Canadian Hydrographic Service, Pacific Region CHC2004 May 24, 2004. Abstract. A discussion of attributes on soundings , sources of error and computation of sounding error. Plus a whole lot more

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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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