© dave wells 2004 26 May 2004Uncertainty Indicators1 of 50 The Increasing Need to Provide Uncertainty Indicators to Users of Hydrographic Products UNB

26 May 2004 Uncertainty Indicators 1 of 50

© dave wells 2004

The Increasing Need to Provide Uncertainty Indicators to Users of

Hydrographic Products

UNB GGE5072 & GGE 6021 students

USM HYD605 students

Rob Hare, Dave Monahan, Dave Wells

Beyond Safety of NavigationMultibeam & Visualization Workshop

Gulfport 26-29 July 2004


© dave wells 2004

“Fit for intended use” (again)The purpose of hydrographic information whether for

safety of navigation, or for applications “beyond safety of navigation”, is to

support informed decision making

The quality of the information will affect the quality of the decisions made based upon it.

End users must be informed about the quality of the data upon which they base their decisions.


© dave wells 2004

ISO 19113 Data quality subelementscompleteness

commission: excess data / omission: data absent

logical consistencyconceptual consistency / domain consistency / format consistency /

topological consistency

positional accuracyabsolute or external accuracy / relative or internal accuracy / gridded data

position accuracy

temporal accuracyaccuracy of a time measurement / temporal consistency / temporal validity

thematic accuracyclassification correctness / non-quantitative attribute correctness /

quantitative attribute accuracy


© dave wells 2004

OutlineExerpts from

CHC2004 Workshop / Tutorial

Uncertainty Management in Hydrography

24 May 2004

Organized by Sue Sebastian

Quality Assurance Branch Head

Naval Oceanographic Office

Full proceedings available at

ftp://moray.dms.usm.edu/CHC2004_Uncertainty_Workshop


© dave wells 2004

The way it was

In past decades, positioning methods used for hydrographic surveys were always more accurate than chart users could navigate.

Navigators were trained to allow a margin of error for the inaccuracy of their own positioning.

Inaccuracies in the charts did not need to be taken into account.

Hans van Opstal, Melaha 2004


© dave wells 2004

The way it is nowAlmost all navigators use the Global Positioning System

(GPS), or even differentially-corrected GPS (DGPS).

These provide far better accuracy than was available when the surveys for most existing charts were performed.

Many users of nautical charts have no idea how uncertain is the information shown on the charts they are using.

This lack of appropriate information leads to inappropriate navigation decisions, groundings, and perhaps loss of life.

Statistics: Over half of the inshore US NOAA nautical charts were acquired by lead-line and sextant surveying prior to 1940. Over 25% of the charted areas on the Canadian British Columbia coast are based on lead-line and sextant surveys.

Hans van Opstal, Melaha 2004


© dave wells 2004

The way it will beHigh-density survey methods (multibeam sonar, LIDAR,

acoustic sweeps) can now provide complete seafloor coverage in digital form.

This allows a much more sophisticated approach to uncertainty management.

New ideas have emerged that may fundamentally change the way hydrographic data is managed (e.g. CUBE).

BUT

Most charts, for many many years, will remain based on legacy data, for which these new approaches have limited application.


© dave wells 2004

Uncertainty management steps Specification: What decisions (navigation and non-

navigation) will be based on hydrographic data? What 95% confidence level uncertainty do these decisions require?

Design: Select equipment, survey procedures and data processing methods which will likely meet the specification.

Attribution: Measure and/or model parameters that describe the uncertainty of the collected data.

Assurance: Include redundancy and calibration procedures to permit assessing actual uncertainties and 95% confidence regions, against predicted uncertainties.

Presentation: Provide uncertainty results in an easily understood way to those making decisions based on multibeam survey results. A big difference here between legacy and high-density data.


© dave wells 2004

TPE (attribution)Total Propagated Error: an attempt to account for all

influences on depth and position uncertaintyInput: all sensor (and processing) uncertaintiesUsing: Law of propagation of variancesOutput: predicted uncertainties in depth and position.

Example: depth uncertainty depends on Measurement uncertainties: echosounder range and beam angle,

heave, pitch, roll, water level and sound speed profile. Configuration uncertainties: sensor geometric and time offsets, and

misalignments. Model uncertainties: draft, squat, load, induced heave, temporal and

spatial variability in water level and sound speed, echosounder beam steering.

These uncertainties vary with beam angle, depth, sea state, bottom type etc.


© dave wells 2004

Example: Depth error estimates

From Rob Hare

EM1002 depth error estimates (95%)

(view looking aft, EA mode)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-250 -200 -150 -100 -50 0 50 100 150 200 250

Cross-track distance (m)

Sounder Heave Roll Pitch

Refraction Total depth measurement Dynamic draft Water level

Total reduced depth (EA mode) IHO Special Order IHO Order 1


© dave wells 2004

Example: position error estimates

From Rob Hare

EM1002 position error estimates (2MSEP)(view looking aft, EA mode)

0

2

4

6

8

10

12

-250 -200 -150 -100 -50 0 50 100 150 200 250

Cross-track distance (m)Positioning System Sounding Refraction Roll Pitch Heading Sounding position IHO Special Order IHO Order 1


© dave wells 2004

The Hare-Calder-Smith method to manage uncertainty

1 (Hare - TPE) Compute 3D uncertainty attributes for every depth data point, using theoretical or empirical models for sensor errors.

2 (Calder - CUBE) Propagate depths & uncertainties to nodes. Allow alternative hypotheses. Use several “disambiguation” metrics.

3 (Smith - Nav Surface) Maintain rare “golden” shoal depths. Defocus shoals to represent georef uncertainty. Generalize by 3D double-buffering.


© dave wells 2004

Representing uncertainty on legacy (paper) charts

Option 1 - No information provided / availableOption 2 - Source Diagram (SD) describes parameters of the

field survey (e.g. date, line spacing, agency, depth sensor, georeferencing sensor, etc.)

Option 3 - Reliability Diagram (RD) advises on preferred areas for navigation, and provides uncertainty assessment (e.g. sounding accuracy, line spacing, survey classification - controlled, lead-line, sounder, shoals examined, sonar swept, etc.)

Option 4 - Represent hazards (rocks, soundings, contours) directly on chart by use of colour and / or pecked lines around hazards


© dave wells 2004

3 Methods of Representing Uncertainty

Two methods, a Source Diagram (SD), and a Reliability Diagram (RD) are graphical insets on a paper chartShowing the geographical limits for each surveyA table describing the attributes of each survey area in the

diagram

Zone of Confidence (ZOC) methods used on Electronic Navigational Charts (ENC)

No standard method exists for Raster Navigational Charts (RNC) - the most widely used form of chart display (being used by the Gordon Reid).


© dave wells 2004

Source Diagram (SD) information

Organization performing survey

Date of survey

Scale of survey

Line spacing information


© dave wells 2004

Mod

el S

ourc

e D

iagr

am

IHB M4


© dave wells 2004

Reliability Diagrams

Give an assessment of accuracy as well as advising on preferred areas for navigation

Examples of the attributes Estimated accuracy of soundingsDistance between survey sounding linesClassification of the survey (e.g. reconnaissance or incomplete;

controlled; sounded by lead line; sounded by echosounder; shoals have been examined; has been sonar swept)


© dave wells 2004IHB M4M

odel

Rel

iabi

lity

Dia

gram


© dave wells 2004

Zones of Confidence (ZOC)

ZOC values assigned to areas on an ENCA1/A2: Full bottom ensonification with

depths determined for all

significant features

B: Uncharted hazards may exist

C: Uncharted depth anomalies are

expected

D: Large depth anomalies are expected

U: Unassessed

Specific depth and position uncertainties are assigned to each ZOC classification.


© dave wells 2004

Survey of 21* hydrographic agencies

Asked 5 questions about representing uncertainty on charts

Very diverse responses

*Australia, Canada, Denmark, Finland, Greece, Hong Kong, Iceland, Italy, Mexico, Netherlands, New Zealand, Norway, Poland, Portugal, Saudi Aramco, South Africa, Sweden, Turkey, UK, US NGA, US NOAA


© dave wells 2004

How is uncertainty represented on paper charts?

Some agencies do not use SDs or RDs at all (one agency removed SDs from their charts, since they were not kept updated, and are useless for RNCs).

Some agencies use SDs only on larger scale charts (larger scale being defined very differently in different countries).

Most agencies are in the process of adding SDs to their charts.

One agency claimed all their charts had SDs.


© dave wells 2004

Are ZOC values on your ENCs fully attributed?

40% have full ZOC attribution on all ENCs (however half of these use only ZOC values B and C, or in one case only B on all ENCs).

30% use only U attribution so far.

30% are partway to full attribution.

Reasons for not having full ZOC attribution were lack of resources,

lack of metadata upon which to base the ZOC, and

the liability implied by assigning a ZOC.


© dave wells 2004

How else do you communicate information on chart uncertainties to users?

60% use Notices to Mariners

25% used each of web-pages, other nautical publications, and presentations to user groups.

The Danish hydrographic office booklet “Behind the Nautical Chart” is free for downloading from its website. This booklet explains the uncertainty associated with hydrographic survey methods over the years (and is soon to be translated from Danish to English)


© dave wells 2004

Email from Ole Berg 15 July 2004I promised you an informal translation of the publication. It has been

undergoing revision, and is not quite complete. Nevertheless, I forward you a copy of the publication in its present form. I do apologize for the English language, especially in the preface and the introduction, it is quite horrible. This will be dealt with before the final version is published. The intention is to publish it in Danish and in English language versions, both being official publications. When complete I will forward you a new copy.

Ole Berg Senior Adviser / Geodata - Strategy & DevelopmentMinistry of The Environment Kort & Matrikelstyrelsen Rentemestervej 8 / DK-2400 København NVTel +45 3587 5050/ direct +45 3587 5112 / mobile +45 4010 2120 / fax 3587 5059Email: [email protected] / official email: [email protected] / web: www.kms.dk


© dave wells 2004

Are you satisfied with your current policies and practices?

30% answered yes,

60% answered with a qualified or unqualified no, and

10% dodged the question (e.g. “we will always try to improve”).

The qualified no answers were based on a desire for better methods than SDs, RDs or ZOCs, and on liability issues associated with RDs and ZOCs.


© dave wells 2004

Are you considering any changes to these policies and practices?

40% intend to work towards completion of SDs on all paper charts, and / or full ZOC attribution on all ENCs.

35% seek improvements to their entire hydrographic data management strategy, incorporating better ways of managing and displaying uncertainty information to the user.

10% answered they would comply with any new international standards that might emerge.

15% answered no.


© dave wells 2004

What to do?1. Define what the end user wants / needs

Commercial ShippingFishing and Natural resourcesRecreational usersMilitary/Coast GuardGIS applications

Each group may have different needs and different preferences for uncertainty representation.

Establish product enhancements based on input from user groups.

Common factor must be improved situational awareness.What enhancements will most improve the safety and decision

making of the Navigator?


© dave wells 2004

What to do?2. Address operational needs

How and under what circumstances will uncertainty representation be used?Voyage planning.

Weather maneuvering.

Shipboard medical emergency.

Result: Time critical decision making requires clear depiction of reliability.


© dave wells 2004

Source DiagramBest for voyage planning


© dave wells 2004Rocks, soundings and depth contours printed in red (Gulf of Finland)

Primary chart depictionBest for time-critical decision making.


© dave wells 2004

What to do?3. Source Diagram attributes

Survey details including:Date of surveySurvey and positioning technology used Line spacing/amount of coverage

Given by either exact survey details, or by a classification scheme to maximize clarity and simplicity

Timeless, without the use of descriptive quality terms such as “modern standards” or “current technology”


© dave wells 2004

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© dave wells 2004 26 May 2004Uncertainty Indicators1 of 50 The Increasing Need to Provide Uncertainty Indicators to Users of Hydrographic Products UNB