Providing Uncertainty:Mariner’s Perspective

CAPT Lee Alexander, USNR (ret)

Three Aspects

1. What does “Uncertainty” mean? • Hydrographer vs. Mariner• Awareness and understanding

2. Use of• “Thinking like a Mariner”

3. Portrayal• Some considerations• Some examples

A Recommendation…

Hydrographic Uncertainty

IHO S-44 - Specifies the minimum standards for hydrographic surveys• So that the data collected are sufficiently

accurate• spatial uncertainty is adequately quantified

to be safely used by mariners (as primary users of this information)

As Defined in IHO S-44

Confidence level - The probability that an error will not exceed the specified minimum value.

Error - The difference between an observed or computed value of a quantity and the ideal or true value of that quantity

Spatial uncertainty - [not defined]

S-57 ENC Product Specification

Object - M_QUAL

Attribute - CATZOC

Attribute values - POSACC, SOUACC, & TECSOU

No specific mention of uncertainty. Instead, confidence in terms of so-called “Zone of Confidence” (CATZOC)

Not really a zone but an assigned category within a defined area.

CATZOC “values” assigned in terms of:

1. Position accuracy

2. Depth accuracy

3. Seafloor coverage

4. Seafloor characteristics

Mariner’s Basic Assumption

The charted information provided is sufficient for safe navigation.*• Sufficiently accurate (depth and position)• Adequately quantified• Errors are known

• within defined confidence level• spatial uncertainty is accounted for

* If not, why not?

The Reality…Mariners are:• Less knowledgeable about charts than

hydrographers.*• Not really interested in the survey, itself

• what type, when conducted, or by who

• More concerned about depths• known/unknown, coverage, density, and accuracy

• Keen to obtain information in which to make informed decisions• Decision support based on current situation/task-at-hand

*CAPT Robert Ward (HI “Insiders View, Sept. 04)

RADM Tim McGee (USHydro 05)

Navigation Display

Cargo

Regulatory

Tasks

Currents

Deep-seaNavigation

ShipConfiguration

Sea State

AIS

Ice

HumanFactors

Speed

Weather

Voyage plan

Draft

Manoeuvrability

Chart in use

Height ofeye

VTS

Load Conditions

Visibility

Radar

TrafficDensity

Coastal navigation

Pilotage

MIOsTime-to-go

Bridge Layout

Uncertainty?

Perceptions & Misperceptions

• The more soundings you see, the better the area was surveyed.

• The number of soundings shown are less than what is known.

• Soundings in deep water are less accurate than those in shallow water.

• Soundings are fixed, while depths are dynamic.

Thinking Like a Mariner

• Hydrographers collect and provide, while mariners “digest” and use.• what may be interesting to a Hydrographer, may not be

particularly useful to Mariners.• more information is not necessarily better

• Give a Mariner good information and he/she will figure out how to use it • they decide what is/is not important

• Good information is that which contributes to informed decision-making.• interesting is not the same critical • too much is often worse than too little

 

Thinking Like a Mariner (cont.)

• The main use is decision support (i.e., “what if” or “can I”).• route/voyage planning

Civil/CommercialGovernment/Military

• current situation/task-at-hand

 • The display of uncertainty/confidence is less

important than how it can/should be used.• source diagrams• numerical values (e.g., CATZOC)• color-coded graphics

Uses for Uncertainty/Confidence

• Dynamic seafloor areas• Assign “changeability coefficient” (value or

color scheme) 

• Superceded Data • Previous replaced with new/better• Old superceded by new• Higher confidence than before

Source: LT Shep Smith

Depth & Uncertainty

Combined DepthGrid

CompanionUncertainty Grid

Combined Depth and Uncertainty

Advantages:

- intuitive

- psudo-3D perspective

- supplements nautical and cartographic info

Uncertainty Modeling

Traditionally, the measurement error of a given sounding is the value reported as the uncertainty of the depth. In other words: How good was that measurement?

 

But, what mariners really want to know is: How well is the depth known at this location?

Defocusing (Horizontal Error)

CHS Pacific EM3000 – Esquimalt - 1m Surface defocused using 5m horizontal uncertainty estimate

Source: “Total Propagated Error, BASE Surfaces and CARIS HIPS 5.4”, Lamey, B. et al., Proc. 3 rd Int. Conf. On High-ResolutionSurveys in Shallow Water, Sydney, Australia, 2003.

Reson 8101 at Survey Density (NOAA Ship WHITING)

Smoothsheet Density Shoal-Biased Selected Soundings

Soundings Extracted from the Navigation Surface

Smooth Sheet Soundings from a Navigation Surface

Soundings and Contours from Navigation Surface

S-57 Ed. 3.1 ENC from Navigation Surface

Tide-Aware ENC

7Cs Tide Demo

CATZOC

• For most ENCs that have been produced, the assigned CATZOC value is “U” (Un-assessed)• Unknown• Unable• Unwilling

• CATZOC not widely understood or used by Mariners

ENC UncertaintyS-57 ENC objects could be attributed with an

estimated error. But,for depths:• depths are defined by a variety of measurements• some areas were interpolated without

measurement

 One approach: require that the bathymetric portions of an ENC have uniform reported error. If there are areas that exceed the reported error:• classified as or “poorly surveyed”• assign a lower CATZOC value

 

Provision and Portrayal Strategy

Recommendation: Focus on New, not Old

• Rather than trying to give uncertainty or confidence information on what occurred in

the past (old surveys current charts),

• Provide new/better information that resulted

from modern surveys (new surveys better charts.