411
1 INTRODUCTION
Electronic Chart Display and Information System
(ECDIS) is a complex navigation tool developed “to
assist the mariner in route planning and route
monitoringanddisplayadditionalnavigation‐related
information if required”, as specified in the
PerformanceStandardsforECDIS[IMO,2006].
ThehistoryofECDISstartedinthe
1990swhena
coupleofcompaniesofferedelectronicchartsystems
tobeusedonboardvessels.Recognizingtheneedto
prepare performance standards for ECDIS, the
International Maritime Organization (IMO) adopted
theresolutionPerformancestandardsofECDIS[IMO,
1995].Thisresolutionsetoutthe minimum levels of
requirementstobe
metinordertousetheECDISas
bridge equipment on board conventional ships. In
year 2000, with the adoption of amendments to the
InternationalConventionfortheSafetyofLifeatSea,
the ECDIS was accepted as complying with the
provisionsoftheSOLASConvention[IMO,2000].
Later on, in
2009, the IMO established the
implementation timeline as depicted in Figure 1.
Nowadays, the period of implementation of the
ECDIS on board vessels came to the end, making
mandatory the fitting of vessels with ECDIS
equipment.
With few exceptions, every vessel shall carry
ECDIS equipment in accordance with the IMO
performance standards and the Safety of Life at Sea
Impact of Chart Data Accuracy on the Safety of
Navigation
N.Acomi
ConstantaMaritimeUniversity,Constanta,Romania
ABSTRACT:Conductingnavigationbyusingelectronicchartsisnotanoptionanymore.Withfewexceptions,
vessels shall carry on board electronic navigational charts and Electronic Chart Display and Information
Systems.TheofficialelectronicchartsareissuedbyoronbehalfoftheauthorityofaGovernment,
authorized
Hydrographic Office or other relevant government institutions. These nautical charts are compiled from
multipledatasources,somemodernandverycomprehensive,whileothersolder.Theaccuracyofdata,named
“CategoryZonesofConfidence–CATZOC”,differsamongvariousnavigationareas.Thenavigationofficersof
thewatchrelyonthe
chartdatatocalculatethesafetyparametersandtoplantherouteinadvance for the
intendedvoyage.Theaimofthispaperistoemphasizetheimpactwhichthedataaccuracyhasonthesafetyof
navigation.Forthispurpose,amodelvesselwasconsideredina StraitofDover
bridgesimulationscenario,
assuming good weather conditions without swell or current. The Safety Contour was defined using a
mathematicalformulawhichincorporatedtheunderkeelclearance,thesquateffectandthetidelevels.Then,
theSafetyContourwasexaminedconsideringthechartdataaccuracy.Theresultsofthisanalysiscontribute
to
increasing awareness and better understanding of CATZOC influences on the identification of safe waters
duringnavigation.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 14
Number 2
June 2020
DOI:10.12716/1001.14.02.19
412
Convention (SOLAS). Ships which will be
permanentlytakenoutofservicewithintwoyearsof
atriggerdatefromtheimplementationtimelinemay
beexemptedbytheFlagStates.Thereisnoprovision
forcargoships(otherthantankers)oflessthan10,000
GTtobefittedwithECDIS.
Figure1.ECDISimplementationtimeline[IMO,2008].
The1
st
of July 2018 markedsymbolicallythat the
ship navigation has accomplished the shift from
paper charts to the era of digital chart navigation
[Weintrit,2018a].
Thereweresixyearsoftransition,fromJuly2012
to July 2018, while the ship‐owners fitted the ships
with ECDIS system in compliance with
the
international requirements [IMO, 1995]. Within this
period, various ECDIS equipment manufacturers
went beyond the performance standards and
providedsystemswithadditionalfunctionalitiesable
to support the officers in the process of decision
making. The functionalities come together with
software design solutions which differ from one
provider to others, and sometimes
from one version
to an updated one.The complexity of functions and
theapparenteasinessofoperatingthesystemseemto
increasetheriskofoverrelianceonECDIS.
Certainly, if properly used by well‐prepared
officers, there are many benefits of ECDIS use on
board. Nevertheless, we cannot deny the
fact that
ECDIS complexity might make things to go wrong.
For example, if wrong settings are put in place, the
information provided to navigators will not be
accurate.Feedinginwrongparametersforthesafety
criticalsettingssuchastheSafetyDepthsandSafety
Contours can give a false sense of
safety [Weintrit,
2018b].
ThevalueofSafetyContour, calculated on board
by the deck officer and set on the ECDIS, rapidly
separates the safe waters from the unsafe waters on
theelectronicnavigationalchart.
Themainquestionarisingis:howsafeisthesafe
water?
The officer must know the
answer.
Recommendations were formulated [Bhuiyan, 2012]
forassistingthedeckofficertounderstandthevalues
forSafetyContourandtoconsiderdatasourcewhile
determining the safety settings. These are of
assistanceonproperlysettingtheECDIStoachievea
sensibleandwellthought‐outimplication.
It has been noted [Weintrit,
2018c; Rutkowski,
2018] that many navigators have a tendency to put
much reliance on ECDIS. This may contribute to
accidentsratherthanpreventingthem.Fromaccident
analyses Chhabra [2014] concluded that the main
causesarenotECDISsystemfailures,butmorelikely
operationalfailures.Oneofthesetypesoffailures
is
theimproperuseofsourcedata.
In this context, the subject of this paper is
represented by the source data used for creating
electronic navigational charts. The ECDIS provides
navigators with a facility to examine reliability and
quality of source data presented on the chart by
meansofCategoryZone
ofConfidence(CATZOC).
The electronic charts for various areas are
generatedusingdataofdifferentaccuracylevels.The
purposeofthefollowingsectionsistoemphasizethe
impactofthedataaccuracyonthesafetyparameters.
Taking into consideration the category zones of
confidence CATZOC, this case study highlights the
differencesinSafetyContourvaluesobtainedwithor
withoutconsideringthechartdataaccuracy,allother
variablesremainingequal.
2 DETERMININGTHESAFETYCONTOUR
“Appropriate safety settings are of paramount
importance for the safety of navigation” [Bhuiyan,
2010].Becauseseveralvariablefactorsareinfluencing
ship’s behaviour (e.g. environmental, topographic,
loading,ships
nauticalqualities)andbecauseallthese
contribute to the safety of navigation, there is no
all‐inclusivemathematicalformulaforcalculatingthe
SafetyContour.Instead,acoupleofrecommendations
were formulated by Bhuiyan [2010] and several
papers from marine professionals agreed with these
[Mukherjee,2018]:
Figure2. Recommended Safety Settings on ECDIS
[Bhuiyan,2010].
Safety Contour value will be calculated for a
model vessel and analysed from the perspective of
variousCATZOCfeatures.
This study is based on the characteristics of a
Coastal tanker model vessel of 21,515 DWT, 144.0m
length between perpendiculars, at even keel with
9.1m draft (Figure 3). The navigation area for
this
scenarioistheEnglishChannel.
413
Figure3.Coastaltankermodelvessel,TRANSASsimulator.
2.1 Underkeelclearance
The minimum Under Keel Clearance (UKC) is
required by the internal procedures of shipping
companies. The procedure of a Romanian company
which remains anonymous requires the vessels to
maintain minimum of 0.5m at all times alongside,
0.65mbelowdeepestdraft(includingexpectedsquat)
while underway in confined
waters, and 15% of
deepestdraftinexposedoropenwaters.Inthegiven
scenario, the model vessel navigates in the confined
water of the Strait of Dover and therefore the
minimum UKS is considered 0.65m (including
expectedsquat).
2.2 Squateffectcalculation
For the purpose of this scenario, the
most
conservative ship conditions were considered, i.e.
maximumspeedanddraft.
The.xlsxsquateffect calculation sheet and formulas
forconfinedwaters(eq.1)andforopenwaters(eq.2)
followbellow:
Squatconfinedwaters=(CbxV2)/50m (1)
Squatopenwaters=(CbxV2)/100m (2)
where:
C
b–blockcoefficient(eq.3)
V – speedthrough the waterinknots (14.5knots in
confinedwaters)
C
b=displacement/(LxBxDxW.D.) (3)
where:
L‐length, B‐breadth, D‐draft of the shipand W.D. is
waterdensityof1.025Kg/l.
For the given scenario, the ship’s characteristics
were taken from the general information given at
Figure 3. The resulting squat effect is 3.25m for
the
vesselinconfinedwaters.
Thecurrentscenariodidnotconsideranycurrent,
which would otherwise be expected in the Strait of
Dover.However,accordingtotheaboveformulas(1)
and (2), the squat is dependent on the ship’s speed
through the water. Therefore, the presence of the
current in the
Strait of Dover is not expected to
introducesignificant changes tothe calculated squat
value.
2.3 Tidelevels
The tide in the Strait of Dover is also a factor
influencingthesafetyofnavigationinconfinedwater.
WhilethechartdepthdepictstheMeanLowestLow
Water, higher tide water
give additional depth
availablefor the ship. Therefore, the Safety Contour
may incorporate the tide evolution, but would be
limitedfortherespectivetimeofstraittransit.
Ifitwerenecessaryordesiredtoconsiderthetidal
gains and set the Safety Contour to smaller water
depth,thetimeconstraintshall
beclearlystatedand
followedup.
For the purpose of this study, the most
conservative scenario was considered, i.e. with the
vesseltransitingtheStraitofDoveratLowWater.
2.4 SafetyContourcalculation
Consideringtheconservativescenarioforthetide,the
Safety Contour is set at the water depth
which is
depicted by the sum between the ship’s draft, the
minimumunderkeelclearanceandthesquat:
S
c=D+UKC+Squat (4)
where:
S
c–SafetyContour
TheresultingSafetyContouris13.0m.
It worth mentioning that other company
proceduresmaygiveinstructionsand/orformulasfor
fresh water allowance, heel allowance and swell,
whichwerenotincludedinthecurrentscenario.
3 THECHARTDATAACCURACY
Thechartdataaccuracyisknownascategory
zonesof
confidence, CATZOC. The data is obtained from a
rangeof sources and methodsincluded below, from
thehighesttothelowestprecisions:
DGPS;
Minimumthreelinesofposition;
Multibeamchannelormechanicalsweepsystem
Echo sounder and sonar or mechanical sweep
system;
Echo
sounder but no sonar or mechanical sweep
system;
Soundings.
The resulting level of accuracy is represented by
values assigned to geographical areas. The position
and depth accuracy is indicated for each navigation
area.
Dependingonthecharacteristicsofthesurvey,the
accuracyisprovidedtotheuserthroughsix
typesof
quality indicators: A1, A2, B, C, D and U. The
accuracyofthepositionmayvaryfrom+/‐5mtomore
than+/‐500m,whiletheaccuracyofdepthmayvary
from less than 0.5m to 2m or more, as depicted in
Table1.
Depending on the ECDIS manufacturer
and
model, various safety settings are available: safety
414
depth, Safety Contour, deep contour and shallow
contour. If properly calculated and set, these
parameterswouldprovideindicationandalarm.This
wouldassistthenauticalofficerinconductingasafe
navigationwatch.
Inordertoproperlysetthesafetyparameters,the
userisrequiredtocheckandinterprettheCATZOC
values. The accuracy of these values shall be
consideredwhilesettingthechartSafetyContourthat
distinguishsafefromunsafewater.
Table1.CategoryZonesofConfidence
_______________________________________________
Zoneof Positionaccuracy Depthaccuracy
confidence
_______________________________________________
A1+/‐5m+5%depth =0.50+1%depth
Depth(m)Accuracy(m)
10+/‐0.6
30+/‐0.8
100+/‐1.5
1000+/‐10.5
_______________________________________________
A2+/‐20m=1.00+2%depth
Depth(m)Accuracy(m)
10+/‐1.2
30+/‐1.6
100+/‐3.0
1000+/‐21.0
_______________________________________________
B+/‐50m=1.00+2%depth
Depth(m)Accuracy(m)
10+/‐1.2
30+/‐1.6
100+/‐3.0
1000+/‐21.0
_______________________________________________
C+/‐500m=2.00+5%depth
Depth(m)Accuracy(m)
10+/‐2.5
30+/‐3.5
100+/‐7.0
1000+/‐52.0
_______________________________________________
DWorsethanZOCC WorsethanZOCC
_______________________________________________
UUnassessed
_______________________________________________
4 DISCUSSION
In order to highlight the safe (white) and unsafe
waters(blue)ontheelectronicchart,thedeckofficer
would set the value of the Safety Contour in the
monitoringwindow.Thiscasestudyconsideredgood
weather conditions without swell or current. This
assumptionwouldallowthecomparisonof
CATZOC
impactonthesafetyparameters.
Figure4.Safetyparameters,TRANSASsimulator.
FortheStraitofDovernavigationareaconsidered
in the current scenario, according to the category
zonesofconfidence,thepossiblevaluesofthedepth
weredetermined.CATZOCvariesfrom6‐star inthe
A1areato5‐starinA2and4‐starintheBnavigation
area, Figure 4.
This might impact the value of the
safetyparametersaspresentedinTable2.
Forthe13.0mvalueoftheSafetyContourobtained
from (eq. 4), the CATZOC depths are tabulated as
follows:
Table2.Depthpossiblevalues
_______________________________________________
CATZOC–DepthfromtheENC Possiblevalues
Categoryzones ‐Electronicofthedepth
ofconfidence NavigationalChart accordingtothe
(m)CATZOC(m)
_______________________________________________
A113.012.37–13.63
A213.011.74–14.26
B13.011.74–14.26
_______________________________________________
The results depicted in the right side column of
Table2aboveshowpositiveandnegativevariations
of the depth. Setting the Safety Contour to 13.0m
wouldintroducedelusiveconfidencewhiletheactual
depthmayvaryfrom11.74m,inA2andBCATZOC
to14.26minBCATZOCareaofnavigation.
Thus, even though the Safety Contour had
previouslybeendeterminedat13.0m,thatdepthmay
beunsafeforthevesseliftheconditionsofmaximum
squateffectandminimumCATZOCdepthforA1,A2
and B category zones of confidence were met.
Moreover, while in case of A1 the vessel
would be
closetoaground(by0.02m,giventheUKCof0.65m
which was considered), in the A2 and B cases the
vesselwouldbeagroundby0.61m.
Therefore,itmaybeprudentfortheofficerofthe
watch to consider the above by setting the Safety
Contour to a minimum
given by the maximum of
CATZOCdepth.Furthermore,because the CATZOC
depth value is often a decimal number, the round
value of the next bathymetric line available for that
area(15.0morgreater)shallbechosenfortheSafety
Contour.
The above analysis clearlyindicates the
importanceofCATZOC,which
shouldbeconsidered
forthesafetyofnavigation.
415
5 CONCLUSIONS
Thecorrectsettingofsafetyparametersisessentialfor
thesafetyofnavigation.Beingawareoftheaccuracy
ofthedataallowstheofficersofthewatchtoproperly
evaluate the situation and to consider negative
variations for depth. The results of this study are
intendedtoincrease
theawarenessofnauticalofficers
regarding the limitations of ECDIS systems with
regards to the Safety Contour, as well as the
importanceofthechartdata.
Inadditionto the above, further area ofstudyin
the field of CATZOC may include the position
accuracy, which was not factored into
the current
analysis.
Moreover, future studies may analyse in further
depththeinfluenceofCATZOCwhiletheshipbeing
inmorecomplexandrealisticconditionssuchasdraft
restrictions during low and high tide, current, fresh
waterallowance,heelallowanceandswell.
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