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5.3 ZOC-diagram

Kartgrunnlaget vil for mange områder være basert på sjømålingsdata hvor posisjonsnøyaktigheten er dårligere enn det som i dag er mulig ved bruk av moderne posisjoneringssystemer. I en ENC er datakvaliteten for hydrografiske data angitt i kvalitetssoner (Zones of Confidence) hvor kvaliteten er vurdert ut fra tre faktorer; posisjonsnøyaktighet, nøyaktighet i dybde og dekningsgrad av havbunnen.

ZOC-diagrammet forteller om kvaliteten på dybdedataene i de forskjellige områdene, og tar for seg fem kvalitetskategorier (A1 til D).

Fram til høsten 2013 ble hovedsakelig kategoriene B og C benyttet for norske kystfarvann ut fra følgende klassifisering:
ENC-er med kildedata fra eldre sjømåling (før ca. 1950) er gitt ZOC-verdi C, mens ENC-er med kildedata fra sjømåling yngre enn ca. 1950 er gitt ZOC-verdi B.

Fra 1. januar 2014 blir områder som er målt med multistråleekkolodd og som ellers tilfredsstiller kravene angitt med kategoriene A1 eller A2. Avgrensningen for de forskjellige soner blir lagt til kartdataene slik at man til enhver tid kan se på skjermen hvilken sone man er i.

Navigatører må vise stor forsiktighet ved anvendelse av (D)GPS og elektroniske sjøkart i områder med gamle sjømålingsdata, da nøyaktighet og fullstendighet i dybdeangivelser ikke er i samsvar med moderne standard.

For øvrig bør navigatørene sørge for at navigeringen til enhver tid foregår med gode marginer og i samsvar med forsvarlig navigasjonsmessig praksis.

For fullstendig beskrivelse henvises det til publikasjonen S57 IHO Transfer Standard for Digital Hydrographic Data.

ZOC Category
1245
ZOC1 Position Accurancy2 Depth Accurancy3 Seafloor Coverage Typical Survey Characteristics5
A1 ± 5 m + 5% depth = 0.50 + 1%d Full area search undertaken. Significant seafloor features detected4 and depths measured. Controlled, systematic survey6 high position and depth accuracy achieved using DGPS or a minimum three high quality lines of position (LOP) and a multibeam, channel or mechanical sweep system.
Depth (m) Accurancy (m)

10
30
100
1000

± 0.6
± 0.8
± 1.5
± 10.5

A2 ± 20 m = 1.00 + 2%d Full area search undertaken. Significant seafloor features detected4 and depths measured. Controlled, systematic survey6 achieving position and depth accuracy less than ZOC A1 and using a modern survey echosounder7 and a sonar or mechanical sweep system
 Depth (m) Accurancy (m)
10
30
100
1000
± 1.2
± 1.6
± 3.0
± 21.0
B ± 50 m = 1.00 + 2%d   Full area search not achieved; uncharted features, hazardous to surface navigation are not expected but may exist. Controlled, systematic survey achieving similar depth but lesser position accuracies than ZOC A2, using a modern survey echosounder5, but no sonar or mechanical sweep system.
Depth (m) Accurancy (m)
10
30
100
1000

± 1.2
± 1.6
± 3.0
± 21.0

C ± 500 m = 2.00 + 5%d Full area search not achieved, depth anomalies may be expected. Low accuracy survey or data collected on an opportunity basis such as soundings on passage.   
Depth (m) Accurancy (m)
10
30
100 1000

± 2.5
± 3.5
± 7.0
± 52.0

D worse than ZOC C worse than ZOC C Full area search not achieved, large depth anomalies may be expected. Poor quality data or data that cannot be quality assessed due to lack of information.
U Unassessed – The quality of the bathymetric data has yet to be assessed

Remarks:

To decide on a ZOC Category, all conditions outlined in columns 2 to 4 of the table must be met.

Explanatory notes quoted in the table:

1 The allocation of a ZOC indicates that particular data meets minimum criteria for position and depth accuracy and seafloor coverage defined in this Table. ZOC categories reflect a charting standard and not just a hydrographic survey standard. Depth and position accuracies specified for each ZOC category refer to the errors of the final depicted soundings and include not only survey errors but also other errors introduced in the chart production process. Data may be further qualified by Object Class 'Quality of Data' (M_QUAL) sub-attributes as follows:

a) Positional Accuracy (POSACC) and Sounding Accuracy (SOUACC) may be used to indicate that a higher position or depth accuracy has been achieved than defined in this Table (e.g. a survey where full seafloor coverage was not achieved could not be classified higher that ZOC B; however, if the position accuracy was, for instance, ± 15 metres, the sub-attribute POSACC could be used to indicate this).

b) Swept areas where the clearance depth is accurately known but the actual seabed depth is not accurately known may be accorded a 'higher' ZOC (i.e. A1 or A2) providing positional and depth accuracies of the swept depth meets the criteria in this Table. In this instance, Depth Range Value 1 (DRVAL1) may be used to specify the swept depth. The position accuracy criteria apply to the boundaries of swept areas.

c) SURSTA, SUREND and TECSOU may be used to indicate the start and end dates of the survey and the technique of sounding measurement.

2 Position Accuracy of depicted soundings at 95% CI (2.45 sigma) with respect to the given datum. It is the cumulative error and includes survey, transformation and digitizing errors etc. Position accuracy need not be rigorously computed for ZOCs B, C and D but may be estimated based on type of equipment, calibration regime, historical accuracy etc.

3 Depth accuracy of depicted soundings = a + (b*d)/100 at 95% CI (2.00 sigma), where d = depth in metres at the critical depth. Depth accuracy need not be rigorously computed for ZOCs B, C and D but may be estimated based on type of equipment, calibration regime, historical accuracy etc.

4 Significant seafloor features are defined as those rising above depicted depths by more than:

  DepthFeature
a. <40 m 2 m
b. >40 m 10% depth

A full seafloor search indicates that a systematic survey was conducted using detection systems, depth measurement systems, procedures, and trained personnel designed to detect and measure depths on significant seafloor features. Significant features are included on the chart as scale allows. It is impossible to guarantee that no significant feature could remain undetected, and significant features may have become present in the area since the time of the survey.

5 Typical Survey Characteristics - These descriptions should be seen as indicative examples only.

6 Controlled, systematic surveys (ZOC A1, A2 and B) - surveys comprising planned survey lines, on a geodetic datum that can be transformed to WGS 84.

7 Modern survey echosounder - a high precision single beam depth measuring equipment, generally including all survey echosounders designed post 1970.".

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