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Miscellaneous Notices to Mariners

5.3 ZOC-diagram

The data sources for an ENC will for many areas be based on old hydrographic data with inferior positional accuracy compared to what is possible with modern technology. These areas are identified by the use of Zones of Confidence (ZOC).

In digital charts the data quality is specified in Zones of Confidence (ZOC).

The ZOC diagram describes the quality of the bathymetry in the different areas. There are five quality categories in the ZOC diagram (A1 to D).

Until autumn 2013, mainly category B and C are used for Norwegian coastal waters based on the following classifications: ENCs with source data from older surveying (before 1960) are given ZOC value C, while ENCs with source data from surveying younger than ca 1960 are given ZOC value B.

From 1st of January 2014 the areas measured with multibeam sonar and which otherwise meets the requirements will be given the categories A1 or A2. The delimitation of the different zones will be added in the ENCs to always show which zone you are in.

Navigators must show great care when using (D)GPS and electronic charts in areas with older surveys as accuracy and completeness of the depth indicators are not in accordance with modern standards.

Additionally, the navigators should ensure that navigation at all times is conducted with good safety margins and in accordance with proper navigational practices.

For a full description: 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 echo sounder - a high precision single beam depth measuring equipment, generally including all survey echo sounders designed post 1970.".

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