电子海图CATZOC的理解

The IMO Performance Standards for ECDIS defines an ENC as:

'The database standardised as to content, structure and format, issued for use with ECDIS by or on the authority of a Government authorised Hydrographic Office or other relevant government institution, and conforming to IHO standards. The ENC contains all the chart information necessary for safe navigation and may contain supplementary information in addition to that contained in the paper chart (e.g. sailing directions) which may be considered necessary for safe navigation.'

In the early 2000's most ENCs were derived from existing paper charts, digitised according to the relevant standards.The International Hydrographic Organization (IHO) has developed a series of standards used by national hydrographic offices around the world. Collectively they define the professional methods used to produce official navigational charts and publications, and their content, including ENC.With the introduction of electronic navigation, however, this chart data is now presented together on screen in a seamless display, and linked to a system of automated alarms. ENC data taken from several paper charts and several hydrographic offices can be on display side-by-side

The content of ENCs may not always exactly match the information contained on the

ADMIRALTY paper chart of a similar scale in the same area. The ADMIRALTY Information

Overlay (AIO) highlights any significant discrepancies, in particular those related to navigational aids and hazards.Where UKHO finds a large number of significant discrepancies it will not normally include affected ENCs in the AVCS service until these are reconciled in discussion with the relevant producer nation.

In a few cases, producers have included significantly more bathymetric information than appears on the paper chart, by capturing depth information from original surveys to produce a richer digital dataset which supports the inclusion of significantly more depth contours. A good example is the Australian ENC covering the Great Barrier Reef route.

It is easy to be misled into thinking that, because ENC data is in digital format, and the ECDIS is linked to GNSS, that everything seen on the ECDIS screen must be complete and accurate.

However, the ENC data is only as complete and accurate as the original survey data from which it has been derived. In the majority of cases this will be the same as used to derive the equivalent paper charts currently available.

The survey data used in the ENC is often based on information from different sources whose relative accuracies may vary widely. On the paper chart, this information is shown on the source data diagram which the mariner is advised to use at the appraisal stage of voyage planning. This ensures that the mariner can apply an appropriate safety margin when determining their route. Bigger margins should be applied where the survey data is old, or is of insufficient resolution or quality to fully depict the sea floor.

ENC do not contain an equivalent source data diagram, but provide information on the quality of data within the ENC in other ways. The primary mechanism used to communicate this information is through an attribute known as 'CATZOC' (category of zone of confidence in data).

ENC producers divide the ENC data into areas of differing survey quality based on positional accuracy, depth accuracy and seafloor coverage. Each area is included in the ENC data as an area meta-feature object known as 'Quality of Data' (M QUAL) and is allocated one of the six possible CATZOC attribute values. In 'All' / 'Other' display mode, the ECDIS will display the appropriate CATZOC symbol within each area (but not the limits of the areas) depending on its assigned value as detailed in the following table.