随着航运事业的不断发展，越来越多的高科技设备被运用到日常航海中。由纸质海图改为电子海图航行是近年来航海领域的重大改变。现代化船舶，驾驶台设备集成程度高，ECDIS与GPS,AIS,雷达，甚至自动舵的的都能有效的连接，且实现数据共享。
要知道设备本身就具有局限性，正确的管理和使用这些设备将极大的减轻现代航海人的工作压力。但是，如果航海人不能正确的管理和使用，以及对误差了解不充分，会有可能引发航行危险。下面我就电子海图的CATZOC讲一讲自己的理解。
在解读问题之前，我们要了解一个问题，那就是电子海图的水深数据是怎么来的，还有一个问题是为什么要建立一个电子海图的CATZOC。
1
电子海图的数据是怎么来的？
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.'
像内容、结构、格式这类标准数据，是由政府授权的水文机构发布，以供电子海图使用，并应符合IHO的标准。这里有介绍到ENC中含有纸板海图上所有关于航行安全的信息，然后有添加了一些纸板海图没有的信息， 例如航行指南。
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
在2000年初，大多数的ENC数据是从现有的纸质海图衍生而来，只是根据标准进行了数字化。IHO制定了一系列的标准，供世界各地的水文局使用。这些标准共同定义了用于制作官方航海用图和出版物的专业方法和内容。随着电子化航行的引入，这些数据可以在屏幕上无缝显示，并且链接了自动报警功能，且从不同水文局获得的数据可以并排显示。
也就是说电子海图最开始的数据就是纸质海图的数据化，随着使用的普及和制定标准的完善，以及更多的水文局的加入，才演变成今天数字化的局面。但随着时间的推移，各个水文局会形成自己的个性化的制图工序，例如在不同比例尺上绘制信息的密度和精度标准会有所不同。
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.
需要注意的是，在电子海图的实际使用中，我们会发现一些和纸板海图不一样的地方，英国水道测量局在其提供的AIO 服务中，将影响航行安全、且已在纸版海图上标明但未包含在ENC上的危险信息，以及ENC 和纸版海图存在的其它差异，通过EP的形式进行展示说明。如果UKHO发现存在大量重大差异，通常不会将受影响的ENC纳入AVCS服务中，直到这些差异在与相关国的讨论中得到协调。
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.
只有在少数的情况下，生产商所包含的测深信息才比纸板海图多的多，他们通过从原始数据中获取深度信息来制作更丰富的数字数据集，从而包含更多深度轮廓。像现在的澳大利亚大堡礁航线的ENC，马来西亚的PENGERANG 港图，英吉利海峡部分等等。这些米级的等深线制作图在readme有被提及，就是我们所说的HENC，提供给大家使用。
2
为什么要建立这个CATZOC ？足
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.
很容易产生一个误导，ENC是数字格式的，并且链接的GNSS，因此显示在屏幕上的所有信息必须是准确和完整的。实际情况是，ENC数据的精准度只取决于所获得的原始数据。现在大多数情况下，ENC这些数据和现在的纸板海图数据相同。
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.
这里介绍ENC的数据通常基于不同的信息来源，其相对准确性可能存在较大的差异。在纸板海图上，这些信息显示在海图信息说明中，即海图信息图框中，并且建议在航次计划的评估阶段使用，这样就可以在使用的过程中有一个适当的安全边界。
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.
这里介绍的是ENC并没有一个像纸板海图一样的海图信息图框，但是提供了一个类似的方法就是我们所说的CATZOC，同样的方式还有readme 。ENC制造商根据位置精度、深度精度和扫海范围将ENC数据划分为不同测量质量的区域，并分为A1，A2，B，C，D，U六个置信度区类，就是我们在计算富余水深时需要考虑的问题。
这段文字是节选自海图置信精度表格里的备注栏
CATZOC是为ENC数据源可靠性评估的机制，提供一个等同于同等纸板海图的数据框的说明来让我们理解，只不过对水深精度进行了数字量化。
最后的个人总结：
CATZOC的出现更像是UKHO对当下现状的一种免责声明，即没有办法获得最新的海图更新数据，因为目前仅有70个不同的水文局参与该项工作，而且目前很多地方仍然无法使用现代的标准进行勘测，以至于数据的精度收到影响。针对当前电子化的进程及ENC的使用，UKHO对海图的精度做出的一个量化处理来让使用者在航次计划阶段进行评估，让使用者对所获得的信息进行一个综合的判断，同时考虑到当前的具体情况，安全、谨慎的驾驶，以达到一个安全航行的目的。
延伸阅读：灵活取用电子海图和纸板海图的水深
