ADMIRALTY charts are made to the highest possible standards using available hydrographic data and the UKHO makes every effort to maintain the currency and accuracy of its products. Nautical charts are navigational tools. As with any tool, they must be ‘used with prudence’ (as stated in the explanatory notes in the weekly ‘ADMIRALTY Notices to Mariners’ booklet) and are ‘fit for purpose’ provided that they are used with due care. It is expected that the user has received a suitable level of training in their use, and understands the limitations of some of the source data.
Whilst GPS may provide a mariner with a very accurate position, they must be aware that much of the source data was gathered before the availability of GPS and depended on a variety of different fixing methods, of varying accuracies. It is often the case that the positioning of the source data is not as accurate as the mariner’s GPS position; therefore, he must make allowance and give charted dangers a wide berth. Furthermore, original data was usually referred to a local horizontal datum, for which the relationship with the world-wide datum used by GPS (WGS84) may be unknown. Due warnings are given on the paper or ARCS charts and must be heeded by the mariner.
The mariner should note the Source Data Diagram, or Source information on SNCs which will give the date and method of survey and can be used to judge the accuracy of the survey data used to compile the chart. On an ENC mariners can view the Categories of Zones of Confidence (CATZOC) which also indicate the likely accuracy of positional and depth information from the survey.
More complete explanations are contained in various publications, such as the Mariners’ Handbook (NP100), especially Chapter 2, and also in Annual Notice to Mariners, available for reference on the UKHO website.
