Accuracy of bathymetric data in electronic navigational charts

Navigational charts are essential tools for marine navigation. But how accurate are the navigational charts that we use when going sailing? Do we really know how much faith can be placed in them? All charts, whether paper or electronic, contain data, which varies in quality due to the age and accuracy of individual surveys. In general, remote areas away from shipping routes tend to be less well surveyed, and less frequently, while areas of high commercial traffic are re-surveyed frequently to very high levels of accuracy, particularly where under-keel clearances are small. It is quite accurate to consider a chart as a jigsaw of individual surveys pieced together to form a single image. Having the necessary skills to determine how much confidence should be placed in the surveys, which combine to form a chart, should be a requirement for any sailor venturing into unfamiliar waters. When the International Hydrographic Organization (IHO) developed the S-57 standard for Electronic Navigational Charts (ENCs), this problem was recognized and it was decided that the quality of survey data used to compile ENCs had to be encoded within a composite data quality indicator ‘Category of Zone of Confidence’ (CATZOC) to assist seafarers in assessing hydrographic survey data and the associated level of risk of navigating in a particular area. According to IHO S-67, the accuracy of Electronic Navigational Charts is not impressive and leaves much to be desired. The author discusses these apparent shortcomings of ENCs and present erroneous approaches to this problem, so common in the seafaring community.