Conducting navigation by using electronic charts is not an option anymore. With few exceptions, vessels shall carry on board electronic navigational charts and Electronic Chart Display and Information Systems. The official electronic charts are issued by or on behalf of the authority of a Government, authorized Hydrographic Office or other relevant government institutions. These nautical charts are compiled from multiple data sources, some modern and very comprehensive, while others older. The accuracy of data, named “Category Zones of Confidence – CATZOC”, differs among various navigation areas. The navigation officers of the watch rely on the chart data to calculate the safety parameters and to plan the route in advance for the intended voyage. The aim of this paper is to emphasize the impact which the data accuracy has on the safety of navigation. For this purpose, a model vessel was considered in a Strait of Dover bridge simulation scenario, assuming good weather conditions without swell or current. The Safety Contour was defined using a mathematical formula which incorporated the under keel clearance, the squat effect and the tide levels. Then, the Safety Contour was examined considering the chart data accuracy. The results of this analysis contribute to increasing awareness and better understanding of CATZOC influences on the identification of safe waters during navigation.