Certain features are corrected at once on the charts as soon as the information is received, such as dangers reported, and changes in lights and buoys. Where harbor works are in progress the periodic surveys made in this country by the Corps of Engineers furnish data which are applied promptly to the charts. Reported dangers in channels and bars are investigated by special surveys and the information is put on the charts. Examinations are made from time to time for the revision of the features along the coast line. Complete resurveys have been made, at long intervals, of some important portions of the coast where there has been evidence of change, and these, when they become available, are applied to the charts. All parts of the coast where the exposed portions are not of very permanent material will require resurveys at intervals, depending on their importance and the rate of change.
Notwithstanding the great progress made in hydrographic surveys, a considerable number of rocks and shoals dangerous to navigation and not previously shown on the charts are reported, averaging nearly 400 each year for the last six years, according to the British reports. Of the 367 reported in 1906, 11 were discovered by vessels striking them.
Immediate information in the form of Notices to Mariners is published, of the more important corrections to charts which can be made by hand. These corrections show what charts are affected, and give sufficient data for plotting.
In the case of extensive corrections or new surveys a new edition of the chart is printed and all existing copies of the previous edition are canceled.
It is important that the user of the chart shall make certain that he has the latest edition and that all corrections from its date of issue have been applied from the Notices to Mariners.
It is unfortunately true that owing to failure to take proper account of the notices, or to economy, old editions or unconnected charts are sometimes used, and in a number of cases the loss of vessels has been directly due to this cause. Those responsible for the safe navigation of vessels should insist that the latest editions of charts are provided and that all charts to be used are inspected and corrected to date.
[READING AND USING CHARTS.]
Reading charts. A chart is a representation on paper of hydrographic and topographic information by means of various conventional methods and symbols. It is evidently important for those making use of charts to understand the system and conventions used, and to be able to interpret readily the various parts of the chart. The ability to read a chart must include an understanding of all its features, such as scale, projection, geographic position, directions, depths, plane of reference, aids to navigation, tides, currents, elevations, topography, and date of survey and publication.