Need for revision. The making of the survey and the printing of the chart do not complete the problem of the chart maker. Both nature and man are constantly changing the facts the representation of which has been attempted on the charts, and also the needs of man are always varying. The original surveys are made to meet the reasonable requirements of the time, but breakwaters and jetties are built, and channels and harbors dredged and otherwise improved, and cities built, and new paths of commerce are opened which bring vessels into waters previously thought of minor importance.

With the increase of commerce and speed of vessels more direct routes are demanded for reasons of economy. Inside routes not originally used are sometimes developed for defensive reasons. The average draft of the larger vessels has also increased remarkably since the modern hydrographic surveys were commenced, and surveys once made to insure safety for the deepest vessels of that time are now not adequate. The average loaded draft of the 20 largest steamships of the world has increased as follows: 1848, 19 feet; 1873, 24 feet; 1898, 29 feet; 1903, 32 feet. The average length of these vessels was 230 feet in 1848, 390 feet in 1873, 541 feet in 1898, and 640 feet in 1903. The number of vessels drawing as much as 2614 feet rose from 36 in 1902 to 185 in 1904. In 1906 there were 17 vessels afloat, drawing 32 feet and upwards. There are now two steamers on the Atlantic 790 feet long, 88 feet beam, and 3712 feet draft when fully loaded, and larger vessels are already planned.

Great natural agencies are also constantly at work effecting changes in features shown on the charts. The action of currents and waves is continually cutting away or building the shore, particularly on sandy coasts exposed to storms. When surveyed in 1849 Fishing Point on the east coast of Maryland was but a bend in the shore line. By 1887 it had built out about two miles in a southerly direction, and in 1902 about two-thirds of a mile further, curving to the westward. Altogether in about half a century this tongue of land has grown out nearly three miles.

Rivers are bearing vast quantities of sediment and depositing these near their mouths, pushing out the coast line and filling in the bottom. The main mouths of the Mississippi are advancing into the Gulf, but at a comparatively slow rate. A break from the main river at Cubit's Gap just above the head of the passes, however, has done an enormous amount of land making, filling in an area of about 50 square miles between 1852 and 1905.

The mouth of the Columbia River in Oregon shows an interesting example of the movement of an island. The chart of 1851 shows the center of Sand Island 314 miles southeast of Cape Disappointment, the chart of 1870 shows it 234 miles southeast, and the chart of 1905 shows it 114 miles easterly. This island has thus moved 2 miles northwesterly directly across the middle of the river entrance, closing up the former[Pg 99-109] north channel. The southern point of the entrance, Clatsop Spit, has built out about the same distance.

Volcanic action in well authenticated cases has caused islands to rise or disappear. In the present location of Bogoslof Island in Bering Sea the early voyagers described a "sail rock." In this position in 1796 there arose a high island. In 1883 another island appeared near it. In 1906 a high cone arose between the two, and a continuous island was formed over 112 miles long and 500 feet high. The latest report (September, 1907) was that this central peak had suddenly collapsed and disappeared. Bogoslof is an active volcano, and the main changes have been the result of violent volcanic action. The history of this island for over a century past forms a remarkable record of violent transformations in the sea.

Earthquakes sometimes cause sudden displacements, horizontal or vertical, of sufficient amount to affect the information shown on the charts. A careful investigation of the effects of the earthquake in Yakutat Bay, Alaska, in September, 1899, showed that the shore was raised in some parts with a maximum uplift of 47 feet and depressed in other parts, and that at least two reefs and four islets were raised in the water area where none appeared before. Undoubtedly there were changes in the water depths, but definite information is lacking because there had been no previous hydrographic survey. The San Francisco earthquake of 1906 caused little vertical displacement, but there were horizontal changes of relative position as much as 16 feet; so far as known this earthquake did not affect the practical accuracy of the charts. Related to earthquake phenomena are the gradual coast movements of elevation or subsidence which are taking place but at so slow a rate as not to sensibly affect the charts in ordinary intervals of time.

Another agency at work is the coral polyp on the coral reefs; although the rate of growth appears to be very slow, the resulting reefs and keys are an important feature in tropical seas.

Practically all of the land features shown on charts are likewise subject to changes, the more rapid of which are mainly due to the works of man.

The changes of channels and of commercial needs cause many alterations to be made from time to time in the lights and buoys which are shown on the charts.

Methods of correction. The problem of keeping a chart sufficiently up to date is one of much practical importance and one which must be taken into account in planning what should be shown on the chart in the first place so as to bring it within the range of practicable revision.

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.