FIG. 6. SYMBOLS USED ON CHARTS OF THE UNITED STATES COAST AND GEODETIC SURVEY.
The layman who looks at the printed chart probably does not appreciate the amount or the variety of information that must be gathered and sifted and put in proper shape for a single chart.
[COLLECTION OF INFORMATION FOR CHARTS]
Need of thorough surveys. As has been stated, a good chart requires that a thorough and correct survey be first made of the region to be charted. It is said that men are very apt to accept as true anything they see on a map. As to the nautical chart the mariner is likely to be somewhat more critical, however, and it is well that he is. The difficulty of charting an invisible surface such as the bottom of the sea is great, and the proportion of the navigable waters surveyed in sufficient detail to be at all certain of the absence of uncharted dangers is small.
The planning of surveys in a new region, such, for instance, as the Philippine Islands, presents many interesting problems, on the solution of which the effectiveness in chart results and the cost of the work materially depend. Many local conditions must be taken into account. The surveys are made on opposite coasts according to the seasonal winds and rainfall. In some parts fair-sized steamers are necessary; in others launches and small boats can do the work more economically. Shore parties with land transportation are used for portions of the work where the country permits. Natives are employed as far as practicable for the classes of work they can do; the Filipinos, for instance, make good sailors on the vessels and excellent penmen in the office.
The following is a brief outline of the steps of a complete survey for charting purposes, according to the present practice of the United States Coast and Geodetic Survey. These are given in their logical order, though in actual work this order must often be departed from. In this Survey the methods of control have been of a high standard; that is, the main stations have been accurately determined and permanently marked and described, and this has proven an advantage in the joining together of the original surveys and resurveys.
Astronomical observations. To locate on the surface of the earth the area to be charted, astronomical observations are required for the latitude and longitude of one or more points. In the best practice the longitude of a point is obtained by observing the transits of stars to get the local time, and sending time signals by telegraph to obtain the difference from the local time of some other place whose longitude is known. The latitude is observed by measuring the difference of zenith distance of pairs of stars crossing the meridian north and south of the zenith. The azimuth or true direction of some line is also obtained from star observations, usually by observations with a theodolite on a circumpolar star. Much existing chart work depends on positions determined by less accurate methods, as, for instance, longitudes obtained by transporting chronometers between the known station and that to be determined, or by observations of moon culminations, and latitudes obtained by direct observations of the altitudes of stars with theodolite or sextant.