Page 5 preliminary 1125 



1125. The Datums of a Nautical Chart 



A nautical chart may be said to have two kinds of datums; vertical datums to 

 which the depths and elevations are referred, and a horizontal datum to which features 

 are referred by latitude and longitude. 



The sounding datum, or the chart datum as it is sometimes called, is the plane 

 of reference for the charted soundings; it is a tidal plane. The sounding datum is 

 noted prominently in the title of each chart published by the Coast and Geodetic Survey; 

 with minor exceptions, all depths on charts in the Atlantic Ocean are referred to mean 

 low water (MLW) and those in the Pacific Ocean to mean lower low water (MLLW). 

 (See also 2172.) 



The vertical datum for elevations, with minor exceptions, is mean high water 

 (MHW) for all nautical charts of the Coast and Geodetic Survey (see 2173). 



The geographic datum, also called horizontal or geodetic datum, is the adopted 

 position in latitude and longitude of a single point to which the charted features of a 

 vast region are referred. Charts of waters bordering the United States are either on 

 the North American datum or the North American datum of 1927; other charts are on 

 various other datums, not connected with the continental triangulation. (See also 

 2171.) Most charts of the Coast and Geodetic Survey do not indicate the datums 

 they are on, but the legend "N.A. 1927" in the upper right-hand corner of the border of 

 any chart, signifies that it is on the North American datum of 1927. 



1126. Chart Accuracy 



The most important criteria by which the value of a nautical chart may be judged 

 are its accuracy, adequacy, and clarity. A lack of any one of these may result in a 

 marine disaster with consequent loss of life and property. Except for blunders in 

 compilation, accuracy depends directly on the quality of the field surveys; the hydrog- 

 rapher and the cartographer are equally responsible for the adequacy, but the cartog- 

 rapher alone can embody clarity in a chart. 



The nautical charts of the Coast and Geodetic Survey are noted for a clarity of 

 which the cartographers and lithographers may well be proud, a quality toward which 

 the Bureau has ever striven. And where the charts are based on modern surveys, 

 their accuracy and adequacy are second to none. 



The accuracy of a nautical chart is dependent on the accuracy and adequacy of 

 the hydrographic surveys from which it is compiled; it cannot be more accurate. In 

 general, it may be said that the date of a survey is an indication of its thoroughness 

 and accuracy. A century ago there were such vast unsurveyed areas that the hydrog- 

 rapher was sometimes obliged to subordinate completeness and accuracy to speed, 

 for even an inadequate chart is better than none at all. Since that time, decade by 

 decade, standards have become stricter, and new methods and instrumental equipment 

 have been devised and the former ones developed to greater perfection. 



The scales of most hydrographic surveys are at least twice, and often several times, 

 as large as the publication scales of the charts on which the information appears. The 

 effect of this reduction in scale is to reduce the errors of plotting to an amount which, 

 for all practical purposes, is not measurable at the chart scale. 



1127. Dates on Charts 



The dates of the surveys from which a chart has been compiled are an important 

 indication of the present trustworthiness of a chart, and this is especially so in a change- 



