1123 HYDROGRAPHIC MANUAL PaGE 4 



shortest route between any two points. In almost every other respect the projection is 

 faulty. The scale is not constant, but increases from its constructional center outward, 

 so that distances, areas, and shapes are very much distorted near its boundaries. 

 Azimuths, except at the point of tangency, can be plotted only by means of a specially 

 constructed compass rose. 



Gnomonic charts are used principally in connection with great-circle sailing for 

 finding the shortest possible track line between any two points and the course at any 

 point along the track. Polar regions, where the Mercator projection is nearly useless, 

 are also charted on this projection. 



The projection and its mathematical theory are treated more fully in Special 

 Publication No. 68. 



1123. The Design of a Nautical Chart 



A nautical chart is designed solely with reference to the needs of navigat'on in a 

 certain area. But even in one area there is a variety of needs; a transoceanic liner 

 entering a port and restricted to the main channels does not need the same information 

 on a chart as a tugboat which may not follow the regular channels and may be required 

 to venture into unfamiliar places. A variety of scales of charts to meet various needs 

 is required. 



The nautical charts published by the Coast and Geodetic Survey are constructed 

 on certain convenient standard scales, which are almost invariably multiples of the 

 proportion 1:10,000. The standard scale of a Mercator chart generally applies to its 

 middle latitude. For convenience in navigating on adjacent charts, some are grouped, 

 and the standard scale applies to an appropriate latitude of the series; thus the scale of 

 adjacent charts is the same in the overlapping area, but the scale at the middle latitude 

 of each chart of the series may differ from the standard. 



The limits of a nautical chart must be decided by the needs of navigation. Each 

 must include near its limits suitable features for use as points of departure, and adjacent 

 charts must overlap so that they include a common prominent feature. Because of 

 these requirements it is customary to plan charts in a series, even though there is no 

 mmediate prospect of needing more than one or two of the series. 



1124. Compilation of Charts 



Nautical charts are compiled from the field surveys of the Bureau and from miscel- 

 laneous data from various sources. It is extremely rare that a hydrographic survey is 

 directly reproduced as a nautical chart, even at a reduced scale. Compilation is a 

 process of evaluation of the available data, from which selection is made. Those fea- 

 tures that are of little interest to the mariner are generalized or omitted altogether if 

 they interfere with, or obscure, data of navigational value. By contrast, those features 

 of greatest navigational importance are' accentuated so they will not be overlooked. 



Some chart users erroneously believe that a midtitude of soundings on a chart is 

 evidence that the region has been thoroughly and adequately surveyed. Of course, if 

 only a few soundings were taken, only a few can be charted and the reconnaissance 

 nature of the survey is clearly indicated by their arrangement on the chart. But, if 

 there is a sufficient reduction from the scale of the survey to the scale of the chart, the 

 area may appear to be well covered and the mariner is lulled into the belief that the area 

 has been well surveyed and that he may navigate it in safety. 



