123 HYDROGRAPHIC MANUAL PaGE 14 



changeably as descriptive of this scale ratio. On published charts and generally on 

 drawings it is represented as a fraction, as 7777;^ • The same meaning is expressed in 



text either by 1:10,000, 1-10,000, or 1/10,000. 



The basic scale for hydrographic sm-veys of the Coast and Geodetic Survey is- 

 1:20,000 and almost all other scales used have a simple relationship to it. No inshore 

 survey adjacent to the coast shall be on a scale smaller than 1:20,000, except by author- 

 ity of the project instructions. 



Larger scales shall be successive multiples of 1:20,000, each larger scale being 

 double that of the preceding scale (i. e., 1:10,000, 1:5,000, and 1*:2,500). All important 

 harbors, anchorages, channels, and many parts of the coast where dangers are nu- 

 merous or navigation is difficult, shall be plotted on scales of 1:10,000 or larger. 



Scales smaller than 1:20,000 shall be those whose denominators are multiples of 

 20,000 (i. e., 1:40,000, 1:00,000, 1:80,000, 1:100,000, etc.). 



The choice of scale is obviously dictated by the amount of detail required on the 

 hydrographic sheet. The larger the scale the more detail can be shown. The scales 

 selected for important harbors, anchorages, and restricted navigable waterways must 

 be large enough to show the maximum amount of detail desired on the smooth sheet 

 and to permit as close a system of sounding lines and development of the area as may 

 be necessary. 



It is an inflexible rule in chart construction that data on a smooth sheet must 

 never be enlarged to the scale of the published chart. A scale should, therefore, be 

 selected for the smooth sheet larger than — and preferably at least twice as large as— 

 that of the largest-scale published chart of the area. 



For the inshore survey along an open coast, the scale will vary according to the 

 importance of the area and the number of dangers which may be encountered. 



Varying scales must be selected for offshore surveys according to the depth of 

 the water and the distance offshore. In the first zone oft"shore from the inshore surveys 

 the usual scale is 1:40,000, adjoined by surveys on a scale of 1:180,000 or 1:20,000 in 

 the area farther offshore. 



If the survey is comparatively distant from the shore the choice of scale may be 

 limited by the necessity of including shore stations on the smooth sheet, unless circle 

 sheets (see 37) are used. A smaller scale than would otherwise be preferable must be 

 selected occasionally in order to include within the limits to the smooth sheet an ade- 

 quate number and arrangement of shore stations, so that strong sextant fixes may be 

 obtained throughout the area. The advantage thus gained in being able to plot the 

 positions of the survey vessel with assured accuracy more than offsets the labor in- 

 volved in inking congested soundings at the smaller scale, or even the disadvantage 

 of having to omit a percentage of the recorded soundings. 



In summation, the survey scale should be large enough: 



(a) To provide for a sufficiently close spacing of the systematic lines and soundings. 



(5) To provide for an adequately intensive development of the dangers and shoals. 



(c) To provide for a reduction to the scale of the largest-scale published chart- — it should gen- 

 erally be at least twice the scale of the chart. 

 {d) To show all details clearly. 



And the survey scale should be small enough: 



(a) To include the necessary shore control within the limits of the sheet. 



(6) To include control which will give strong well-conditioned fixes. 



