26 



HYDROGRAPHIC MANUAL 



2-19 Survey scales. — The basic scale for 

 hydrographic surveys of the Coast and 

 Geodetic Survey is 1:20,000, and almost all 

 other scales bear a simple relationship to it. 

 No inshore survey adjacent to the coast shall 

 be on a scale smaller than 1 : 20,000, except 

 by authority of the Director. In inshore 

 areas, surveys at 1 :25,000 will sometimes re- 

 sult in a better sheet layout without the use 

 of oversize sheets and still permit adequate 

 development of the bottom features. Before 

 reducing the scale of the field survey, con- 

 sideration should be given to making the sur- 

 vey on an oversize sheet and then reducing 

 the size of the smooth sheet by smooth-plot- 

 ting the survey at a smaller scale (see 6-7). 

 The latter practice is preferable from the 

 view of the -chart compiler. In either case, 

 prior approval must be obtained. 



Scales smaller than 1:20,000 shall gen- 

 erally be those whose denominators are mul- 

 tiples of 20,000 (i.e., 1:40,000, 1:60,000, 

 1 :80,000, etc.) except that surveys at 1 :30,- 

 000 or 1 : 50,000 are authorized where the use 

 of these scales in place of the 1 :40,000 scale 

 will result in better sheet layout without the 

 use of oversize sheets, and/or better develop- 

 ment of bottom features. 



Larger scales shall be multiples of 1:20,- 

 000, each scale being double that of the 

 preceding scale (i.e., 1:10,000, 1:5,000, and 

 1:2,500). All important harbors, anchor- 

 ages, channels, and many parts of the coast 

 where dangers are numerous or piloting is 

 difficult, shall be plotted on scales of 1:10,- 

 000 or larger. 



The choice of scale is obviously dictated 

 by the amount of detail required on the 

 hydrographic sheet. A Chief of Party is 

 authorized to use, at his discretion, scales 

 larger than those required by project in- 

 structions. The scale selected for the smooth 

 sheet must never be smaller than — and pref- 

 erably twice as large as — that of the largest- 

 scale published chart of the area (see 1-6). 



Offshore surveys controlled by Electronic 

 Position Indicator should not be plotted at 

 scales larger than 1:100,000. When Raydist 

 equipment is used a scale should be selected 



which will permit use of a line spacing ade- 

 quate for the area. 



2-20 Sheet Layout. — Since boatsheets of 

 different scales will be required for surveys 

 of different parts of a project area, it is es- 

 sential that a sheet layout be constructed as 

 part of the preliminary planning. In order 

 that these sheets may be planned practicably 

 and economically, a sheet layout should be 

 made on a chart of appropriate scale. A 

 tracing of the sheet layout should be for- 

 warded to the Washington Office for record 

 and approval before the boat sheets are made 

 (see 1-7). 



Each hydrographic sheet should be laid 

 out in such a manner that it will include as 

 large a water area as practicable, at the 

 same time providing for adequate overlap 

 with adjacent sheets and ensuring that the 

 survey can be adequately controlled. The 

 overlap of adjacent sheets should be suffi- 

 cient to provide for a suitable junction with 

 adjacent surveys and to include the neces- 

 sary control. As a result of handling and 

 age some smooth sheets tend to crack along 

 the edges, and soundings near the edge may 

 become illegible. For this reason, it is de- 

 sirable to lay out the sheets so that it will 

 not be necessary to plot soundings closer 

 than three inches to the edge of a sheet. 



A convenient method for making this lay- 

 out is to construct on tracing cloth or clear 

 plastic one or more models of each standard 

 size sheet according to the scale of the chart 

 on which the layout is to be made. The 

 models may then be shifted about on the 

 chart, giving consideration to required over- 

 lap, until the best position for each sheet 

 is determined. Where the area is complex it 

 is frequently necessary to try various lay- 

 outs of sheets before the most practicable 

 and economical one is found. As the best 

 position for each sheet is determined, the 

 four comers may be pricked through to the 

 chart and joined by straight lines on the 

 layout (see Fig. 2). 



The area that will be included on a sheet 

 of given dimensions at a specified scale may 

 be readily determined from Table 5 by mul- 



