20 



HYDROGRAPHIC MANUAL 



(a) Maximum errors : 



(1) to 11 fm. (0 to 20 m.): 1.0 

 ft. (0.3 m.) ; 



(2) 11 to 55 fm. (20 to 100 m.) : 0.5 

 fm. (1.0 m.) ; 



(3) 55 fm. (100 m.) and deeper: one 

 per cent of depth. 



II. Scale of Survey : 



(a) In general, never less than that of 

 the published chart and preferably : 



(1) For rivers, lakes, harbors, chan- 

 nels, and pilot w^aters : 

 —Multiples of 1/1000 for scales 

 to 1/10000 



— Multiples of 1/2500 for scales 

 from 1/10000 to 1/20000 



(2) For coasts and oceans : 



— Depths generally less than 30 



fm. (60 m.) : 1/40000 or larger; 

 — Depths generally less than 100 



fm. (200 m.): 1/100000 or 



larger ; 

 — Depths generally greater than 



100 fm. (200 m.) : 1/250000 or 



larger. 



III. Interval of Sounding Lines : 



(a) Spacing of principal lines: 0.4 in. 

 (1.0 cm.) or less, except where 

 depth and character of the bottom 

 will permit wider spacing ; 



(b) Spacing of cross (check) lines: 3.0 

 in. (7.5 cm.) or less. 



IV. Interval of Plotted Soundings : 



(a) Frequency along sounding lines: 

 0.2 in. (0.5 cm.) or less. 



V. Sampling of Bottom Characteristics: 

 (a) In general sufficient sampling to 



demarcate the limits where one gen- 

 eral type of bottom changes to 

 another. Samples in depths less 

 than 30 fathoms (60 m.) should be 

 obtained at a spacing of 3 inches 

 (8 cm.) or less at the scale of the 

 survey sheet, depending on type of 

 bottom. Beyond that depth special 

 equipment is required; therefore, 

 bottom samples shall be obtained 

 only when conducting oceanograph- 

 ic observations of the area. When 

 surveying deep anchorage areas, 

 depth should be extended beyond 30 

 fathoms in order to determine char- 

 acter of holding ground. 



VI. Position Fixes : 



(a) Spacing on survey sheets: 1 to 1.5 

 in. (2 to 4 cm.) ; 



(b) Maximum error of plotted position 

 (relative to a shore control) : 0.05 

 in. (1.5 mm.) ; 



(c) Location of shore control: from a 

 triangulation scheme of 3rd-order 

 accuracy or better. 



VII. Reference of Sounding to Vertical 

 Datum : 



(a) Location and duration of water 

 stage observations to be such that 

 each sounding can be referenced to 

 the selected vertical datum with an 

 error no greater than one-half that 

 specified for measurement of depth. 



