2 (Change 1) 



HYDROGRAPHIC MANUAL 



1— 3a Project instructions. — The field op- 

 erations required for the liydrograpliic survey 

 of a specified area are assigned a project num- 

 ber such as OPR-406, the letters identifying the 

 project with Operations Division, Office of 

 Oceanography. The numbered project instruc- 

 tions supplement the general instructions in this 

 and other manuals, and their content varies 

 from specific detail to generalized manual-com- 

 pliance depending on locality and nature of 

 project. Instructions for combined-operation 

 projects usually include such subjects as project 

 limits, control, topogi'aphy, hydrography, tides. 

 Coast Pilot, magnetic observations, and miscel- 

 laneous; occasional added subjects are special 

 oceanogi-aphic investigations, current obsei-A'a- 

 tions, and wire drag. See Chapter 2. 



1-4 Data to start survey. — Copies of all 

 prior survey data that are considered neces- 

 sary in connection with the combined opera- 

 tions of the project will be furnished with 

 the project instructions. These will include: 

 descriptions and geographic positions of all 

 triangulation stations and recoverable topo- 

 graphic stations ; copies of prior hydro- 

 graphic and topographic surveys; descrip- 

 tions and elevations of tidal bench marks ; 

 and information as to dangers reported. If 

 prior photogrammetric surveys have been 

 made, copies of the manuscripts and photo- 

 graphs will be furnished. 



1-5 Presurvey review. — The Chart Divi- 

 sion will prepare a presurvey review for each 

 hydrographic survey project (see 6-108). 

 All prior records and the large.st scale charts 

 of the project area are examined. Critical 

 soundings and charted data which are unveri- 

 fied or questionable will be indicated and 

 described on the charts. All items must be 

 thoroughly examined in the field to prove or 

 disprove their existence, and each item shall 

 be specifically mentioned in the descriptive 

 report to accompany the survey. The pre- 

 survey review is not intended to relieve the 

 Chief of Party or the hydrographer from a 

 responsibility to compare the results of the 

 survey with the features shown on 

 the large.st .scale chart of the area. 



1-6 Scale of surveys. — One of the rules 

 of chart construction is that data on a 



smooth sheet shall seldom, if ever, be en- 

 larged to the scale of a published chart. 

 The scale adopted for a survey shall be larger 

 than — preferably at least twice as large as — 

 that of the largest-scale published or pro- 

 posed chart of the area. 



The basic scale for hydrographic surveys 

 of the Coast and Geodetic Survey is 1:20,000 

 and almost all other scales used have a sim- 

 ple relationship to it. No inshore survey 

 adjacent to the coast shall be plotted on a 

 scale smaller than 1 :20,000, except by au- 

 thority of the Director. 



All important harbors, anchorages, re- 

 stricted navigable waterways, and many 

 parts of the coast where dangers are numer- 

 ous shall be plotted on scales of 1:10,000 

 or larger. 



1-7 Sheet Layout. — Prior to beginning 

 field work, a hydrographic sheet layout (Fig- 

 ure 2) shall be prepared and forwarded 

 to the Washington Office for record and ap- 

 proval. The layout should generally be made 

 at the scale of the largest scale chart cover- 

 ing the project area. Each sheet should be 

 laid out to include as large a water area as 

 practicable, at the .same time providing for 

 adequate overlap with adjacent sheets and 

 ensuring that all required control stations 

 will be included (see 2-20). The overlaps of 

 sheets should be such that soundings will 

 seldom be plotted closer than 3 inches to the 

 edge of a sheet. 



Sheets containing small detached areas of 

 hydrography shall be avoided if practicable. 

 This can usually be accomplished by placing 

 a subpian, or inset, on the smooth sheet at 

 the same or an enlarged scale (see 6-6). 



All hydrographic sheets shall be laid out 

 so that the projection lines are approximately 

 parallel with the edges of the sheet, except 

 when such a layout is extremely uneconomic 

 or impractical. Skewed projections should 

 not be used without prior authority of the 

 Director. 



1-8 Sheet sizes. — The standard size for 

 all hydrographic smooth sheets is 36 by 

 54 inches. Chiefs of Party are authorized to 

 increase the size of the sheet to 36 by 60 

 inches in exceptional cases, but approval 

 shall be obtained from the Director before 

 using a larger sheet. The maximum size, 



