118 



HYDROGRAPHIC MANUAL 



metry, a planetable shall be used to locate 

 the supplemental control. In such cases the 

 photographs should be examined to ascertain 

 whether all topographic detail can be com- 

 piled at the enlarged scale. Such features 

 as piling, small buildings and details around 

 piers and slips shall be located by the plane- 

 table survey if they are not clear in the 

 photographs. 



When the high water line is obscured by 

 overhanging trees or by shadows and there 

 are an insufficient number of identifiable 

 points to permit photogrammetric location 

 of all signals, planetable graphic control 

 methods should be used. Triangulation sta- 

 tions, if available, shall be used to control 

 the survey. In the absence of triangulation, 

 photo-points may be transferred from the 

 manuscript and used to control the graphic 

 survey, provided that all such points can be 

 accurately identified on the ground. 



4-21 Accuracy of topographic signal lo- 

 cations. — An aluminum-mounted sheet 24 by 

 31 inches, which is identical with the size 

 of the planetable board, shall be used for 

 graphic control surveys and for planetable 

 topographic surveys. The methods and in- 

 struments used shall be such that 90 percent 

 of the control stations located shall be within 

 0.5 mm. of correct geographic position at the 

 scale of the sheet, and no station shall be in 

 error more than 0.8 mm. Closing errors of 

 planetable traverses, prior to adjustment, 

 shall not exceed 0.4 mm. per mile at the 

 scale of the sheet, and in no case shall the 

 total closing error which may be adjusted 

 exceed 2.0 mm. at the scale of the sheet (see 

 1-17). 



4-22 Shoreline by planetable. — When 

 planetable methods are used to locate minor 

 control stations, the highwater line should 

 be located by at least three rod readings 

 near each planetable setup. Each rod read- 

 ing should be indicated by a black dot and 

 the highwater line drawn in black between 

 the readings leaving a short hiatus on each 

 side of the dot. The high-water line at a 

 rocky point which is difficult to interpret in 

 the photographs should be located by plane- 

 table when time permits. 



4-23 Planetable sheet numbers and re- 

 ports. — Each planetable sheet shall be desig- 

 nated by a capital letter assigned in alpha- 

 betical order during the season. A new series 

 starting with "A" shall be used each season. 

 The complete designation shall be composed 

 of the first two letters of the name of the 

 survey vessel, or other assigned designator, 

 followed by the capital letter, followed 

 by the last two digits of the year, all 

 connected by hyphens. For example SU-C- 

 59 would indicate the third topographic sheet 

 initiated by the Ship SURVEYOR in 1959. 

 The location of each sheet shall be shown on 

 the sheet layout sketch for the project. 



If the survey contains important shoreline 

 information, a registry number will be as- 

 signed on request to the Washington Office. 

 If the survey contains only minor shoreline 

 details or corrections, or if it is a graphic 

 control survey for location of signals only, 

 the data will be transferred to the smooth 

 hydrographic sheet and no registry number 

 will be assigned. Such sheets will be de- 

 stroyed after the smooth sheet has been veri- 

 fied, inked, and reviewed. 



A descriptive report shall be written to 

 accompany each topographic sheet as speci- 

 fied on pages 11 and 12 of the Topographic 

 Manual. 



4-24 Marked topographic stations. — To 



provide for future use in revision surveys, 

 the topographer or photogrammetrist shall 

 supplement existing control by establishing 

 additional recoverable stations at intervals 

 which will vary with the nature of the area. 

 In areas where revision surveys will be re- 

 quired because of natural or artificial 

 changes, stations should be established at 

 intervals of one mile along the coast. In 

 isolated areas, along difficult coastlines, and 

 where changes are not expected, recoverable 

 stations shall be established at not less than 

 2-mile intervals. 



These recoverable stations may be natural 

 or artificial objects when available, or stand- 

 ard topographic station disks set especially 

 for this purpose. Stations should be estab- 

 lished where they can be easily identified 

 and in the most prominent places, such^^s 



