5. HYDROGRAPHY 



129 



plied the four corners of the projection are 

 located and the principle lines may be sub- 

 divided to draw the rest of the projection. 

 Curvature values shall be included where 

 necessary according to the scale of the 

 projection. 



5-8 Duplicate boat sheets. — When more 

 than one survey unit is to work in the area 

 covered by one boat sheet, a duplicate boat 

 sheet can be made by pricking through the 

 projection intersections with a fine needle. 

 A long steel straight edge should be placed 

 along each line as the points are pricked, 

 being careful not to disturb the relation be- 

 tween the two sheets as the straight edge is 

 moved. 



Field numbers are assigned as in 1-13. 

 The letter A should be added to the number 

 on the original sheet and duplicates indi- 

 cated by adding letters B, C, etc., to the 

 field numbers. 



When the area to be surveyed lies en- 

 tirely within the limits of a photogram- 

 metric manuscript, a boat sheet may be 

 made by transferring the projection and 

 shoreline from the blueline tracing of the 

 manuscript. Joining two or more manu- 

 scripts for this purpose shall not be 

 attempted. 



5-9 Calibration sheets. — If a projection 

 is required for calibration of an electronic 

 control system it shall be constructed on an 

 aluminum-mounted sheet of appropriate size. 

 It is desirable that the scale of the projection 

 be twice as large as the largest scale survey 

 on which the system is to be used, and it 

 must never be smaller than the largest scale 

 survey sheet. The method of construction 

 and plotting control are the same as for 

 other projections, but the work must be very 

 accurately done. The sheets are not num- 

 bered. They may be discarded after all data 

 have been tabulated and verified. 



5-10 Control stations. — All control sta- 

 tions, whose positions are known at the start 

 of the survey, should be plotted on or trans- 

 ferred to the boat sheet and shall be shown 

 by standard symbols (Fig. 79). The names 

 of stations may be lettered on the boat sheet 



in freehand, provided they are unmistakably 

 legible. They should not be placed in water 

 areas or obliterate any essential detail on the 

 boat sheet and must be placed so that they 

 are clearly associated with the correct sym- 

 bols (see 6-15). Existing names of control 

 stations must be retained with their exact 

 spelling. If a station is in the water area, 

 such as a beacon or offlying rock, the name 

 may be inked on land area nearby and an 

 arrow or leader used to indicate the station 

 to which the name refers. For each control 

 station in the water area, notation should 

 be made on the boat sheet as to whether the 

 feature on which it is erected is permanent 

 or temporary, and a short description should 

 be added. 



When control stations are numerous, as 

 on an inshore hydrographic sheet, identifi- 

 cation will be aided and confusion avoided 

 if brief descriptions of the signals are noted 

 on the boat sheet. 



Each natural object used as a control sta- 

 tion shall be described on the boat sheet. 

 When it is conspicuous enough for use as a 

 landmark, that fact should be included in 

 the description. 



As new control stations are located, they 

 shall be plotted on the boat sheet by direct 

 transfer or other accurate method. The plot- 

 ting of all control stations should be verified 

 before the station symbols are inked, and 

 the fact noted on the boat sheet. 



5-11 Electronic distance circles. — When 

 an electronic system is to be used for the 

 control of hydrography and the locations of 

 the shore stations are known, arcs of dis- 

 tance circles should be drawn as soon as the 

 projection has been verified. If the arcs must 

 be drawn at a later date as the survey pro- 

 gresses, the projection should be checked for 

 distortion and significant amounts equally 

 distributed by procedures described in the 

 following paragraphs. 



When the shore stations fall on the sheet, 

 the arcs can be drawn directly with a beam 

 compass. Colored inks shall be used with a 

 distinctive color assigned to each station. A 

 circle of the same color about 5 mm in di- 

 ameter, shall be drawn around the station 



