4. CONTROL AND SIGNAL BUILDING 



115 



of a hydrographic survey. When project 

 plans permit, this work will be done one 

 year in advance of the hydrographic survey. 

 In areas where control indentification and 

 shoreline inspection must be done by a hy- 

 drographic party, the field work should be 

 done in sufficient time to permit completion 

 of incomplete or advance manuscripts prior 

 to the beginning of hydrographic surveys (see 

 1-16). A lead time of 3 to 6 months is re- 

 quired, depending upon the size and com- 

 plexity of the areas to be mapped. 



4-11 Control identification. — Instructions 

 for identification of horizontal control sta- 

 tions are contained in Photogrammetric In- 

 struction No. 22, Revision 1. When a hy- 

 drographic party is required to identify con- 

 trol in advance of a photogrammetric plot, 

 a sketch of the project area will be furnished 

 showing the approximate sites where control 

 points are desired. If there is an existing 

 scheme of triangulation, the instructions 

 may require identification of only a few 

 selected stations. The identification of 

 the station, or a substitute point close by, 

 must be positive and very carefully done. 

 There is no objection to the identification of 

 stations in excess of requirements if such 

 identification is equally accurate. The point 

 must be identified within 0.15 mm. of its 

 true location, at the scale of the photograph 

 (see 1-16). Incorrect or poor identification 

 of control will have the same effect on the 

 compilation as similar inaccuracies in geo- 

 graphic positions or plotting of triangula- 

 tion stations. 



A Control Station Identification (C.S.I.) 

 card. Form 152, shall be prepared for each 

 horizontal control station identified to con- 

 trol a photogrammetric plot, regardless of 

 the method of identification. 



If the project instructions specify estab- 

 lishment of additional triangulation in ad- 

 vance of photogrammetric surveys, all new 

 stations shall be identified on the photographs. 

 When photographs are to be taken soon after 

 establishment of control, a white ground 

 mark at each station will facilitate identi- 

 fication. 



When the substitute method is used, at 



least two substitute points should be identi- 

 fied. Small offlying rocks near stations along 

 the coast are excellent substitute stations. 

 If distances cannot be taped, stadia dis- 

 tances are satisfactory if accurate to about 

 1 meter. 



Identification of any point shall be done 

 after careful study of the photograph under 

 a steroscope. A fine pointed needle shall be 

 used to prick the point in the emulsion. 

 Under no circumstances shall a hole be 

 punched through the paper. 



4-12 Classification of photogrammetric 

 map manuscripts. — Photogrammetric map 

 manuscripts are classified and labeled in ac- 

 cordance with Photogrammetric Instruction 

 No. 48, dated 9-3-54, as follows : 



(a) Preliminary manuscript. This manu- 

 script has been prepared without prior field 

 identification of control or field inspection. 

 It will be completely recompiled after re- 

 ceipt of control identification and inspection 

 data. 



(b) Incomplete manuscript. This manu- 

 script is based on field identified control 

 and is a final photogrammetric plot, but 

 the delineation of details is incomplete. It will 

 be completed after additional field inspection 

 data are received. 



(c) Advance manuscript. This manu- 

 script is based on field identified control and 

 field inspection data. It is subject to correction 

 if errors are found in the field or during 

 office review. 



If the manuscript is not labeled in one of 

 the above categories, it is a final manuscript. 



4-13 Use of preliminary manuscripts. — 



A preliminary manuscript is based on office- 

 identified control, but in making the photo- 

 grammetric plot the photographs are rigidly 

 held in relation to each other rather than 

 being held to the triangulation stations. 

 This results in a plot which is consistent 

 within itself, but will be out of position by 

 varying amounts as compared to the tri- 

 angulation control. Photo-hydro stations 

 located on a preliminary manuscript may be 

 used to control hydrography, but such sta- 

 tions should not be used in conjunction with 



