4. CONTROL AND SIGNAL BUILDING 



117 



be shown on the boat sheet by a solid red 

 line. 



Where the revised shoreline has been lo- 

 cated by less accurate methods it shall be 

 shown by a broken red line. 



4-17 Field inspection report. — All hydro- 

 graphic parties which field inspect shoreline 

 or identify control, either horizontal or ver- 

 tical, on the photographs, shall submit a 

 field inspection report in accordance with 

 Sections 718 and 724 of Topographic Manual, 

 Part II. A progress sketch shall be forwarded 

 with the report showing previously estab- 

 lished horizontal and vertical control stations 

 which are recovered and identified, and new 

 stations established and identified. 



4—18 Location of photo-hydro stations. — 



Stations for control of hydrography located 

 by photogrammetric methods, known as 

 photo-hydro stations, shall be established 

 in accordance with Photogrammetry Instruc- 

 tion 45, Revision 1, dated 15 March 1954. 

 The identification of the photo-hydro point 

 should be pricked on the photograph within 

 0.2 mm. of its true position. 



Photogrammetric methods shall be used to 

 locate supplemental stations for control of 

 hydrography whenever practicable (see 1- 

 16). In areas where photographs do not 

 contain sufficient detail to permit photogram- 

 metric positioning of signals at minimum 

 intervals required for control, various other 

 methods may be used to supplement the 

 photogrammetry. In narrow inlets, bays, and 

 passages, planetable graphic control surveys, 

 based on triangulation control or on points 

 located by the photogrammetric plot, will 

 provide control of sufficient accuracy. A 

 planetable traverse starting at a station 

 located by the photogrammetric plot and 

 closing at another similarly located station, 

 may be used, but the length of the traverse 

 should seldom exceed one mile. A stadia dis- 

 tance and azimuth from a known point to a 

 signal may be used, but the distance in meters 

 and the azimuth line should be inked on the 

 boat sheet. Three well-intersected sextant 

 cuts from points located by the photogram- 

 metric plot may be used to locate a signal. 



The orientation station used for each cut 

 should be as distant as possible. In all the 

 above cases the identification of the photo- 

 grammetric points used must be positive. 

 Identifiable points, such as small rocks, 

 corners of buildings or piers, and forks of 

 streams may be used as hydrographic signals. 

 Supplemental stations may be located by ref- 

 erence measurements to such detail, if the 

 objects themselves are not convenient for 

 use as signals or as sites for erection of sig- 

 nals. (Photogrammetry Instruction 22, 

 Revision 1.) 



4-19 Photogrammetric data. — A hydro- 

 graphic party will usually be furnished field 

 and office photographs, blackline impressions, 

 paper prints, and blueline tracings of photo- 

 grammetric manuscripts. 



If preliminary manuscripts are furnished, 

 all photographs, C.S.I, cards, shoreline in- 

 spection reports, and associated data shall be 

 forwarded to a photogrammetric office to be 

 specified by the Washington Office. The data 

 should be forwarded as rapidly as field work 

 is completed on one or two manuscripts and 

 should not be retained until the end of the 

 season. Office processing will be scheduled 

 to provide blueline tracings of advance 

 manuscripts as needed for smooth plotting 

 hydrographic surveys. 



If significant changes are found on incom- 

 plete or advance manuscripts, all photo- 

 graphs and field inspection reports should 

 be forwarded to the Washington Office. New 

 blueline tracings will be furnished after the 

 manuscript has been corrected. 



If office revision of the map manuscripts 

 is not required, the photographs shall be 

 returned to the Washington Office, and all 

 other photogrammetric data shall be submit- 

 ted with the hydrographic records. 



4-20 Planetable surveys. — The hydro- 

 graphic party will rarely find it necessary 

 to make a topographic survey by planetable 

 methods. However, it may be expedient to 

 locate signals for control of hydrography by 

 planetable graphic control surveys. 



When a hydrographic survey is required 

 at a scale larger than that of the photogram- 



