Page 101 control and signal building 2393 



air photographic surveys will be accompanied by ozalid prints (see 7332) containing 

 descriptions of the marks and other data for the location on the ground of the hydro- 

 graphic signal sites selected by the air photographic party. Prints of the air photo- 

 graphs for field use will also be furnished. (See also 733.) 



When special conditions require that boat sheets be made by the hydrographic 

 party in the field, copies of any photographic surveys available will be furnished with 

 other survey data from the Office. 



2393. Additional Control Stations 



The number of stations for hydrographic signals located by the air photographic 

 surveys made in advance of the hydrography will vary from practically all that are 

 needed, where plenty of objects of an enduring nature are identifiable on the photo- 

 graphs, as on irregular rocky coasts or in the vicinity of city harbors, to relatively few 

 on straight sand beaches with only featureless detail. Wliere suitable locations are 

 clearly identifiable at the time the photographs are inspected in the field but are subject 

 to change before the construction of the hydrographic signals, inexpensive concrete 

 marks or stakes will be set by the field inspection party to facilitate exact recovery of 

 the points selected for stations in the photographic plot. 



In general, approximately 75 percent of the stations required for hydrographic 

 signals will be located by the radial plot of the air photographic survey. Additional 

 stations must be located by the hydrographic party to supplement these, in order to 

 have sufficient control for the hydrographic survey. Usually these additional stations 

 will not be far from the stations located by the radial plot, and the latter may be used 

 as control or starting points from which to locate the former. Since each additional 

 station ordinarily will be located independently of the others and there are no accumu- 

 lative errors, unconventional methods will frequently suffice to give sufficiently accurate 

 locations. The methods which may. be used to locate additional control and the 

 symbols to be shown on the hydrographic sheets for each method are as follows: 



(a) Conventional geodetic methods, usually traverse, starting from stations whose positions have 

 been determined by computations, from which the latitude and longitude of the new station can be 

 computed and the position plotted by means of dms. and dps. (see 7411). Stations so located are 

 symbolized on the boat and smooth sheets by triangles in red ink (see 743) if they are of third-order 

 accuracy and by red ink circles, 3 mm in diameter, if of lower accuracy. 



(6) Planetable traverse, starting at a station located by the radial plot and closing at another 

 similarly located station; or single stadia rod readings from a station located b}^ the radial plot, in 

 which case a closed traverse is not required, but the stadia distance in meters and the azimuth line 

 shall be shown in ink on the boat sheet. All stations located by planetable traverse shall be symbolized 

 by red circles, 3 mm in diameter, on the boat and smooth sheets. 



(c) Graphic planetable triangulation, using stations located by the radial plot for control. The 

 lines of such triangulation must be kept as short as possible. Stations so located shall be symbolized 

 by red circles on the boat and smooth sheets as in (b). 



(d) Sextant cuts observed at stations located by the radial plot. There shall be at least three 

 cuts to each new station, two of which intersect at an angle of not less than 60°, in order to obtain a 

 strong intersection and a check of the position of each new station. The orientation station used for 

 each cut should be as distant as practicable, preferably twice as far away as the station being located. 

 Stations so located shall be symbolized by red circles on the boat and smooth sheets as in (6). 



(e) Sextant cuts observed simultaneously with three-point sextant fixes from stationary launches, 

 using stations located by the radial plot for control, the new station being determined by three or more 

 well-intersecting cuts. Stations so located shall be symbolized by circles, 3 mm in diameter, in blue 

 ink on the boat and smooth sheets. 



(/) Combination of methods such as one sextant cut for azimuth, and distance measured by tape, 

 stadia, etc. Stations so located shall be symbolized by blue circles on the boat and smooth sheets 

 as in (e). 



