Page 281 hydrography 363 



If it becomes practicable to have air photographs taken especially to supplement 

 the hydrography, the following conditions are the most favorable: sun behind the 

 camera at an elevation of 40° to 60°, clear sky, sea calm or rough enough to cause 

 breakers, and low tide. 



363. Bare Rocks and Rocks Awash 



Each isolated bare rock and rock awash within the project area, must be located 

 and its height determined by the hydrographer, unless this has been done by the topog- 

 rapher, in which case the hydrographer must verify the data. (See also 782.) The 

 important rocks of a group or rocky area should likewise be located and determined in 

 elevation. 



Where it is practicable to land on such features the location should be determined 

 by a strong three-point fix taken at the rock. Otherwise the rock may be cut in from 

 successive positions of the boat, selected so as to give strong intersections at the rock, 

 or may be located by sextant positions taken from the boat wlien in range with the rock 

 and control stations bearing in several directions. 



The height of each rock above the water must be determined as accurately as 

 practicable and the time noted and entered in the Sounding Record, together with the 

 height, for use in reducing the elevation to the plane of reference. If a landing can be 

 made and the sea is calm, the height of a bare rock can be best measured on a staff 

 whose lower end is held at the water's edge, the height being noted on the staff by lining 

 it in with the horizon with the eye at the top of the rock. If it is not practicable 

 to land on the rock the height should be estimated as accurately as possible from a 

 position nearby. 



The data relative to bare rocks and rocks awash originating with the hydrographic 

 survey and the topographic survey must be in agreement or be supplemental; any 

 discrepancies between the two surveys must be reconciled before the party leaves the 

 working ground at the end of the season. (See also 3244, 381, and 753.) 



Where a rock that has been adequately located by the topographer is passed on 

 a sounding line, a note of this fact shall be entered in the "Remarks" column of the 

 Sounding Record with an estimated distance when the rock is abeam. It should be 

 made clear that these data are not to be used to locate the rock but merely as verifica- 

 tion of its existence. Where such a feature is passed close-to without a reference to it, 

 doubt may arise in some circumstances as to whether it actually exists. (See 3353.) 



If air photographs of the area are available they should be carefully examined for 

 evidence of bare rocks and rocks awash and compared with the hydrographic and 

 topographic survey sheets. (See 3624.) 



If bare rocks or rocks awash shown on prior surveys or on published charts are 

 found to be nonexistent, or in different locations, or with different elevations, a full 

 explanation shall be included in the Descriptive Report, with a recommendation as to 

 the charting procedure to be followed. 



364. Sunken Rocks and Breakers 



Where the existence of a sunken rock or other danger is evidenced by breakers and 

 it is impracticable to locate it by a three-point fix or to obtain a sounding on the spot, 

 it must be located by cuts from nearby positions of the sounding boat chosen to form 

 a good intersection at the object, and the depth must be estimated, A statement 

 should be made in the Sounding Record as to the probable accuracy of the estimated 

 depth, together with the time of the observation. 



465382—44^—20 



