3337 HYDROGRAPHIC MANUAL Page 220 



object may be combined to advantage with two comparatively near objects to form a 

 very strong fix. For the control of offshore hydrography by three-point fixes, indefinite 

 objects are frequently the only ones available. 



a. Hilltops. — The tops of hills or mountains are often used as control for the 

 offshore hydrography. Definite and conspicuous points shall be selected and these 

 shall be accurately located so far as practicable. Round indefinite summits appear 

 smaller and sharper at increased distances. Angles should never be taken to round- 

 topped peaks from such short distances that there is doubt about seeing the summit. 

 At close range, nearer points, such as shoulders and the lower slopes of the summit, 

 are often mistaken for the actual summit, causmg errors in the positions. 



b. Tangents. — Regardless of the number of accurately located definite objects, it 

 is occasionally necessary to include in a fix an angle to the well-defined tangent of an 

 island, point, or other feature. Where the topography is accurate and the shoreline 

 steep, such features can generally be used with tolerable accuracy, if they are distant 

 as compared with the other objects. Tangents to sandy and low-lying points should 

 be avoided. 



Where the angle is measured to a tangent so distant that the juncture of the land 

 and the water is below the horizon and cannot be seen, an error may be introduced 

 owing to the fact that the observation is taken to a part of the feature some distance 

 above the water and consequently some horizontal distance from the true tangent at 

 the waterline. Even for offshore hydrography such a feature should be used only 

 where no better objects are available. If its use is unavoidable the elevation of the 

 feature above the true high-water line should be computed from its distance and the 

 height of eye above the water (see 272), the latter being recorded in the Sounding 

 Record. The observed angle should then be plotted on the tangent of the contour 

 that is probably on the observer's horizon. If the feature has not previously been 

 well contoured it may be necessary to contour it especially for this purpose in order 

 to obtain the desired degree of accuracy of position. If such a tangent needs to be 

 used repeatedly it should be scrutinized carefully to see if there is not some distinctive 

 natural feature at a sufficient elevation to be always visible, which may be specially 

 located for use as a control station. 



3337. Sextant Fixes on Buoys 



The positions of survey buoys cannot be determined as accurately as those of fixed 

 objects on shore, and the scope of the buoy around its anchor is an additional factor 

 of uncertainty in position at any particular observation. Because of this, propor- 

 tionately stronger fixes must be used for sextant observations on buoys or the plotted 

 positions may be considerably in error and difficulty will be experienced in running the 

 sounding lines. 



Considering a zone of hydrography to be controlled by three-point fixes on buoys 

 established in a line, the position of the sounding vessel near the line of buoys can be 

 determined more accurately where they are correct in azimuth with reference to one 

 another, irrespective of the accuracy of their distances apart ; and the positions far from 

 the line of buoys can be determined more accurately where the distances between the 

 buoys are correct, irrespective of the absolute accuracy of their azimuths with reference 

 to one another. If sounding lines are run normal to and through the line of buoys, 

 large jumps will occur when the vessel passes from one side of the line of buoys to the 

 other, unless the buoys are accurately located in azimuth with reference to one another. 

 If the distances between buoys are inaccurate the extreme outer end of a straight 



