Page 225 hydrography 3345 



control points; this method of direct measurement cannot be used satisfactorily when 

 proceeding toward the control point. 



3345. Use of Portable Range Marks 



Where it is impracticable to set ranges on shore for use in running the sounding 

 lines, use can often be made of a small skiff manned by one or two men, on which a 

 flag is erected. The desired spacing of lines is selected and an anchor line marked 

 accordingly. The skiff is anchored to one side of one end of the area to be surveyed. 

 By means of oars the skiff is propelled in a direction normal to the desired system of 

 lines and stopped at each mark on the anchor line. It is held in place normal to the 

 direction of the sounding lines by an occasional djp of the oars while the line is being 

 run. For each successive sounding line the anchor line is slacked away permitting the 

 skiff to move, normal to the direction of the sounding lines, to the next position marked 

 on the anchor line where it is again held in place to serve as the range mark for the next 

 line, and so on until all the desired lines have been run. 



The flag in the skiff serves as the front raiige mark and an extremely distant object 

 may be selected to serve as the rear range mark. If sufficiently distant the same object 

 may serve as a rear range mark for the entire area or different points near it may be 

 selected by estimation. 



In using this method the ranges serve only to control the direction of the lines and 

 keep them straight and do not otherwise serve as part of the control, the positions of 

 the boat being entirely fixed either by directions from shore stations or by the usual 

 sextant three-point fix method. 



335. Positions by Estimation 



It is often impracticable to provide a sufficient number of control stations so that 

 every position of the inshore hydrography can be fixed by a three-point fix. Positions 

 near the beach on the inshore ends of lines, in small bights, and up narrow winding 

 creeks and sloughs often cannot be fixed by sextant angles. It is customary in such 

 cases, and the necessity is recognized, to estimate a smaU percentage of the positions. 

 This is the most difficult part of position determination in inshore hydrography. 



The estimation of distances. at sea is made more difficult by the fact that there are 

 no objects of known size by which to make comparisons. 'Long experience will be of 

 assistance, but the experience of a topographer in estimating distances along the shore 

 does not necessarily make him expert at estimating them from a launch or vessel at sea. 

 In the estimation of short distances the height of eye plays an important part, and one 

 who is accustomed to estimating such distances from the bridge of a large vessel at a 

 considerable height above the water invariably overestimates the same distance from 

 a much lower height of eye in a launch. The general tendency in estimating distances 

 from a moving vessel is to underestimate distances to objects ahead and overestimate 

 distances to objects abeam. 



3351. At Inshore Ends of Lines 



Where it is impracticable to fix the inshore end of a sounding line with a three-point 

 fix when approaching the shore, the hydrographer should obtain a good three-point fix 

 as near to the end as practicable and maintain the same course and speed to the end 

 of the line, if this can be done without endangering the boat and its personnel. If it 

 is necessary to slow down on approaching the shore, a fixed or estimated position should 



