358 fry. SEDIMENTATION IN RESERVOIRS [Ch. 20 



is a plane table to locate the soundings by cutting in horizontal angles. 

 The plane-table point is located as far from the end of the range as 

 the range is wide, or perhaps on an island in the lake if one is avail- 

 able. Plane-table points for a day's sounding are always chosen 

 before the day's work is begun. For this type of work, the Walkie- 

 Talkie radio instrument is invaluable to increase efficiency of the 

 party and speed up the work. Both men on the bank are equipped 

 with Walkie-Talkies, and a third instrument is used on the boat. As 

 the distance wheel is not used on long ranges, the wheel operator 

 becomes the radioman on the boat. To obtain best radio conditions, 

 he takes a position on the bow of the boat removed from the recorder 

 mechanism and motors. The transitman uses his Walkie-Talkie to 

 keep the party on range. The plane-table man follows the sounding 

 boat through the alidade and cuts in position at each sounding by 

 horizontal angle on the plane-table sheet whenever the sounding oper- 

 ator indicates a fix by waving a flag. Flags are alternated, four white 

 flags and one red being used to provide a check for the plane-table 

 man in observing positions and plotting them on his sheet. After 

 the boat has completed its run to the opposite bank, contact with the 

 plane-table man by Walkie-Talkie makes certain that he has ob- 

 tained all the necessary data. The plane-table man in this case has 

 a separate small boat and proceeds independently to the plane-table 

 point from which he will cut in sounding positions for the next range. 

 Meanwhile the main party also proceeds to that range. Contact is 

 again established on the Walkie-Talkie before the new range is 

 started. It is possible to carry on work in this way, perhaps for a 

 whole day, without the main sounding party even catching more than 

 a glimpse of the plane-table man. 



Surveys op Deltas or Other Areas 



In detailed surveys where it is desired to measure the volume of 

 material brought in by tributaries and deposited as a delta, or to 

 determine accurate bottom contours in a zone several hundred feet 

 in radius, the radial-sounding method is highly efficient. An expert 

 boatman is essential for this type of work, as the boat must be held 

 closely in position and must be handled skillfully in order not to break 

 the distance wire. A pivot point is located on the bank adjacent to 

 the area to be surveyed, and a plane table is set up over this point. 

 The distance wheel is initially set at a fixed distance of 25 feet from 

 the pivot point. The pivot point is placed high enough above the 

 water to keep the distance wire out of the water, and, for distances 



