Page 97 



CONTROL AND SIGNAL BUILDING 



2382 



A sketch of the sextometer rod is shown in figure 13. The rod is 15K feet in length, 

 with diamond-shaped targets at both ends, whose centers are exactly 15 feet apart. 

 To assist in holding the rod perpendicular to the line of sight from the topographer, 

 a projection of light wood about 2 feet long is constructed perpendicular to the rod at 

 its center. Sights are put on this so that the rodman can assure himself of the per- 

 pendicularity of the rod by holding it horizontally at eye level and sighting at the 

 topographer. 



The method of use is as follows: The topographer in a launch or boat plots his traverse on the 

 planetable board held on his lap. Beginning at a station of known position, a rodman is sent ahead in 

 a skiff to mark the first station. The topographer measures the orientation angle from some distant 

 signal to the station ahead, and then measures the distance angle between the targets on the 15-foot 

 pole. When its size permits, the distance angle is read on and off the arc and the two values are aver- 

 aged to eliminate index correction. The orientation angle is laid off on the planetable sheet by means 

 of a metal three-arm protractor and the position of the station ahead is pricked on this line at the 



A 



A\ 



I II x 



\/ 



\/ 



-REAR SIGHT 



-FRONT S)GHT 



SCALE IN FEET 



Figure 13.— Sextometer rod. 



distance obtained by the hypsograph. All orientation and distance angles shall be recorded in order 

 that the graphic traverse may be checked at a future convenient time. 



By locating the stations on alternate sides of the waterway, and keeping the dis- 

 tances between stations less than 400 meters, an accuracy closely approaching that 

 obtainable by planetable can be obtained for short distances not exceeding a mile or two. 

 A check should be obtained, where possible, by tying in the end of the traverse to 

 established control stations. Hydrographic signals can be located along the course of 

 the river or stream as desired. 



The horizontal angles subtended by the rod may be converted into distances very simply by the use of a hypsograph, or they 

 may be computed from the formula: 



L e 



D'=-cot- 



In which D' = the computed distance in meters; i=the fixed length in meters between the two targets on the pole; and e = the observed 

 sextant angle. 



A small correction, always subtract ive for small angles, must be applied to the computed sextometer distances owing to the fact 

 that the vertex of the small angle measured by the sextant is actually back of the eyepiece of the telescope. The smaller the angle, 

 the larger is the correction. This correction is dependent on the constants of the sextant used and is computed from the formula: 



a sin (8+0) 



Ad= 



--b 



sin Q 



in which Ad=the correction in meters; a=the distance in meters between the reflecting surfaces of the two sextant mirrors; 6=the 

 distance in meters between the horizon mirror and the eyepiece of the telescope; /3=the fixed angle between the line of sight through 

 the telescope and the line joining the two mirrors; and e=the observed sextant angle. 



For a standard Coast and Geodetic Survey navigating sextant, a=0.11 meters; 6=0.20 meters; and /3=30°. Table 4 gives the 

 computed corrections for such a sextant for values of 8 from 0° to 10°. From these the intermediate corrections may be interpolated 

 with suflicient accuracy. The table also gives the .corresponding hypsograph sextometer distances and the corrected distances for a 

 target interval of 15 feet. It is to be noted that the correction is very nearly proportional to the distance, being m this case approxi- 

 mately 1.2 meters per 100 meters of distance. For distances between targets other than 15 feet, a new table of hypsograph and 

 corrected distances must be prepared. 



