50 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 



ELEVATIONS FROM VERTICAL ANGLES. 



When the distance to a mountain. or other object is 

 known its elevation above the surveyor may be deter- 

 mined. A vertical angle is measured with a clinometer 

 or clinometer-compass, and the difference in elevation 

 can be determined from the table. Information of this 

 character assists greatly in the preparation of a map, 

 and this method should be used when a peak is inacces- 

 sible or not likely to be occupied during the present sur- 

 vey. If both the distance and elevation of a peak are 

 known, and the surveyor desires the elevation of the 

 station which he is then occupying, this process is 

 easily reversed. The table is prepared to miles of dis- 

 tance, and if intermediate fractional miles are needed 

 the ratio may be interpolated. 



The method of determining the distance of a peak or 

 other salient topographic point is illustrated in the 

 various plane-table methods. If compass sights are 

 taken from two or more known points the intersections 

 may be platted with a protractor or computed. 1 



i The following is the method of computing the sides of a triangle when two angles 

 and one side are known: The angle opposite the known side is equal to 180 minus 

 the sum of the two known angles. The sine of an angle is the same as its departure 

 (in the traverse table) for distance 1. A and B represent the two known angles 

 and their distance apart; C is the opposite angle: 



Distance ^^Xsine of angle B 



Sine of angle C 

 Or: 



Distance ^.BXsine of angle A 



Sine of angle C distance 1K7 



The traverse table, distance 1, being the same as a table of natural cosines and sines, 

 may be used to change a slope measurement to a horizontal measurement, and also 

 get the difference in elevation. Thus a distance of 10.00 chains up or down a 7 slope 

 would represent 9.92 chains on the level, and 1.22 chains rise or fall. The same method 

 is used in reducing stadia measurements. 



