450 Forestry Quarterly 



chiefly very rough country. Much more time is taken in going 

 to and coming from such a point than in the work when it is 

 reached. The detail or secondary work was done in such cases 

 in conjunction with the primary control, saving a second trip 

 to such stations. Owing to the roughness of the country, it 

 was unnecessary to locate as many points as where the cover 

 types were of greater importance; more could also be sketched 

 because of the wider view. Such a procedure balanced the work 

 very well, as otherwise the secondary control would have fallen 

 behind the primary. The primary control points for the country 

 not completed in this manner were transferred to sheets 15 inches 

 square, on which the details were located by use of the traverse 

 board and small alidade. 



One great advantage of having a good number of signals lo- 

 cated is that it makes it possible to do all the secondary work 

 without running a traverse. The three-point problem can be 

 used wherever it is necessary to occupy a station not previously 

 located. The secondary points can be so selected that they will 

 serve both to locate contours and to control the sketching of 

 cover types. Where much of this work is to be done, it would 

 be well worth while to try the Baldwin Solar Chart^ in orienting 

 the table. It is exceptional to find country in the mountains where 

 the magnetic needle can be depended upon for orienting the 

 table. The plane table can be used as a range finder to advan- 

 tage. (Wilson's "Topographic Surveying," p. 291.) 



In mountainous country, and for as long sights as are used 

 in this work, the small boxwood scale alidades are not satis- 

 factory. The brass alidade with 7" blade and sights 6" high 

 is fairly satisfactory, but one having a 10" blade would be much 

 better. 



Primary Vertical Control. — The primary vertical control can 

 be carried with very satisfactory accuracy by means of vertical 

 angles. It is impossible to keep the table level enough for the 

 vernier to read on zero, so two readings must be made, one sight- 

 ing on the signal and the other with the tube level. Before 

 any important angles are read, a test should be made by reading 

 the telescope with the level direct and reversed, the telescope 

 being both upright and turned 180° in the collar. A correction 

 can then be applied to the level reading if necessary. Since 



* See "Topographic Instruction of the United States Geological Survey, 1913." 



