Plane Table and Forest Maps 449 



The plan of work should consider whether a considerable area 

 may be covered from one camp, or whether frequent changes of 

 camp will be desirable. A ride of six miles from camp and back 

 is ordinarily as far as can be made and leave time to accomplish 

 work. If there are good trails and not much climbing, rides up 

 to 10 miles may be made for a few stations if necessary to avoid 

 moving camp. Side camps for a few days will often make it 

 unnecessary to move main camp. Small crews are the best for 

 this work, a crew of three men being a very convenient size. One 

 man can work entirely with the large table, one with the tra- 

 verse board, and the other man set signals, move camp, etc. All 

 three men can work together on stadia traverse work, or the 

 third man may at times accompany the man using the telescopic 

 alidade, acting as recorder. The speed of the work depends so 

 much on the roughness of the country that an exact estimate is 

 difificult. In our case, for a complete map, the time over consid- 

 erable areas ran from one to three sections per day per man, 

 including both primary and secondary work. 



Plane Table Primary Control. — In making a forest map, the 

 plane table and telescopic alidade can safely be used for the pri- 

 mary work. In one case a triangulation system was extended 

 from a land office survey for 25 miles, and the location of the 

 signals checked by a relocation from a new base (corners on a 

 standard parallel). The greatest difference in the six points 

 located in common, on sheets platted to a scale of 2 inches equal 

 one mile, when one set was transposed over the other, was one 

 half chain for one of the signal points, and one and one half 

 chains for a point located by sighting on a sharp peak. In order 

 to carry accuracy to this extent, it is necessary to make very 

 careful location of the primary points and use signal poles 

 entirely. The greater accuracy obtained by the use of a transit 

 and a careful balancing of the angles in the triangles would not 

 justify the extra expense, since sufficient accuracy for all practi- 

 cal uses of the map may be obtained without this. As far as the 

 primary work is concerned, a station to every 5 square miles 

 would give excellent control. Generally, this number allows 

 the use of the quadrilateral system of composition of the tri- 

 angles giving the greatest possible strength to the locations. 



Secondary Control. — There are often points occupied in the 

 primary work which are difficult of access and which control 



