Plane Table and Forest Maps 453 



If a needle of the same size is used each time and set in the 

 same position, the result is the same as though the blade were 

 made wider by an amount equal to the radius of the needle. In 

 orienting the table and using two points, the needles should be 

 of the same size. If a change is made in the needles, the orienta- 

 tion of the board should be checked. For the most important 

 control points at least, the attempt should be made to bring the 

 accuracy of platting as nearly as possible up to that of the instru- 

 ment. The circular level on the blade is especially handy, as the 

 leveling can be done very quickly, and being always in place, any 

 change from level is quickly detected. 



Double mounted paper is desirable for use on the primary work 

 with the large table. The single mounted paper is satisfactory 

 for secondary work, and the use of the double mounted is never 

 justifiable on the traverse board. Celluloid sheets are desirable 

 for secondary work in wet weather. It is very trying for the 

 eyes to work over the white paper in strong sunlight, and for 

 this reason it is very necessary that the plane table sheets be 

 tinted. If tinted sheets cannot be obtained, tinted glasses are 

 necessary, but are not satisfactory, for if the tint is heavy enough 

 to give good protection, it lessens the clearness of vision through 

 the telescope when taking long, difficult sights. A shade is used 

 sometimes for the table, but is impractical for this work, both 

 on account of bulk in packing and the prevailing high winds on 

 the triangulation points. 



A satisfactory method of marking the stations in the field has 

 not been devised. There is little time to spend in making chisel 

 marks on the rocks, and these are difficult to find on a mountain 

 top which is a mass of loose rocks or ledges. It is also very 

 impractical to attempt to carry wooden posts and mark them. 

 The points established in the past have been monumented by a 

 substantial pile of rocks with a Forest Service blaze on any 

 trees near. A description of these points is kept as a record and 

 notes made on the easiest way to reach them. A practical method 

 of marking these triangulation stations and bench marks might 

 be to scratch the station number on one of the metal Forest 

 Service cruising posters which would be buried in the monument 

 of rocks. There might be two posters left, to guard against 

 one being lost. Where it was possible to calculate the elevation 



