38 



A MANUAL FOR NORTHERN WOODSMEN 



Another way is to take the offsets at regular distances 

 along the base, 10 rods apart for instance. In that 

 case the rule for the area is : Add together all the in- 

 termediate offsets and half the end offsets, and multiply 

 the sum by the constant interval between them. 



By Cross Sectioning. The method of ruling off an area 

 on a map into squares of equal and known size is very 

 convenient, especially for irregular areas like bodies of 

 water. The whole squares can be counted up and the 

 fractions of squares estimated. In such cases it may be 

 best to do the ruling not on the map itself but on a de- 

 tached piece of tracing cloth or of paper. If the map is 

 opaque, the ruled tracing cloth may be laid over it and 

 held firmly till the work is done. If it is transparent, the 

 ruled sheet may be laid underneath. 



By Planimeter. The area of any surface may be 

 quickly and accurately ascertained by an instrument called 

 the planimeter. That instrument is not, however, in the 

 hands of most woodsmen. 



From Traverse. The area 

 enclosed by a balanced sur- 

 vey may be accurately com- 

 puted from the latitude and 

 departure of its courses. 

 The general scheme will be 

 grasped at once from the 

 figure, in which ABODE 

 represents the survey whose 

 notes are given on page 35, 

 e b is a meridian through its' 

 most westerly point, bB,cC, 

 d D, and e E are lines drawn 

 vertical to it from the angles, 

 and B m, D n, and E o are 

 parallel to it or vertical to c C 

 and d D. In this figure it is 

 evident in the first place that 

 the area of the figure b B C D E e minus the area of the 

 two triangles A E e and A B b equals the area of A B C D 

 E, and secondly that the figure b B C D E e is made up of 



