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 \nterval 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 *)f 
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 Plammeter. The area of any surface may be 
quickly and accurately ascertained by an instrument called 
She plammeter. 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,c C, 
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 
