290 IMPROVING THE ACCURACY OF TIMBER ESTIMATES 



Fig. 60. — Polar planimeter. 



length of strip by its width. The most convenient units are rods, 

 since 160 square rods equals 1 acre, or chains, 10 square chains to 1 

 acre. Distance in chains on strip required for 1 acre may be computed 

 for each width of strip and the area of the strip obtained by dividing 

 its length by this factor. 



226. Method of Separating Areas of Different Types. To determine 



the total area of 

 the type accurately 

 from a map, a -plan- 

 imeter may be used. 

 By the use of 

 this instrument a 

 direct reading on 

 the map is obtained 

 in square inches 

 of the area whose 

 boundary is traced 

 by the needle, moving clockwise. The stationary pin is placed outside 

 of the area to be traced. When placed within the area so that the 

 movable pin finally encircles the pivot before returning to its point of 

 origin, a deduction or correction must be made in the indicated area, the 

 size of which depends upon the make of instrument used. 



The equivalent in acres for square inches, as determined by scale 

 of the map, gives the acreage. Lacking a planimeter, the area of types 

 can be computed by the method of approximation through triangles 

 or the sum of small squares. For the latter purpose a map should be 

 plotted on fine cross-section paper. 

 The area of these types is required 

 only to a reasonable degree of 

 accuracy since the determination 

 of their field boundaries is a 

 matter of inspection and sketching 

 and the total area of the tract is 

 not involved. 



As an illustration of the effect of 

 using type areas in estimating, the follow- 

 ing example may be cited: Area of 

 tract, 200 acres, divided into two types 

 containing 100 acres each. The stand 

 on the first type is 30,000 board feet per 

 acre, and on the second 10,000 board feet. 

 The total stand is therefore 4 million 



board feet. Twenty-five per cent of this area or 50 acres is to be covered by 

 strips. The result of the cruise is shown in Fig. 61. 



Fiq. 61. — Relation of areas of types to 

 strips in timber estimating. 



