179 



It is obvious, then, that if such an instru- 

 ment be sighted along one saw cut exactly 

 in line with two poles, by looking along 

 the other saw cut a perpendicular may be 

 set off or obtained. 



6. An ordinary compass for obtaining 

 the aspect. 



7. A bundle of laths for lining out. 



With this equipment we are in a posi- 

 tion to consider the three propositions 

 which are most likely to present them- 

 selves. They are as follows : — 



(i) To make a plan of a small area 



to scale, 

 (ii) To find the area of a given plot or 



field, 

 (iii) To subdivide areas into any re- 

 quired size. 



(i) To Make a Plan of a Small Area to 

 Scale. 

 Before starting be sure to see that the 

 chain in use is correct. If the measure- 

 ments be made with a stretched chain the 



Fig. 1 gives an idea of the method of 

 procedure : — ■ 



indicates the position of the ash poles and 

 the chain lines are measured successively 

 1, 2, 3, etc., as indicated in the sketch. 

 Offsets are measured from the chain lines 

 to the boundary where shown. An extra 

 line, called the check line, is measured, 

 which, when the plan has been drawn on 

 paper, should exactly fit in, and thus prove 

 the accuracy of the work. 



A specimen of entry into the Field or 

 Note-book is as follows : — 



Offsets from chain 

 line to boundary 

 where required by 

 its irregularity. 



Fig. 3, 



result is worthless unless the area be 

 mathematically adjusted afterwards — a 

 contingency better avoided. 



Next bear in mind that the basis of all 

 measurement of area is the triangle, 

 because it is the only geometrical figure 

 which cannot change its shape without at 

 the same time altering the length of its 

 sides. Consequently, if we ascertain the 

 length of the sides by measurement, its 

 shape and size are determinate. 



The figures in blacker type represent 

 the position of the poles or stations as 

 they are called. 



An exact plan can be drawn to scale 

 from such field work. The correct area 

 may be obtained with the scale, by 

 first calculating the areas of the two 

 main triangles and then adding the 

 offsetted areas which are made into 

 trapezoids and triangles and calculated 

 accordingly. 



m2 



