180 



(ii) To Find the Area of a Given Plot 

 OR Field. 

 This is simple. Fig. 3 gives the idea : — 

 Run a line A B diagonally across the field 

 and on this line measure two perpendicu- 

 lars, found by means of the cross-staff, to 

 the other corners of the field C and D. 

 Sufficient data has then been obtained. 

 A B multiplied by C E divided by 2 gives 

 us the area of that side of the field, and 

 A B X F D divided by 2 gives us the area 

 of the other side. Note that when the 

 field is bounded by irregular fences the 

 lines must be laid down on a give and take 

 principle. The result, of course, in this 

 case is not so accurate as in example No. 1 

 but it is sufficiently accurate when only an 



to A D. Suppose the fence A D to be 5 

 chains long. 



Divide the required area by A D and wo 

 get the length of the sides to be measured 

 along A B and D C. 



5 chains = 500 links. 



2 acres = 200,000 square links. 



Then 200,000 -r- 500 = 400 links, i.e., 4 

 chains. Measure off along A B and D C 

 4 chains and put in pegs. A fence erected 

 between these pegs will cut off an area 

 equal to 2 acres as required. 



Example II. (Fig. 4). 

 When the plot of land has irregular 

 boundaries the procedure is somewhat 

 similar, but first of all the irregular boun- 



p » 



3 



3 



4 



I 1 — f 



Fig. 4. 



■pta 



b 



approximate idea . of the acreage is re- 

 quired. 



(iii) To Sub-divide Areas into any 

 Required Size. 



Before actually setting out the work in 

 the field and measuring the distances for 

 the purpose of sub-dividing areas, laying 

 out plots ,etc., a tracing of the area to 

 be divided should be taken from the 

 ordnance map. On this tracing the sub- 

 division should be worked out, and the 

 actual measurements in the field made 

 from it. 



The easiest case of sub-division is that 

 of a rectangular plot. 



Example I. (Fig. 2). 



A B C D is the given plot : it is required 

 to cut off 2 acres by a line running parallel 



daries must be straightened. If we 

 take an example where one of the sides of 

 the field is crooked and the other two are 

 straight and parallel, it will serve to show 

 the method to be adopted. 

 A B C D is the given field : it is required 

 to cut off a plot equal to 2 acres by a line 

 running from the side A D to the side B C. 

 On the tracing draw a line joining A B, 

 and work out the area of the piece of land 

 thus cut off between it and the curved 

 boundary The piece marked No. 1 is 

 worked out according to the rule for tri- 

 angles and the pieces 2, 3, and 4 according 

 to the rule for trapezoids. Suppose this 

 area already cut off equals 10,000 square 

 links, then the area which remains still 

 to be cut off to complete the 2 acres is 

 200,000 less 10,000— that is, 190,000 square 

 links. 



