570 



LAND SURVEYING. 



the quadrilateral) and the perpendiculars are measured. If the 

 field is conrtained under a greater number of sides than four, it 

 will be necessary to intersect it with two or more lines from each 

 of which perpendiculars must be erected to the angular points 

 along the boundary. Very frequently also, there will be what is 

 termed off sets, or the irregularities of a crooked boundary. In 

 this case it is necessary to take a straight line between two angles 

 of the field where these occur, and measure offsets or perpendi- 

 culars from that line towards every bend or angle in the bound- 

 ary, by which means a number of small triangles or trapezoids 

 are obtained, to be included in or deducted from the total area, 

 according as they are situated within or without the boundary 

 line. 



By the aid of a single diagram we shall endeavour to give a 

 few illustrative examples 

 of field-work, which will 

 remove all misapprehen- 

 sion and difficulty. 



Let it be required to 

 find the content of the 

 triangular field ABC. 

 The field measurements 

 will be the line A B 

 = 900 links, or rather 

 A F = 300, and F B 

 =600, and the perpen- 

 dicular F C = 480 ; 

 from these the subse- 

 quent operations of plot- 

 ting and estimating the 

 content can readily be 

 performed, as will pre- 

 sently be shown. 



We may here remark that the plan observed in keeping the 

 field-book is by no means uniform, nearly every surveyor having 

 his own peculiar method ; and so long as there is method, and the 

 memoranda be arranged in a way that the measurer can himself 

 fully comprehend them when he leaves the field, it is of small 

 consequence what plan may be adopted. Usually, however, the 

 page of the field-book is divided into three columns ; the centre 

 one is occupied by the measurements of the principal lines, and 

 in the right and left hand columns are entered the perpendicu- 

 lars and offsets from those lines. Let us suppose that it is re- 

 quired to find the measurement of a quadrilateral field A B C D. 

 The data to be obtained in the field are these ; the diagonal A B, 

 000 links, the perpendicular F C, 480, and the perpendicular 

 D E, 510. Again, we will suppose one side of the field has the 

 crooked and irregular boundary D d ef B ; in this case, in addi- 

 tion to those already mentioned, the field measurements will be 



