SURVEYING 



821 



determine the length of the perpendicular line AD, 

 and this is usually done (when possible) on the 

 A ground by means of a 



simple instrument called 

 a cross, which consists of 

 two sights or fine grooves 

 at right angles to each 

 other ; which being placed 

 on the line BC (keeping 

 -- c B and C visible in one of 

 the sights), nearly opposite 



the angle A, is moved gradually till the angle 

 A is intersected by the other sight. The line 

 AD can also be laid down on the drawing, and 

 its length found by scale and afterwards verified 

 on the ground, or it may be at once laid down on 



PAOE 2. 



Prom A 



The line ab may be similarly booked and platted. 



the ground by the use of the chain alone. An 

 improved reflecting instrument, called an optical 

 square, ia also often used for this purpose. Any 

 boundaries along the lines or sides of the triangle, 

 ABC, can be determined by the use of offsets or 

 insets, as they occur on riglit or left of line. No 



matter what the form of the surface to be surveyed 

 may be polygon, trapezium, or trapezoid it may 

 thus be determined by a judicious subdivision into 

 triangles ; and when the survey is not of a very 

 extended nature or character, and when no serious 

 obstructions exist, chain surveying is both accurate 

 and expeditious, especially if proof or tie lines are 

 properly introduced for the purpose of testing the 

 accuracy of the work. 



In every description of surveying it is best to 

 make the original triangle as large as possible, and 

 to work from a whole downwards rather than 

 build up a large triangle by the addition of several 

 small ones. It would be impossible here to lay 

 down rules to meet the many difficulties which 

 arise in the practice of surveying ; indeed the best 

 test of a good surveyor is the ease with 

 which he will overcome local obstructions 

 which appear almost insurmountable to a 

 novice, or even to a theoretical surveyor 

 with little field practice. Where buildings 

 or other impediments are found in the 

 measurement of a straight line, they are 

 generally passed by the erection of short 

 perpendiculars sufficient to clear the ob- 

 stacles, and a line parallel to the original 

 measured as far as they exist, when the 

 original line can be again resumed. Differ- 

 ences of level occurring in measuring a line 

 where no instruments are used are gener- 

 ally compensated or allowed for by the judg- 

 ment of the surveyor. 



In registering the dimensions taken on 

 the ground, such as sides of triangles, off- 

 sets, intersections of roads, fences, &c., 

 and everything necessary to make a perfect 

 delineation or plan of the surface, sur- 

 veyors use what is called a field-book, the 

 mode of keeping which varies very much 

 with individual practice. Some surveyors 

 use hand sketches or rough outlines of the 

 form of the ground, and mark the dimen- 

 sions on them, while others use the ordi- 

 nary form of field-book, or a combination 

 of the two methods, which perhaps is the 

 Stream, best when any difficult complications 

 happen on the ground, such as the frequent 

 occurrence of buildings, enclosures, water, 

 &c. along the line. In the ordinary field- 

 book the centre column, commencing from 

 the bottom, represents the length of any 

 line or side of a triangle, and the figures 

 in the column the distance at which the 

 offsets to the riglit or left are taken, or 

 where roads, streams, fences, &c. cross the 

 line, or buildings adjoin the same. We 

 give herewith the field-book of the assumed 

 survey of the triangle ABC, with the differ- 

 ent offsets and insets on its sides, and 

 where roads, fences, streams, &c. cross 

 them, the detail of which can be obtained 

 by subdividing the triangle into smaller 

 internal ones. The figure can 'thus be laid 

 down from the book, and its area calcu- 

 lated by the formula AB * CD ' and the off- 

 sets and insets calculated, and added or 

 deducted. 



Ponds, plantations, and enclosures of dif- 

 go to A. ferent kinds may be surveyed with a chain, 

 especially if their form be such that they 

 can be conveniently included in the area of a 

 triangle, the correctness of which being proved by 

 proper tie-lines, the form, area, &c. may be ascer- 

 tained by offsets, or rather insets from the sides. 



See ORDNANCE SURVEY, LEVELLING, MENSURATION, 

 THEODOLITE, CONTOUR, Ac. ; and for the United States 

 Coast Survey, see CHART. 



Road. 



t0. 



Fence. 

 Stream. 



