886 PROCEEDINGS OF SECTION J. 



of buildings and permanently luavked points on the ground, 

 are determined by intersecting observations from the triangu- 

 lation ; these intersections, being chosen in convenient positions 

 over the whole area to be surveyed, afford means for main- 

 taining azimuth and reducing the whole survey to the same 

 unit of length as that of the trigonometrical survey. 



The values of all the points of the triangulation, both 

 observing stations and intersections, are carefully computed 

 to one common origin or zero, and the value of each station 

 subsequently fixed in the detail survey being connected with 

 the trigonometrical survey is also computed to the same origin ; 

 consequently as the coordinates of every single detail or 

 survey mark, angle of building or other prominent landmark 

 are referable to one zero, the bearing and distance between 

 any two points of the survey, however far apart, can be 

 readily computed in but a line or two of figures. 



In order to ensure against the accumulation of instrumental 

 or chainage errors which might result from one surveyor's 

 work joining on to anothers without their being confined to a 

 certain limit of error, it is necessary to run a carefully carried 

 out standard or base line survey on which the detail survey 

 proper is dependent. This is generally conducted by the 

 supervising surveyor, as on the care and accuracy of this 

 particular part of the work the correctness of the whole 

 survey chiefly depends. 



The standard survey is run as a sort of network over the 

 whole area to be detailed, and is connected wherever possible 

 with the points already fixed by the trigonometrical survey. 

 The lines should be, as far as possible, along main streets, the 

 stations being fixed at crossings or other points convenient for 

 starting fresh traverses from. The surrounds should enclose 

 areas that can be conveniently divided into from three to six 

 sheets or blocks of survey, as it is quite unnecessary to run 

 the standard traverse along every street. 



In the instrumental work a six-inch transit theodolite should 

 be used, both verniers being read, and the angles repeated not 

 less than four times, angles being also taken to one or more 

 prominent landmarks, such as church steeples, in order that 

 each station may have a reference bearing from it. The 

 azimuth adopted throughout must, of course, be identical with 

 that of the triangulation, A convenient and rapid way of 

 reading the angles is to carefully set the theodolite to the 

 back bearing of the last line, then read the bearings fi-om left 

 to right of each of the lines requii'ed, leaving- the instrument 



