430 HOW MAPS ARE MADE. 



Foyle, wliicli was 7 miles long, was measured with specially designed 

 compound metal rods of brass and iron, 10 feet long, compensating like 

 the balance and spring of a clironometer, so as to be independent of 

 expansion and contraction, and their contact a<ljusted with microscopes. 

 From this base once fixed, its latitude and longitude being most care- 

 fully taken, the surveyor measures the angles of suitably laid out tri- 

 angles, ami computes the length of their sides. p]ach of these sides 

 in its turn becomes the base of a new triangle. The surveyor plants 

 his instrument on the spot fixed on and measures new triaugles, and 

 gradually covers the surface of his island with a net^^ork of great 

 triangles. The length of these sides are all calculated from the angles 

 not measured, but, as a matter of fact, the lengths of these sides so 

 computed from angular measurements are infinitely more accurate than 

 if they were actually measured with a chain. 



So accurate, indeed, was thetriangulation of this country that when 

 the ordnance surveys verified their calculations thirty-three years after, 

 in 1827, by actually measuring the check base on Lough Foyle, as already 

 described, the greatest possible error was found to be less than 5 

 inches. This, be it remembered, was calculated from the base in Salis- 

 bury Plain, on\y 7 miles long, at a distance of over 300 miles. The 

 mean length of the sides of the triangles was 35 miles, and the longest 

 side was 111 miles. The history of the triangulation is quite a romance, 

 but Sir Charles Wilson referred to all this at length last month.* 



The instrument with which the angles are measured is the theodolite. 

 This network of triangles so laid down is the backbone of all details of 

 map-making. All these imaginary sides of triangles are, like the par- 

 allels of latitudes and meridians on large maps, the lines to which all 

 filling in of detail is referred. Every point on this network is abso- 

 lutely fixed, and from these points, as from the line of lamp-posts we 

 considered at the beginning, all details are measured. The great trian- 

 gulation in the Ordnance Survey being complete, the officers then lay 

 off from the great triangles what are called secondary triangles, the 

 sides of which are about 5 miles in length, and where necessary, ter- 

 tiary triangles, with sides of about 1 inile in length, and from them the 

 surveyor breaks up the interior of the triangle with a network of cross 

 lines, all self-checking when laid on the paper, and this is the begin- 

 ning of ordinary land survey. 



Land siirrei/inf/. — The filling in of a survey is like writing a book. 

 Men work differently. No two surveyors use exactly the same method 

 of working, and it very much depends on the nature of the ground, the 

 extent of his resources, and the accuracy of detail required what method 

 the surveyor employs. In a theoretically jierfect survey the triangular 

 system would be pursued throughout, but in practice this is not neces- 

 sary, nor is it done. 



*The Scottish Geographical Magazine, vol. vii, p. 248. An arlrairable popular 

 account of the operations of the Ordnance Survey will be found in The Ordnance 

 Survey of the United Kingdom, l>y Lieut. C:ol. T. P. White, R. E. (Blackwood, 1886.) 



