152 MAPS AND THEIR MAKERS 



measurement of base lines by metal tapes, and for the reduction 

 of the standards of length employed in the field to the official 

 national standard. By the further careful comparison of 

 national standards such as the yard and the metre, it was 

 possible to determine, from the results of surveys in many 

 parts of the world, the figure of the earth with considerable 

 accuracy. The shape of the earth approximates to a spheroid, 

 flattened at the Poles; the International Union of Geodesy in 

 1924 adopted a figure for the major semi-axis of 6,378,388 

 metres and of 1 in 297 for the flattening. Correction for the 

 figure of the earth is of course essential in the computation of 

 the triangles. 



It should be noted that maps are drawn as though pro- 

 jected on to the plane of the sea-level surface, so that distances 

 measured on them are independent of irregularities in the 

 relief. 



The stages in a systematic topographical survey, before the 

 introduction of aerial photography, may be briefly summarized 

 as: (i) determination of mean sea level at one point at least, to 

 which all altitudes are referred; (ii) a preliminary plane table 

 reconnaissance to select suitable points for the triangulation, 

 and the erection of beacons over them; (iii) determination of 

 initial latitude, longitude and azimuth (for direction), which 

 will 'tie' the map to the earth's surface; (iv) careful measure- 

 ment of the base or bases with a tape or wire of a special alloy; 

 (v) triangulation, the theodolite being used to observe hori- 

 zontal angles from the base and beaconed points, and to 

 measure altitudes by readings of vertical angles ; (vi) calculation 

 of the triangulation and heights, and the transference of the 

 trig, points to the sheets issued to the plane tablers; (vii) the 

 filling-in on the sheets by the plane tablers of the required 

 topographical detail — contour lines, rivers, woods, settle- 

 ments, routes, and names. 



In the present century, the measurement of long meridian 

 arcs has tended to go out of fashion, partly because they of 

 necessity take no account of local topographical features and 

 consequently may not be particularly useful for local surveys. 

 In areas of excessive local gravity anomalies, they do not 

 furnish the expected data for determining the figure of the 



