NATIONAL SURVEYS AND MODERN ATLASES 153 



earth. On the whole, methods of survey have become much 

 more flexible, and are adapted to local conditions. It is found 

 that *'it is better to be content with small triangles easily 

 accessible than to make enormous efforts at rays longer than 

 nature easily allows". With improved equipment, the necessary 

 base lines can be measured rapidly and accurately. 



Instrumental development has also given the surveyor 

 greater freedom. In place of the old cumbrous theodolites, 

 light and accurate instruments have been evolved, such as the 

 3J-inch 'Tavistock' theodolite, which allows the mean of read- 

 ings on each side of the glass circle to be read directly through 

 the same microscope to one second of arc. The invention of 

 wireless has also simplified the troublesome problem of deter- 

 mining longitudes. It is relatively simple to receive Greenwich 

 mean time by time signal and to compare it with local time. 

 The development which has perhaps attracted most public 

 attention has been that of air survey, though often its merits 

 have been exaggerated. As long ago as 1858, the value of 

 vertical air photographs, taken from balloons, was appreciated, 

 but there were obvious difficulties in obtaining them. A com- 

 bination of the camera and theodolite was later successfully 

 used in ground survey, particularly in Canada. Experience of 

 air photography gained in the first world war gave a consider- 

 able impetus to research into air survey, and by the end of the 

 last war, owing to the demand for the rapid mapping of terri- 

 tories inaccessible to land surveyors, standard methods had 

 been evolved. 



The problems of air survey are concerned with (1) obtaining 

 suitable photographs, (2) providing the necessary ground 

 control for the framework of the map, and (3) filling in the 

 detail from the photographs. First, the area to be mapped 

 must be covered by overlapping strips of photographs, taken 

 at a constant altitude and in favourable conditions. In England 

 there are on the average only thirty days a year suitable for air 

 photography. These photographs are examined stereoscopically 

 in pairs. Vertical, or nearly vertical, photographs simplify the 

 later stages of the work. To produce a map on a relatively small 

 scale and covering an area with slight surface variations, the 

 centres of photographs with a tilt of not more than 2° can be 



