NATIONAL SURVEYS AND iMODERN ATLASES 163 



The topographical maps issued to the public are on three 

 standard scales, according to the importance of the particular 

 area; 1:31,680; 1:62,500; and 1:125,000. The sheet lines are 

 based on quadrangles formed by parallels of latitude and 

 meridians of longitude, the sheets of the 1 : 62,500 series covering 

 15' of latitude and longitude. At this scale the contour interval 

 ranges from 10 to 50 feet. Every fourth or fifth contour line is 

 strengthened, and numerous spot heights and bench marks 

 are given. The sheets are printed in three colours; cultural 

 features (roads, settlements, etc.) and names in black, water 

 features in blue, and contours in brown. On some, a green tint 

 is used for woodlands. The general effect is clean and sharp, 

 with no over-crowding of detail. 



About twenty-five per cent of the area of the United States is 

 covered by 'acceptable topographic maps', but for almost 

 forty per cent of the area no topographic maps of any kind 

 exist. Recently, however, progress has been expedited, largely 

 by the adoption of air survey methods. Since 1936, the Geological 

 Survey has made increasing use of air photographs and carried 

 out extensive research in techniques and instruments. Photo- 

 graphic cover of varying standards exists for all but five per cent 

 of the country, and is used for many administrative purposes. 

 It is anticipated that within twenty years all the United States 

 and Alaska will be covered by standard topographic maps. The 

 Survey has also produced geological maps on the scale of 

 1 : 62,500, or larger, for ten per cent of the country. 



The International Map of the World on the Scale o/ 1/1 Million 

 The value of a map of the world on a uniform projection 

 and scale with a standard set of conventions to many types of 

 map users is obvious, but equally obviously its production on 

 any scale larger than those employed in atlases could not 

 usefully be contemplated until a significant proportion of the 

 earth's surface had been mapped topographically. The idea of 

 such a map was first advanced by Professor Albrecht Penck 

 at the International Geographical Congress, Berne, 1891, when 

 he proposed that it should be compiled on the scale of 1/1 

 Million (approximately 1 inch to 15.8 miles). Little was 



