56 
tude and 1 degree in latitude, There are some 500 of these areas in continental Aust- 
ralia, and it may take up to six months for a geological party to map one of them, With- 
out aerial photographs, the task of mapping the whole of Australia, already formidable, 
would be almost insuperable, 
Geophysical surveys also make use of aerial photographs,.photomosaiecs and.maps 
on photo-seale before and during a survey. Geophysical exploration is, by and large, an 
expensive method of working, It is therefore not always expedient to wait for the pro- 
duction of a planimetric map which can be used as a base map, and photomaps provide a good 
Similarly survey planning can be more flexible if based on aerial photographs 
substitute, 
or photomaps, 
Ce 
One of the most important recent advances in geophysical exploration in Australia ti] 
has been the application of airborne magnetic radiometric and electromagnetic surveys, ma 
pae 
are 
Aerial photographs and photomosaies are used for navigation during survey flights 
and for plotting of results, In fact, the low-level surveys use aerial photographs at all are 
stages - for planning, for navigation and plotting of courses flown, for plotting and exhi- ehe 
bition of results in the field, and as part of the final published map, of 
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