ently plotted 
ous studies using 
on provided by 
pical conditions 
lusions. These 
en planning and 
nd the temperate 
wide geographic 
he user with remote 
as and with a 
comparable 
best serviced 
le with pre- 
now as offering 
ierra Leone 
le aerial 
er available from 
other 'civilian' 
Sierra Leone study 
a from the 
P photographs at 
of photographs 
pinations and over 
3 indicated in the 
ohotography 
and colour for 
re to be mapped 
y from 
tropical hazy 
bral Africa, 
in quality than 
shed studies, the 
zraphs over a 
content from 
, irrespective of 
= 1415 - 
scale, there is possibly little gain in the usable information content 
of good quality photographs until a photographic scale of about 
1:15,000/1:20,000 or larger are obtained; and this will be offset usually 
by the inoreased costs and, possibly, by the loss of the synoptic view 
provided by the high altitude aerial photography. At very small scales 
using colour infrared film, there is probably a threshold of information 
content for some purposes (e.g. forestry) at scales approaching 1:100,000. 
Similarly for urban traffic studies in Australia, the first author observed 
some years ago that an information threshold existed between 1:60,000 and 
1:80,000 when using normal colour films. 
In conclusion, it is strongly recommended that, for the planning of aerial 
surveys in the future, separate and careful consideration, including cost 
benefit analysis, should be applied to the feasibility of using high 
altitude aerial photography before choosing the most appropriate remote 
sensing system, The additional cost, including interviews with potential 
users and the assessment of local conditions, will be small compared with 
the total cost of providing the imagery. The cost of this approach to 
planning remote sensing in Sierra Leone was less than 0.5% of the cost 
of the aerial photography (excluding interpretation).