- 1414 - 
The following relevant features were interpreted and subsequently plotted 
on a stable map-base: 
scale, the 
of good 
= Secondary dirt roads and tracks 1:15,000/1 
— All-weather dirt roads by the inc 
—- Main road (tarmac) = but in low contrast with dry provided t 
grassland on photographs using colc 
= Railroads content fc 
— Towns and villages Similarly 
- Isolated farms and buildings some years 
— Bridges and ferry crossing 1:80,000 v 
VI. DISCUSSION AND CONCLUSIONS In conclue 
The comparison provided in this paper by the review of previous studies using HE i 
high altitude photography in North America and the information provided by altitude c 
the results of the work being undertaken under moist/wet tropical conditions sensing sj 
covering the whole of Sierra Leone leads to a number of conclusions. These users and 
should be helpful in the future when choosing sensors and when planning and the total 
executing surveys. planning 1 
of the aeı 
The lack of conflict in the results from the moist tropics and the temperate 
zone is seen as inferring that high altitude photography has wide geographic 
applicability. Further high altitude photography provides the user with remote 
sensing data quickly and relatively cheaply of extensive areas and with a 
resolution far superior to any other sensor operating under comparable 
conditions, For some problem areas of the world considered best serviced 
previously by SLAR, high altitude aerial photography (possible with pre- 
flight planning using environmental satellite data) is seen now as offering 
an extremely attractive alternative at reasonable cost. 
Reference to published results and the work carried out in Sierra Leone 
leaves no doubt in the minds of the writers that high altitude aerial 
photography provides a quality of detailed information neither available from 
Landsat nor likely to be available at regular intervals from other ‘civilian! 
satellites in the next few years. Also as confirmed in the Sierra Leone study 
the cost of thematic mapping and the extraction of other data from the 
photography is considerably reduced as compared to the use of photographs at 
moderate scales, due primarily to the greatly reduced number of photographs 
and stereoscopic models requiring to be handled. 
In the examination of photographs of several film-filter combinations and over 
a wide range of scales, colour infrared film was observed, as indicated in the 
text, to be the best general purpose film for high altitude photography 
because of its relatively high resolution, tolerance to haze and colour for 
ease of interpretation. Where small areas (e.g. 1 ha) require to be mapped 
from the photographs, it may be an advantage to work directly from 
enlargements. Further, as observed by the writers,under the tropical hazy 
conditions existing in southern West Africa and east and central Africa, 
infrared black-and-white photographs are usually much better in quality than 
panchromatic black-and-white photographs, 
Somewhat surprising, as confirmed by an examination of published studies, the 
usable information content of good quality high flight photographs over a 
range of scales, seems comparable to the usable information content from 
aerial photographs at much larger scales. For many purposes, irrespective of