undertaken
| 305 mm focal
le C-factor was
1igh contrast
e
information
19 to atmospheric
id tilt. However,
arp (about 0.02%
the accuracy of
of the flying
. The error in
of 13,400 m was
gle lens (i.e. 15 cm
points were
animetric maps
1:50,000 from
t topographic
racy falling
ed too from the
» 1977).
rage by Landsat
ite sensinge
the land surface
ing less *than
about 45% was
| of side-looking
l weather
ird advantage of
+ and hence provide
restry as in
IR is either 10-20 m
vison, the
a is about 80 m
erial camera
nd wide-angle camera
yted to be better
provided the
on the extent and
and reduced time
t will be appreciated
altitude aerial
ind that aerial
nds and their
- 1497 -
shadows on any single photograph and not more than an average of 1% for the
entire photographic coverage.
Sierra Leone
For Sierra Leone, the concept of high flight photography was attractive and
offered excellent prospects. At least in theory, ,the country could be
covered by high flight photography in about four days. Examination of black-
and-white IR photographs of nearby Francophone countries showed that (despite the
haze problems in the dry season) acceptable quality images can be obtained,
which are superior to panchromatic black-and-white photography taken under
hasy conditions, although of a lower resolution than black-and-white panchromatic
photographs teken under non-hazy conditions (cf. Fig. 3a, 3b). Also, based
on US experience of high altitude photography,products of acceptable quality
for forest and land-use studies and with a high resolution, can best be
obtained with colour IR film, provided a wide angle lens and not a super wide
angle lens is used.
At the time of planning the aerial photography for Sierra Leone (1975),
consideration was also given to relying only on the 1951/64 panchromatic black-
and-white photographs. Although these old photographs are of historical
interest (e.g. for land-use dynamic studies, see Fig. 1) and provide some
details used in studies of the geomorpu^'!ogy, hydrology, soil etc., their
application to land resources survey in Sierra Leone was not practical for
the following reasons:
- Firstly, the thematic information relating to present vegetation
and land-use of the country could not be satisfactorily extracted
from old photographs (up to 24 years old at the outset of the
project activities), but also due to the considerable time span
between adjoining missions (up to 13 years).
— Secondly, due to major changes in land-use, the identification of
tie-in ground points would be tedious and sometimes impossible,
while orientation problems in the field were bound to occurs
— Thirdly, the total amount of photographs to be handled (3,600) together
with the number of different photographic missions involved (at least
8) would have made their systematic use awkward and time consuming,
particularly when compared to a single coverage of high flight
photographs consisting of 1,100 and 350 exposures only with wide-
angle and super-wide angle cameras for scales of 1:70,000 and
1:120,000 respectively.
Concerning cloud-cover and length of season for aerial photography, daily
detailed information on the seasonal distribution of rainfall was available
from synoptic weather stations (e.g. at airports); and for the first time
this type of information was combined with information on cloud cover obtainable
by examination of the low-resolution NOAA imagery (Fige2).