Under this methodology, the satellite images
were geocoded and edge enhanced. The
inter-scene colour and contrast were balanced.
Thereafter, a digital mosaic was created.
The map sheets were then extracted and co-
ordinate overlay and map legends were
added for photo products. In the case of
map revision, following the extraction of the
map sheets from the digital mosaic, coordinate
overlay, details of existing maps and map
legends were added for photo products. The
final product could be film negative, or paper
print, or digital data on a CCT, or floppy
sidk, or digital cassette.
EXECUTION OF STUDY
The input was SPOT data. The images were
radiometrically corrected by applying
calibration coefficients to each individual
detector in the satellite sensor. The Sate-
llite Image Maps (SIMs) were geometrically
rectified to correspond to the UTM map
projection and Clarks Reference Ellipsoid,
normally adopted for mapping in Nigeria.
In a case of insufficient ground controls,
control point correction of the SIMs was
performed, using ground control points
from maps of adjoining areas.
In order to achieve an enhanced interpreta-
tion accuracy, while retaining the geometric
accuracy associated with the Panchromatic
mode, the SPOT data source used was a
combination of SPOT Pan and SPOT XS.
The SIMs were produced each with an image
area of 50 cm x 50 cm over an area of
104,363 sq.km.
RESULTS OF STUDY AND ANALYSIS
The results so far obtained from this prelimi-
nary study clearly demonstrate the following
advantages over the adoption of the conven-
tional approach.
(a) The dependence of possibility of
acquisition of data for the mapping on
the availability of good weather was
greatly reduced. This arises from the
repetitiveness of the satellite coverage
as against an exposure from a single
flight of the Photographic Aircraft.
(b) The possibility of eliminating dependence
of flight over a neighbouring territory
to acquire mapping data upon the
granting of permission by the appro-
priate Authorities through the purchase
of available satellite imagery.
(c) The possibility of acquiring mapping
data from an orbiting satellite, which
can be as up-to-date as possible and
with relative ease.
(d) The availability of the mapping data in
digital form, which is relatively easier to
334
up-date and compatible with
modern techniques, such as GIS/
LIS.
(e) The mapping method lends itself
more easily to costing.
. LIMITATIONS
(a) Low image contrast generally
resulted in difficulties in edge
matching, poor identification of
details and, in a particular case,
in the loss of an important
feature.
(b) Differences in illumination
between adjoining SPOT scenes
because manifest in the combina-
tion of scenes to produce the
image maps of some sheets.
(c) Differences in colour rendition
between SPOT scenes resulted in
two different colour hues (red
and green) for a given feature,
on adjacent image map sheets.
. PROPOSALS FOR IMPROVED MAPPING
The following proposals, if implemented
in future experimentation on border
mapping, wiil yield improved definition
and contrast and lead to enhanced
accuracy of mapping
(a) The use of SPOT products, pre-
processed to level "IAP", on
Analytical Stereoplotter in the
medium of photographic film, with
auxiliary data recorded on CCT
tapes, or MS-DOS diskettes, will
ensure improved quality of inter-
pretation, higher geometric
accuracy and greater operator's
comfort. Improvement in the
quality of interpretation will
result from
(i) use of a special digital filter,
which considerably enhances
the interpretation of linear
objects through the amplifi-
cation of frequencies close
to half the sampling frequency,
while attenuating the more
"noisy" sampling frequency;
(ii) maintaining photographic
specifications designed to meet
aerial photography standards,
such as:
D. 7 0.2 above back-
min
ground fog and
$e «MY equ (or 1.5)
in the case of very low
contrast landscapes); and
(iii) slight mean scale increase to
1:350,000 from 1:400,000 for
IA level.
