used for 1:50,000 mapping and map revision. 
At the launch of SPOT , 60 stereopairs were evalua- 
ted by IGN France on TRASTER analytical plotter 
and the results yielded standard error of about 6m 
in X, Y, and Z, (IGN,1986). Also, the National 
Research Council (NRC) of Canada Photogrammetric 
Research Laboratory reported that RMS values of the 
residuals on 80 check points for a single panchro- 
matic image, measured on the Anaplot, ranges bet- 
ween 6.7m and 8.2m depending on the control confi- 
guration (Van Wijk, 1987). Similar results were 
Livi by Gugan et al (1988) and Rodriguez et al 
1988). 
In the sphere of digital image processing, Rosen- 
holm (1988) reported accuracy of 6m to 7m in height 
using multipoint matching technique for generating 
digital elevation model (DEM). Similar results 
were reported by Swann et al (1988). 
In the area of map revision, extensive use of Land- 
sat TM, with a resolution of 30m has been made for 
the operational revision of 1:250,000 maps of Cana- 
da by detecting changes on 1:50,000 map series. 
(Turner et al, 1987). 
In general, there are two main, approaches to the extra- 
ction of cartographic data from SPOT imagery viz: 
-- analytical stereo-restitution or 
-- digital image processing. 
For both methods panchromatic stereo pairs with ba- 
se to height ratio of between 0.5 to 1 are required 
for best results. 
3.1 Analytical Stereo Restitution 
Any analytical plotter equipped with necessary soft 
ware can be used, e.g Traster, Kern DSR, Wild Avi- 
olyt ACI, Anaplot etc. This method has the advanta- 
ge that both height and planimetric details can be 
plotted. However, it has the disadvantage that de- 
tails are not always easily and correctly identifiable. .Ano- 
ther major disadvantage is that films used for res- 
titution are distorted during film writing from di- 
gital data. 
5.2 Digital Image Processing 
Digital stereopairs are processed to obtain digital 
elevation model (DEM) either by automatic image 
correlation or by multipoint matching (Rosenholm, 
1988). Contours are then obtained by interpolation. 
This method is faster and has the advantage that 
most mapping processes like image enhancement, DEM 
generation, orthoimage generation, interpolation 
and map drafting can be automated. Final maps are 
obtained by superimposition of height information 
on planimetric details from precision corrected 
imagery and toponymic information. Digital image 
processing has the disadvantage that details cannot 
be plotted. However contours can be superimposed on 
visually interpreted and extracted details from 
precision corrected imagery and topographical line 
maps or directly on orthoimage maps. 
4. NIGERIAN EXPERIENCE 
The National Population Commission mapping project 
is a pioneering work in the application of space 
imagery to topographical mapping in Nigeria. It con 
consists of the production of:- 
-- 1:50,000 annotated SPOT image maps 
-- 1:50,000 planimetric line maps from SPOT imagery. 
The SPOT image mosaics which were to be produced 
within 8 weeks of the ward of the contract was to 
be used as map substitutes for planning the Enumer- 
ation Area (EA), pending the time they will be 
117 
replaced by line maps, which were to be delivered 
within nine (9) months. 
The mapping covers all the areas of the country 
where base maps at 1:50,000 were not available. 
See figure 1. The total area mapped is 1355,192km*. 
It is covered by 177 map sheets, of size 15' x 15' 
of arc each. The project is divided into 7 blocks 
details of which are shown in Table 1. Seven sur- 
vey companies participated in the project. Sola 
Atilola Associates carried out the mapping of 
Block 6. 
TABLE 1: No of Map Sheets and Area Covered by 
Block Numbers. 
  
  
  
  
Block No No of Map Sheets Area Km? 
1 48 36,120 
2 23 17,308 
3 29 21,822 
4 17 12,792 
5 15 11,288 
6 22 16,555 
7 25 17.3507 
Total 177 133,192 
4.1 Operational Procedure 
  
The operational procedure for executing the project 
included: - 
-- Acquisition of SPOT (imagery) scenes 
Provision of control points by geodetic measu- 
rement, or extraction of-control points from 
existing topographical maps within and outside 
the area to be mapped. 
Computer-base digital precision correction and 
(Processing) transformation of CCT data into 
Universal Transverse Mercator Projection system 
using the control points. 
Digital combination of two or more scenes to 
produce digital mosaics. 
Cutting of mosaics into map sheets and writing 
of films and production of bromide prints of 
map sheets. 
Cartographic annotation 
Field verification and completion 
Preparation of final mauscripts 
4.2 Acquisition and Processing of SPOT Imagery 
Reception and Processing of space imagery for topo- 
graphic mapping employs the state of the art 
"High-Tech" equipment and sophisticated softwares. 
Only very few organisations the world over are ful- 
ly equiped to receive and process such data to 
high degree of geometric fidelity for mapping 
purposes. Other organisations depend on these feu 
companies for imagery at required level depending 
on application for which they are intended. There- 
fore this aspect of the project was subcontracted 
to Sweedish Space Corporation (SSC) in order to 
facilitate the speedy completion of the project. 
4.2.1 Choice of spoT Imagery For cartographic 
applications, panchromatic imagery with a high 
resolution of 10m is most appropriate. However at 
the beginning of the project no panchromatic scen- 
es of the project areas were available both at 
Kiruna and Toulouse. Programming SPOT for Panchro- 
matic imagery would also take a few weeks. There- 
fore in view of the urgency of the project, the 
available multispectral (XS) scenes had to be used. 
  
4.2.2 Precision Correction of SPOT Imagery 
The selected scenes were precision corrected to 
level 2 product. For the geometric rectification, 
about 4 to 6 control points are required per scene.