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ground control, time dependent collinearity equations must be used and 
linear or polynomial constraints must be used to link the successive 
sensor positions. 
SPOT will be launched in 1985 (Chevrel et al 1981) and already a great 
deal of work has been done on simulated SPOT data. MOMS (Bodachtel 
1983) has already been flown on Shuttle Transport System (STS) mission 
no.7 on the free flying space platform SPAS. Stereo MOMS is being 
developed together with a software system to map from the data 
(Hofmann et al 1982). The Mapsat concept has been described by 
Colvocorresses (1982) and this includes automated mapping. The 
Canadian Centre for Remote Sensing has developed an airborne push 
broom system MEIS which is being developed to obtain stereo image 
(Neville 1983) and this should be considered as a future source of data. 
At present synthetic aperture radar (SAR) cannot be considered as a 
serious source of data topographic mapping because of poor resolution 
and complex distortion. However, some stereoscopic was obtained from 
Seasat and there is considerable interest in the Shuttle Imaging Radar 
(SIR) already flown on STS2 and to be flown again on STSI7 A European 
microwave satellite ERSI will be launched in the late 1980's. Details 
of these systems and of the geometry of radar can be found in Allan 
(1983) and Leber] (1976). 
Control 
An essential element in using any data for topographic mapping is the 
determination of the elements of exterior orientation. The number of 
elements required is six for a single photograph. With push broom 
sensors six elements are needed for each line of data since the 
satellite will have moved between recording each line. The number of 
unknowns to be found is reduced by relating successive perspective 
centres and attitudes by linear or non-linear functions. 
Existing earth observation satellites cannot be fixed in position and 
attitude with a sufficient degree of accuracy to provide the elements of 
exterior orientation and therefore ground control must be used. The 
provision of ground control in the traditional manner is an essential 
part of mapping from metric camera photography and will be from SPOT data. 
It is argued that much of the value of mapping from satellite data is 
lost if control has to be fixed on the ground, therefore with several new 
systems, provision is made for the determination of the elements of 
exterior orientation by on board sensors. The requirements for the 
accuracy of such systems are discussed by Welch and Marko (1981), 
Colvocoresses (1982) and Doyle (1982) and it is concluded that position is 
required to better than 5m and attitude to better than 5" with changes in 
attitude measured to 10 5 deg sec !. 
The means of acquiring the position and attitude of the satellite are the 
Global Positioning System (GPS) which will give position to 7m in X, 7m in 
Y and 10m in Z, and star cameras giving attitude to 5" or electro optical 
stellar sensors to 2". A combination of these methods is proposed for the 
large format camera on STS-17 (Doyle 1982) and for Mapsat (Colvocoresses 
1982). Such systems are not yet proven and it seems likely that some ground 
control will be needed for some considerable time yet.