15 
Photograph 
Fig. 6 
Configuration of a D. M. P. system 
These m^y operate on-line, off-line or partly on- and partly off-line. In the on-line 
version the data processing is time-constrained; therefore the programme organisation 
is stringent and appropriate interface equipment is required. The chief merit of the 
on-line version is its enablement to inspect the output immediately. 
The off-line approach is easier to programme, tut it requires intermediate recording 
of the data. The work can be phased and carried out at different places. Existing equip 
ment may be employed moreover. 
In the preparation stage the computer should determine the orientation data (e. g. by 
resection in space). Then the transformation of the locational data (x, y) on the photo 
graph into the corresponding map data (X, Y) can be accomplished. The stream of 
photo-co-ordinates can be transformed into the corresponding stream of map co 
ordinates in a single stage, or in two successive stages. In the latter case the photo 
data are first transformed into the model system, making use of the D. T. M. - and of 
the orientation data. The model co-ordinates obtained are then transformed into the map 
system. Ih the case of a direct (single stage) transformation, however, the D. T. M. 
data should first be transformed into the map system. 
Ih both cases the central problem is to combine the three groups of input data: the 
orientation (or transformation) parameters, the stream of photo-co-ordinates fx, y), 
and the D. T. M. data ( Z = f(X, Y)). A simplified flow chart for the two-stage trans- 
fdrmation is shown in fig. 7. 
The rate of sampling heights from the D. T. M. data may be much lower than the rate 
of the transformation computations. The sampling rate can be adapted to local terrain 
slope and to a specified accuracy.