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digital data base. A "graphic data file" (or "plot file") contains only 
selected data items, provided with symbols, text and drafting instruct 
ions. Moreover, the features can also be generalised. 
As a byproduct of the "graphical" variant of updating, the intermediate 
digital products i.e., the changed data files, may be collected with the 
aim of gradually forming a digital data base, while the graphical one 
remains preserved. An outline of the procedure for the transition to a 
digital data base, however, is beyond the sope of this paper. 
Overall Process Definition 
The overall process of updating is carried out in several sequential 
stages (fig. 4). The initial stages, such as reconnaissance, planning and 
air mission, and the cartographic reproduction, are not considered in 
this paper. In DMP, the most significant and delicate stages are the 
preparation, the mensuration and the subsequent transformations. 
PREPARATION 
Control data, correction data 
Existing maps (or data files) 
Photographs 
FIELD: verification 
completion 
MENSURATION on: 
Existing maps 
New photographs 
EXTERNAL ORIENTATION 
Rotation matrix 
sequence may 
TRANSFORMATION^(A correction«) 
Photo to model 
I 
TRANSFORMATION (& corrections) 
Model to map (or field) 
Changed data files 
X 
'data ElilTINC & 
CONDITIONING; 
(for transformations) 
I 
PLOT 
changes on existing 
Conditioned control data 
Edited changed data files 
(in both digitisers' system? 
TRANSFORMATIONS j& corrections) of: 
control data to map (or field) system, 
changed data to photo system 
L 
Fig. 4: Sequential stages of updating by DMP. 
PREPARE : 
graphic data files 
PLOT: 
Two Realms in Data Flow 
The data and operations can be virtually separated into the semantic and 
the geometric parts. The semantic realm covers operations such as iden 
tification, extraction, encoding and indexing, and symbolising data 
items, whereas the geometric realm concerns the locational data of the 
elements of semantic information, the associated transformations and 
corrections, and positioning. In the following, the operations of the 
two realms will be reviewed. 
FLOW OF SEMANTIC INFORMATION 
Changes 
Though the identification and extraction of changes is not unique for 
DMP, attention is given to it in this paper because it tends to dominate 
the overall performance and reliability of updating. The primary data 
source is new survey photographs (or other images). Before discussing 
the extraction of relevant changes, however, it seems appropriate to 
review some issues concerning the changes in terrain. Most of the factors 
listed in figure 5 can be differentiated further, though this is not 
necessary in the context of the paper.