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Basic versus Control Data 
In the data flow, two different ingredients are involved, i.e., the "basic" 
and the "control" data. The "basic" data locate the elements of the seman 
tic information in the different coordinate systems, whereas the "control" 
data are used merely for the internal process control. Users of the infor 
mation products are in principle not interested in the control data - un 
less they upgrade these products further. 
Examples of the control data input are the locations of fiducial points, 
principal distance, control points (rotation matrix), tie points (for local 
transformations), correction data (for deterministic errors) and the DTM. 
The latter, however, serves in DMP merely for process control. 
Process control in DMP can be differently synthesised, depending upon the 
type of the control data input. Normally, the external orientation is 
determined by means of the control points - by resection in space. In the 
case of aerial triangulation, however, the rotation matrices may be direct 
ly available, though a new aerial triangulation is not justified for up 
dating. 
Operations in the preparation stage are restricted to the control data. 
The basic data are generated in the mensuration stage, which also includes 
some control data. The differential heights of new man-made objects are used 
for locational corrections and thus act as control data. Geometric pre 
editing of the basic data is connected with measurements; it covers verifi 
cation of the geometric correctness and consistancy, and the corresponding 
corrections. 
The transformation stage concerns mainly the basic data. The control data, 
however, are also involved in transformations of the control and check 
points, tie points and in external orientation. Other subsequent stages 
in updating, i.e., editing, generalisation, and plotting are as in the 
original (digital) mapping (fig. 8). 
In the following, attention is given only to those stages which imply the 
operations specific for updating by means of DMP. 
Operations 
The preparation stage concerns primarily identification of the control and 
tie points. Special care is required in identifying conjugated points on 
generalised maps on which some detail is displaced. Redundant control and 
tie points should therefore be provided, which permit a least squares fit 
and detection of gross errors. These points should be marked, and indexed. 
New object points with differential heights should also be indexed if 
parallaxes are to be measured separately. 
The mensuration/digitising stage concerns both the "new" and the "old" data 
sources. The control data are measured on both sources, whereas the basic 
data are measured on the new photographs. If the DTM is not a priori given, 
the contour lines and/or spot heights should be sampled locally on the 
existing topographic maps, i.e., in the zones of the changes. Differential 
heights can be derived from the x-parallaxes. These may be measured either 
simultaneously with the changed planimetric features, or separately (e.g., 
by means of a stereoscope and parallax bar). Simultaneous measurement is 
preferable because it is faster and more reliable than separate (Radwan, 
1980). The pre-editing of the geometric data is as in the original mapping. 
The transformation stage represents the nucleus of DMP. It comprises three 
successive transformation steps, i.e., from the digitiser to the photo 
coordinate system, then to the model coordinate system, and finally to 
the map (or field) coordinate system. Each transformation step may be 
accompanied by the correction routines for deterministic errors. The con 
trol data are needed to determine the transformation coefficients in each 
step. 
In the first step, affine or similarity transformation is usually applied 
to both the control and the basic data.