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Data management 
The digitized data are first stored into a cyclic graphics memory buf 
fer up to nearly 1000 points. This enables quick and easy access to 
the data as long as they are still "fresh". When the buffer is full, 
the oldest data in the buffer are sequentially transferred to the per 
manent data file on tape, viz. pointwise and parallel to digitization 
on a first-in/first-out basis. The program has to keep track of sever 
al buffer counters (Fig. 5). To achieve access to complete features, 
the list of two-dimensional coordinate values of all points of an ob 
ject is preceded by the feature code plus object mode, and by a house 
keeping code consisting of a serial number of the object and the number 
of points in this object. 
User definable keys 
The 10 user definable keys (UDK's) on the 4054 keyboard allow for a 
total of 20 subprograms (UNSHIFT/SHIFT) to be directly activated by 
the operator. Fig. 6 exhibits the currently available layout. The keys 
most frequently used are SET FEATURE for entering a feature name for 
digitization, SET SCALE for selecting the scale of the display plot, 
and CENTER WINDOW. New feature names may be appended to the feature 
list, or a completely new feature table be defined by DEFINE FEATURE, 
while PLOT FILE enables plotting of selected features (overlays) from 
file. With HOLD MODEL the current model data are saved on a temporary 
file, thus permitting power shutdown. CLOSE MODEL terminates the curr 
ent model with recording all buffer-resident data on the permanent 
data file. It must be realized that a suitable number of UDK's reserv 
ed for optimally designed key-functions can only be approximated by 
practical experience. 
Figure 6. 
Layout of UDK's 
Data editing 
Contrary to existing graphics systems an interactive stereocompilation 
graphics system does not require sophisticated editing and correcting 
facilities. It is generally recognized that the majority of measure 
ment errors are detected almost immediately after their occurrence. 
I.e. the system must first provide for a very simple and quick means 
of rejecting detected errors. In the present system four function keys 
support this task. With REJECT MEASUREMENT the previously digitized 
point within the current object is deleted from the buffer, and, to 
gether with its vector, will be earmarked on the display by a suitable 
symbol (Fig. 7). Entire objects may be deleted by the REJECT OBJECT key,