185 
Interaction 
Particular emphasis has been placed on developing an interactive system 
with as high ergonomic standards as is feasible by the available hard 
ware. The underlying concept towards such a solution is that the major 
task of the human operator, viz. to interpret and to measure in the 
stereomodel, should not be impeded by the system in any reasonable way. 
This entails a proper definition of a limited but sufficiently large 
set of useful functions and a consistent layout of a corresponding 
number of special function keys. Different features should preferrably 
be entered by names rather than coded numbers. Also, the system must 
respond by visual and audio signals in the right time. For interrupting 
the real-time program, a footswitch is considered as essential inter 
active device. It is also important that the graphics picture of digit 
ized objects be presented in a convenient scale comparable to the virt 
ual scale presented through the stereo-oculars, and that errors or 
mistakes recognized by the operator can be easily corrected. Closed 
polygons are supposed to close automatically without interference from 
the operator. 
Fig. 3 shows two interlocked real-time loops for measurement, display 
and data logging. The primary loop RTRO provides updates of the current 
model coordinates and of the position of the "stereocursor" represent 
ing the measuring mark in the stereoplotter. As soon as the stereo 
cursor passes over the window border, the window is automatically 
centered on the new position, and the graphics data are replotted. 
This principle of updating the window was first used by Simonsson 
(1980). Interruption of RTRO is only possible by pressing either a spe 
cial function key or the footswitch. The latter triggers the recording 
of an object. As long as the end of the object (E00) has not been 
reached, a secondary real-time loop RTR1 has control. Here, the foot 
switch signals the recording and plotting of a point as member of the 
object. A typical display is shown in Fig. 4. E00 is obtained by a 
series of conditions depending on the type of object and its geometry. 
Different audio signals as computer prompts are used for normal record 
ing, EOO's, or inconsistent mensuration states. Thus the operator hard 
ly needs to look away from the oculars. Only occasionally he has to 
view the screen and to check the validity of the displayed data and its 
compatibility with the stereomodel. In the present version continuous 
digitization, according to time or distance intervals is not included.