186 
Object mode 
In photogrammetric mapping generally three kinds of objects are to be 
expected, viz. singular point objects, linear or polygon objects, and 
area objects. While points and lines are digitized directly, and there 
fore can be labelled with attributes immediately prior to digitization, 
area objects are derived from their surrounding digitized linear seg 
ments. Labelling an area object may therefore only be meaningful if the 
object were completely surrounded by a closed polygon. In this way, 
however, each line or segment in the stereomodel has to be digitized 
twice, thus yielding double space requirements. In the contrary, it is 
traditional stereocompilation practice to plot all lines only once, and 
to label any area objects by simple annotations. This method has been 
adopted for the digitization process, since it represents the most ob 
vious solution to the problem. Therefore area objects are digitized as 
single points, and shown on the screen by the feature's name. As the 
object mode is part of an object, it has been integrated in the total 
feature code. 
Feature classification 
Every digitized feature consists of a sequence of interconnected points 
represented by a pair of coordinates each. The feature code added at 
the beginning of this sequence of coordinates prepares the data for 
future processing, be it in graphical or in statistical form. From a 
graphical viewpoint this code initiates the pen-down state. The classi 
fication scheme was developed to easily meet different user require 
ments. Actually it is only necessary for the user to establish a table 
of arbitrarily ordered names of features. Each name may consist of up 
to four letters of the alphabet. In a separate program the feature name 
table will be transformed in numerical data - each letter being repre 
sented by its ASCII-code - and stored on a separate file. Up to 100 
different features may be used concurrently. On feature definition the 
object mode (0 for point, 1 for line, and 2 for area) must be appended 
to the feature name by the operator. Prior to digitizing an object it 
is necessary to enter its feature name on the keyboard. 
Record on 
tape cassette 
Store 
current point 
Figure 5. 
Cyclic graphics 
memory buffer 
begincount