Data structure editing refers to the editing of topological data and includes 
commands to: 
- connect lines into a continuous line 
- join lines together to form stream-line segments; 
- break a line into separate lines; 
- define, rechain, rename, or delete levels in the data hierarchy; and 
- add elements to levels, rearrange elements among levels or delete 
elements from levels of the data hierarchy. 
5. COMPUTER GRAPHICS WORKSTATION 
In the Surveys and Mapping Branch, various types of computer graphic terminals 
are in use. 
The main components of the terminal are the digital memory, or frame buffers in 
which the displayed image is stored as a matrix of intensity values, the display 
"monitor" (CRT), the display controller that passes the contents of the frame 
buffer to the monitor and the keyboard. 
The picture-generation techniques are used in graphic terminals (Machover, 1983). 
In raster scan displays, the electron beam is moved continuously in a fixed 
pattern, very rapidly in the x-axis and much more slowly in the y-axis. At the end 
of a vertical scan, the beam resumes its horizontal scanning, as shown in Figure 2. 
In stroke-writing (Figure 2-B), the beam can be moved simultaneously in arbitrary 
x and y directions. 
Today, CRT computer graphics terminals use stroke-refresh, stroke-storage or 
raster-refresh. In the stroke-storage display type, all the information is written 
on the CRT and they do not have to be refreshed constantly (phosphorescent light 
will continue to be emitted for several seconds). 
Refresh display may use long persistence phosphors that refresh at low frame 
rates (10 to 30 frames/seconds). They may also use shorter-persistence phosphors 
that refresh the image faster (30 to 60 frames/seconds). In raster-refresh display, 
the image pixels are transmitted in a raster format, independent of the logical 
connection between the image points on the disply. 
While stroke (vector) storage- and stroke-refresh displays are still in use in 
interactive graphic terminals, color raster display systems are clearly emerging as 
the dominant display technology with higher resolution (more than 1000 x 1000 
pixels resolution) and flicker free screens. 
In computer graphics terminal, the control of the display and editing functions is 
done with the "command menu". This menu may be contained in a menu tablet or 
on the graphic display. The command menu is activated with the cursor, light pen 
or trackball. 
6. CONCLUSIONS 
Digital mapping and geographic information systems are continuing to exhibit 
growth and future potential, particularly in the areas of raster scan technology, 
computer graphics, displays and editing. 
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