use of this function would be adding some straight lines to 
a map. 
The stroke function is used for such things as tracing 
hand-drawn curves for computer entry. Using this method, 
users input points by holding down a button on the puck 
while moving it across the tablet; in response, the termi- 
nal sends a stream of xy-coordi nates of each point the puck 
passes over, along with a code identifying which button on 
the puck the user held down, to the host computer. A pos- 
sible use of this function would be adding a river to a 
map. 
The pick function is used to select a segment by positio- 
ning the cursor on the desired object. Using this method, 
users identify an object by positioning the cursor over it 
and press a puck button; in response, the terminal sends 
xy-coodi nates, button code, segment number, and the pick ID 
to the host computer. ( When you define a segment, you 
specify an segment number and assign pick ID to identify 
parts of the segment.) A possible use of this function 
would be deleting an object from a map. 
2.6 Zoom and pan 
Zoom is a terminal] feature that allows you to enlarge part 
of a graphics image, so you can see the details on a map 
displaying on the screen in any scale, without any host in- 
teraction. Pan is another terminal feature that allows you 
to select a part of the screen for detailed viewing while 
using zoom feature. With these two features, digital maps 
can be easily edited in any detail even using a low resolu- 
tion terminal, and the Programmer don't need to worry about 
the difficult algorithms for windowing and clipping more. 
2.7 Hardware, firmware, and software 
The above mentioned new techniques are usually carried out 
by the hardware with firmware, typically by an intelligent 
color graphics raster terminal and a graphics tablet con- 
nected to the terminal. In case the hardware/firmware can 
not satisfy fully the requirements of some neu concepts, 
the manufacturer of the hardware will provide the software 
to support it. As examples are the firmware of Tektronix 
4109 terminal /7/ and the software of HP Advanced Graphics 
Package AGP-3D /5/. 
3. FILE STRUCTURE OF DIGITAL MAPS 
Although the real data base could be the best data struc- 
ture for digital maps, the manufacturers of most analytical 
stereoplotters are still using the traditional conservative 
file structure. Normally they are using direct access bi- 
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