triangulation or to be connected to a semi-automatic electrically 
driven plotting table instead of the mechanical coordinatograph. 
Instruments of this type include: 
  
  
  
Table 1 
Supplier Instrument Model Plotting Table 
WILD A10, AMH/U,AG1 TA, TA2, TA10 
ZEISS D3, E3 Datatech 
KERN PG2 Datatech 
  
  
The factory encoding systems installed were mechanically sound, 
however the encoder resolutions were intended for hard copy map 
plotting and were usually not fine enough for typical digital 
mapping applications. Often, the encoder output were non-T TL or 
the encoders would require a special power supply. In either 
case, the casing for the factory encoders were large enough that 
fitting a replacement was never a problem. The factory encoder 
resolution may be determined by the encoder model numbers. 
ISM maintains a reference list for all popular encoders. 
ISM normally recommends that the encoders be replaced with a 
modern TTL/+5volt unit so that power may be supplied directly 
from the computer. The recommended resolution is 10um in 
model space. Finer resolution can, of course be effected, 
however the encoders would most likely be measuring the "noise" 
or the imperfection of the mechanical systems. 
Most digital mapping system entails that the map be entirely 
compiled and edited in the digital domain. Hard copy, if required, 
is then plotted with a pen or ink-jet plotter driven directly by the 
computer. The co-ordinatograph or plotting table attached to the 
instrument is now redundant. 
Many photogrammetrists however, still feel uncomfortable without 
a plotting table directly driven by the instrument. Often, they would 
request that the digital up-grade be configured in such a way that 
the plotting table and the computer system operate independently 
but simultaneously. This type of installation often induces 
electronic interference problems that are difficult to isolate and 
resolve, however a switching mechanism can be inserted into the 
system so that either digital or mechanical plotting may be 
performed conveniently. 
In addition to instrument conditions, site conditions are also very 
important. |t cannot be overly stressed that clean and 
uninterrupted electrical power be maintained. 
3.0 ENCODING SYSTEMS 
There are many designs for encoding systems. In general, they 
can be classified broadly as photo coordinate systems and model 
coordinate systems. The mechanical principles are essentially 
simple. A photo coordinate based system will measure the 
positions of the left and right viewing microscopes (x'y'x"y") as 
they traverse the photo space. A model coordinate based system 
will measure the position of the measuring mark (x,y,z) as it 
82 
traverses the model space. Of course, the interfacing software 
must also be able to accept either the photo or model coordinate 
inputs in order to effect either system. 
3.1 ENCODERS 
Modern TTL encoders are available in the Rotary or Linear 
types. A Rotary encoder utilizes a glass disc with fine 
markings subdividing a complete rotation (360 degrees) into 
equal sectors. If mounted concentric to the lead-screw, the 
encoder will then subdivide each complete lead-screw rotation 
(pitch) into fine equal parts. 
eg.: 
If the model stage lead-screw pitch is 2mm(2000um), 
then a 250 count/rev. encoder will yield a model space 
resolution of 2mm/250 or 8um. 
A Linear Encoder is essentially a glass scale with very fine 
markings. 5 or 10um spacing per marking (least count) is 
generally sufficient. A Linear Encoder can be used to provide 
continuous encoder counts instead of a rack-and-pinion/rotary 
encoder system. 
A modern TTL encoder is normally rated at 50,000 hours life or 
more, however if malfunctioning occurs, it is cheaper to replace 
the entire device. 
3.2 MOUNTING SYSTEMS 
The mounting of encoders to the instrument can be problematic 
and costly. A good mounting system will provide easy and 
accurate mounting with the least disturbance to the instrument. 
Some of the design considerations discussed are: 
a. Accuracy 
The encoders are expected to measure the fine motions 
of either the model stage or the viewing microscopes. If a 
Linear Encoder is used, it is important that it is mounted 
parallel to the guide-rail or lead-screw. If a Rotary 
Encoder is used, then the axis of encoder rotation should 
be parallel to the rotary shaft or lead-screw. 
To minimize time wasted in adjusting or calibrating the 
errors, ISM has developed various mounting jigs for most 
instrument types. The technician simply attached the jig 
onto certain critical instrument component (eg. a main 
guide rail) and drill and tap screw holes for attaching the 
entire encoder mounting bracket set. This method 
ensures that the encoder mounting is within the precision 
tolerance and easy to install. 
b. Standardization 
ISM has selected as few encoder types and resolutions as 
possible for the instrument types encountered. This 
enables bulk purchasing of the devices and maintaining 
adequate stock. 
For the mounting bracket systems, many of the 
component are interchangeable between instrument 
types. Sizes of machine bolts and other fasteners are 
also m 
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instrument t 
Instruments 
WILD A7 
WILD A7 
WILD A8 
WILD A8 
WILD B8/B€ 
WILD A10 
WILD AG1 
ZEISS D2/3 
ZEISS E2/3 
ZEISS F2/3 
Jena 
Topocarts 
Jena SMG's 
SANTONI C 
3-Axis 
4-Axis 
  
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WILD A7's, 
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lead-screw 
instrument i: 
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