G. Stereoplotters

This Section is a very brief overview of stereoplotters. It's included to round out the discussion on stereophotography. Detailed geometry, stereomodel orintation, C_factors, etc, are not coverd for two reasons:

• The details are beyond the scope of the the basic photogrammetric theme of this topic. 
• Analog systems have been largely replaced by digital systems (scanners, softcopy p[hotogrammetry, etc) so most of this information is primarly of historical nature.

For greater detail, please refer to one of the Photogrammetry textbooks.

1. What

Maps were compiled from analog aerial photos using a stereoplotter. A stereoplotter allowed the operator to directly view and map overlapping photos in 3D.

Instead of creating paper positives from film negatives, the images are developed on glass plates. The developed plates are called diapositives. Glass is rigid and more stable than paper-based pictures and allows light to pass through.

A stereoplotter consists of two projectors, each holding one dispositive of a stereopair. When oriented correctly the projectors replicate camera position and geometry. Light passing through the diapositives project corresponding images into a 3D viewing system. 

There are three primary types of analog plotters:

• Optical
• Mechanical
• Opti-Mechanical

They are to photogrammetry what transits and total stations are to survey measurements - both accomplish the same things based on the same principles, but do so in a different fashion. Optical plotters are the older design of the two and have an open design (like a transit); mechanical plotters are enclosed, have a more efficient viewing system, and are more easily integrated with computers and digital data capture. Because they are simpler and easier to visualize, optical plotters will be used to describe the basic concepts.