THE PDS DIFFERENTIAL RECTIFIER 
James Horton 
The Perkin-Elmer Corporation 
Applied Optics Operations 
7421 Orangewood Avenue 
Garden Grove, CA 92641 
ABSTRACT 
The digital sampling and playback capabilities of the PDS Micro-D and the 
rapid random access capability of an image dissector tube can be combined to 
differentially rectify photographic images to produce orthophotos and other 
simulated-perspective reconstructions. A method is described whereby input 
imagery could be non-orthogonally scanned, eliminating the necessity for re- 
sampling the digitized pixel data. 
INTRODUCTION 
Orthophotos are photographic reproductions of terrain wherein all features 
appear in their proper spatial relationships, as in a map, rather than being 
radially-displaced, as in an aerial photograph. Since such displacements 
are feature-dependent, an individual compensation must be made for each 
elemental area in order to transform an aerial photograph into an orthophoto. 
A method is presented whereby the process of photodigitizing could be uti- 
lized to physically move the image of a feature from its photo location to 
its true datum position. 
The same method of digital differential rectification could be used to 
generate stereomates and simulated-perspective reconstructions as well as 
orthophotos. In addition, compensation for the effects of microrelief could 
be factored into the process if desired. Manipulating photographic data in 
the digital domain would also enable image enhancement techniques to be 
employed so that optimum-contrast reproductions could be generated from low- 
contrast aerial photographs. 
FEATURE DISPLACEMENT 
Feature displacement renders an aerial photograph unsuitable for use as a 
map. This is caused by camera lens distortions and by film shrinkage and 
lack of flatness. It also results from geometric displacements due to 
terrain relief, camera roll and pitch, earth curvature, and atmospheric 
refraction. The net result is that images of terrain features are incorrectly 
located in the photograph. 
Relief displacement is a result of the centrally-projective character of a 
photograph (Fig. 1). Since this is a function of terrain elevation, and 
therefore localized in nature, it cannot be compensated for by any overall 
manipulation of the photograph. Instead, each feature must be treated 
individually, and a process of differential rectification used in order to 
create a planimetrically-corrected photographic reproduction. The image of 
a feature would be displaced from its photo location to its true planimetric 
location to form the orthophoto (Fig. 2). 
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