5. Differential Rectification
The accuracy analysis for scanner data shows, that the geometric properties of such images are
inferior to standard in using scanners must therefore lie in interpreting and analyzing them for
thematic information. This must be done comparing images of the same geometry. For different sen-
sors (multispectral) and for flights with different orientation parameters (multitemporal) one
image must therefore be transformed to the geometry of the other. Or it is even better to trans-
form both images to the geometry of a map. Due to the large local displacements of the images
from an orthographic view because of locally varying orientation parameters and because of un-
dulating terrain such a transformation is only possible by differential rectification.
5.1. Analog differential rectification
Special optical rectification devices to approximately remove panoramic distortions from line
Scanner images and slant range distortious from radar images have been suggested by Leberl /59/,
but they were not built.
Special electronic rectification devices however have been built experimentally for radar res-
titution, such as the Radar Orthographic Restitutor on the principle of a cathode ray tube image
transfer with its deflection controlled by a digital computer (Yositomo /88/, Graham /32/ , Boo-
man et al /12/). Cathode ray tube output devices of standard electronic image correlation systems,
such as the Unamace and the Gestalt Orthophotomapper have also been used to demonstrate the
feasibility of rectifying remote sensing imagery by computer control.
Because of increased resolution capabilities the electron beam recorder has become the image
generation device for satellite photos.at NASA and at the USGS . Because of its design for high
density digital tapes it has not been used for rectifications other than for first and second
order corrections along ech scan line, producing a semi-corrected bulk image.
(Forrest /30/, Kratky /55/, Steiner /80/ ).
Laser beam recorders under construction will permit a moderate deflection across the scan line
in the future. Expensive devices of this nature will however only be justified for operations
with an extremely large throughput (bernstein /8/ ). Such devices may also better be operated in
a digital fashion, however.
For the differential rectification of scanner images on an occasional, less costly basis standard
orthophotodevices have been used. Masry and Gibbons /71/ demonstrated the use of the orthoprinter
OP/C of Ottico Mecchanica Italiana for this purpose. Kraus /56/ and Otepka /74/ reported on the use
of the Wild Avioplan OR 1 for the fitting of scanner images to a map base. Experiences /74/ show
that the fit corresponds to the practical accuracy limitations of scanners shown in table 2. The
controls for the orthophoto device are desived from the orientation of the flight Height infor-
mation from a digital terrain model could also be included.
5.2. Digital Differential Rectification
The first digital rectification approach on a pixel by pixel basis has been demonstrated by
Sharp of I.B.M. at the 1964 ISP congress. Since that time computer technology has evolved to
the state where processing times or costs of digital rectification methods have come into the same
order of magnitude as the use of optical or electronic analog differential rectification pro-
cedures.
Three types of output dvices may be used:
1. a line printer, available to almost every computer user. Character overprinting
may be programmed in such a manner as to generate a number of grey levels in the
output.
