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Demystifying Advancements in Digital Orthophotography
by Surdex Corporation, St. Louis, MO
About Surdex Corp.
Scanning from Aerial Negatives
In order to be used for digital orthophotography, any aerial photograph exposed in an
analog camera must be translated into digital format. This is accomplished by scanning
the aerial images with a radiometrically and geometrically precise digital aerial film
scanner. When scanned, each pixel of the aerial image consists of a radiometric value
plus an XY coordinate set unique to that image. Until recently the most accepted
methodology to do this was to scan from film diapositives; a diapositive is a second-
generation positive reproduction of the original film negative with positive tone. Once
scanned, the photogrammetrist develops a histogram to adjust radiometric contrast to
insure that the raw data has a more pleasing overall image tone.
The problems associated with this process are three-fold. First, scanning from a second-
generation product creates a third-generation scan. It should be noted that each
replication of an image is destructive to the overall accuracy, precision, and quality of the
original aerial image. Imagine taking a photocopy of a photocopy of a photocopy … it’s
easy to visualize the rapid loss of resolution. When photogrammetrists scan from a film
diapositive, they are using a “copy of a copy” as their source material, the source material
upon which the entire basemap is based. Secondly, the process of scanning from a film
diapositive is extremely laborious; the photogrammetrist must load each individual
diapositive into a single frame flatbed scanner. Conversely, technicians can scan
continuous aerial photography film directly from the spool in an automated fashion.
Consequently the costs for scanning from hand-placed diapositives, in comparison to
scanning direct from film, is significantly higher. Finally, the chance of image
degradation is increased significantly due to processing blemishes or excessive handling
of film diapositives.
To combat the problems associated with scanning from second-generation film
diapositives, Surdex scans directly from the archive-quality aerial negatives with our
Vexcel VX4000 Digital Imaging Systems, radiometrically and geometrically precise
digital aerial film scanners. The VX4000 is capable of scanning an entire roll of aerial
film at one time, significantly automating the digital imaging process. The VX4000 will
accept rolls of film up to 500’ in length. It has a Geometric Accuracy of 1/3 pixel RMS.
For example, the geometric accuracy of a 7.5-micron scan will be 2.5 microns. The
Radiometric Accuracy is 2 gray values RMS, and the pixel depth is 8 bits monochrome
and 24 bits color (8 bits per color).
Our testing universally affirmed our premise that scanning directly from film improved
the quality of the scans, plus all subsequent mapping products, as well as significantly
improved schedules. Additionally, the Vexcel scanners allow us to preprogram scanning
requirements for a full 500’ roll of aerial film. Histogram and scanning parameters are
applied to the entire roll insuring that the project imagery is radiometrically precise. In
some cases, it may be necessary to scan individual frames as opposed to the entire roll.
This process too can be programmed, thereby yielding the same time and cost savings.
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