STANDARDS FOR IMAGE SCANNERS USED IN DIGITAL PHOTOGRAMMETRY 
Scott Miller and Alex Dam 
Helava Associates Inc., 10965 Via Frontera, San Diego CA 92127 
ABSTRACT 
Digital photogrammetric products can be produced from virtually any type of imagery. Presently, most digital 
photogrammetric products are produced from scanned diapositives. The image scanner can typically be the most 
influencing factor on the accuracy of data collection. A Digital Photogrammetric Workstation (DPW) cannot recover 
from a poor quality scanner. This paper discusses methods for geometrically calibrating scanners and suggests that 
a standard should be made. 
KEY WORDS: Scanner, Accuracy, Standards, Digital Image 
INTRODUCTION 
Photogrammetric products can be produced from 
virtually any type of image. The image can be from hand 
held 35 millimeter cameras, video cameras, satellite 
imagery, panoramic cameras and many others. The 
accuracy of products produced is largely affected by 
the input source. There is a general expectation that 
photogrammetric products produced from high quality 
mapping cameras, with distortions in the micron range, 
would produce products commensurate with input image 
quality. If we are to use scanners to digitize high quality 
films for photogrammetric data production, we must be 
careful to preserve radiometric and geometric quality if 
we are to compete with existing photogrammetric 
products such as analytical stereo plotters and high 
quality comparators. If one does not wish to preserve 
the fidelity of the input image and is cognizant of this 
(i.e., it is not required) then there is no problem. If one is 
not cognizant, and believes they are getting a product 
equivalent to the film camera output, then there is a 
serious problem. What standards are required? Should 
we have a rating system? |n this paper, we are limiting 
our discussions to geometric quality, there is of course 
a very related issue of radiometric quality. 
WHAT PHOTOGRAMMETRY EXPECTS 
Today, many firms view photogrammetry in terms of C- 
factor or similar rules. Through experience and 
practice, firms learn what is achievable from a given 
instrument type. 
C-factor = Flying Height of Image / Contour 
Interval of Map 
Analytical plotters are typically achieving in the 1200 to 
well over 2000 range for C-factor. Stage accuracy's for 
first order analytical plotters are generally better than 3 
microns. For the most part, a DPW should be limited in 
accuracy only by the quality of pixels given to it and the 
quality of control points used. Assuming the pixels are 
of good quality, algorithms and the human can easily 
measure to better than 0.5 pixels. Many studies prove 
that precision better than 0.25 pixels are routinely 
possible. To obtain accuracy, we need not just 
134 
precision, but absolute knowledge of where those pixels 
are with-respect-to the original film focal plane of the 
camera. Much has been done in photogrammetry to 
assure good geometric fidelity of film when placed on 
the stage plate of an analog or analytical plotter. Why 
should photogrammetry require camera calibrations if 
the scanner does not require it? A scanner should 
preserve and adhere to the quality of analytical plotters 
and/or should state the accuracy to be expected under 
typical or standard operating conditions. If the 
scanners geometric quality is as good as the analytical 
plotter stages, then a user's C-factor concepts can be 
used for DPWs as well (assuming the pixel radiometric 
quality and size is good enough). If we scan an image 
with a 25 micron pixel size, and assume that 
measurements can be made to 0.25 of a pixel, we get a 
desired precision of 6.25 microns. This tells us that our 
scanner geometric error should be significantly less 
than 6 microns to achieve the best results and not 
unduly influence accuracy from a 25 micron pixel scan 
image. 
General Scanner Types And Error Sources 
Geometric qualities of scanned film primarily results 
from the scanner design of which there are several 
types. Stage types of scanners have a big impact on 
scan quality. Drum scanners that primarily are derived 
from the graphics art industries, typically use tension to 
hold the film flat against the drum. This combined with 
drum "roundness" make it difficult to maintain focus and 
geometric position in relation to the scan head. Flat bed 
stage designs can use hard optically flat cover plates to 
hold the film flat during scanning. This is an easier 
design to hold to higher geometric accuracy. Flat bed 
stages with cover plates are less susceptible to 
operator error in mounting the film. Scanners designed 
for a typical application are normally designed with 
sufficient accuracy for that application. In the graphic 
art applications, desk top flat bed and drum scanners 
are typical. Their accuracy is generally sufficient for the 
application they were designed for. The Sharp JX-600 
uses a flat bed design but does not use a flat cover 
plate and does not use a high precision stage drive or 
low distortion lens. Its geometric accuracy is in the 80 
micron range (Sarjakoski 1992). It produces reasonably 
good pixels and could be rated for photogrammetry, 
perhaps by stating its limitation on C-factor. 
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