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INTERPRETABILITY OF SCANNED AERIAL PHOTOGRAPHS 
K.A. Grabmaier, K. Tempfli, R. Ackermann: Scientific Staff, ITC, The Netherlands 
Girma Messelu: Head Photogrammetry, Ethiopian Mapping Authority, Ethiopia 
Commission IV, Working Group 3 
KEY WORDS: Interpretation, Orthoimage, Pixel, Resolution, Feature Extraction, Digital Image, Scanning. 
ABSTRACT 
Digital photogrammetry offers a number of advantages, especially towards automation. One of the major sources of digital images 
for topographic mapping and data collection for spatial databases are aerial photographs. Although interpretability is an important 
property of images used for this purpose, there is little known about the loss of interpretability due to scanning of the aerial 
photographs. We studied the influence of scanning pixel size on the interpretability of wide angle aerial photographs of scales 
1:30,000 and 1:60,000. From the specifications for the French topographic maps 1:50,000 we selected some critical point-, line- and 
area-features. Exclusively for these critical features we compared the stereo interpretation of digital images on a Digital Photo- 
grammetric Workstation (Matra Traster T10) with the one of the original analogue images on an Analytical Plotter (Zeiss Planicomp 
C120). Interpretability of the analogue originals was clearly superior to their digital offsprings with pixel sizes of 60 um, 30 um and 
even 15 um. Point features suffered the clearest reduction in interpretability. 
To study the influence of pixel size on the usefulness of 1:50,000 orthophoto maps (without annotation), we also produced digital 
orthophotos from the digital images using different input and output resolutions. We printed them at scale 1:50,000 and tried to 
interpret from these monoscopic orthophotos the same features as in the stereo interpretation tests, this time with unaided eyes. Point 
features were not recognizable in this test series, but for line- and area-features the influence of photoscale and pixel sizes was clear. 
KURZFASSUNG 
Die Digitale Photogrammetrie bietet einige Vorteile, insbesondere bezüglich Automatisierung. Der Großteil der digitalen Bilder, die 
für topographische Kartierung und zur Datenerfassung für ráumlichen Datenbanken verwendet werden, sind digitalisierte Luftbilder. 
Obwohl die Interpretierbarkeit von Bildern, die für diesen Zweck verwendet werden, wichtig ist, ist über den Verlust an Interpretier- 
barkeit durch das Scannen wenig bekannt. Wir untersuchten den Einfluf der Pixelgrófe auf die Interpretierbarkeit von Weitwinkel- 
Luftaufnahmen der BildmaBstábe 1 : 30 000 und 1 : 60 000. Von den Spezifikationen der Französischen topographischen Karte 
1 : 50 000 haben wir einige kritische Punkt- Linien- und Flàáchen-Objekte ausgesucht. Ausschlieflich für diese kritischen Objekte 
verglichen wir die Stereo-Interpretierbarkeit der digitalen Bilder an einem Digitalen Auswertegerät (Matra Traster T10) mit der der 
analogen Originalbilder an einem Analytischen Auswertegerät (Zeiss Planicomp C120). Die analogen Bilder waren ihren digitalen 
Derivaten mit PixelgróBen von 60 um, 30 um aber auch 15 um deutlich überlegen in Interpretierbarkeit. Punktobjekte zeigten die 
deutlichste EinbuBe an Interpretierbarkeit. 
Um den Einfluf der Pixelgrófe auf die Nützlichkeit von 1 : 50 000 Orthophotokarten (ohne Annotation) zu untersuchen, haben wir 
von digitalen Bildern verschiedener PixelgróBen digitale Orthophotos ebenfalls unterschiedlicher PixelgróBen hergestellt. Diese wurden 
im MaBstab 1 : 50 000 ausgegeben, und wir versuchten von den monoskopischen Orthophotos mit unbewaffnetem Auge die gleichen 
Objekte zu erfassen wie bei der Stereo-Interpretation. Die Punktobjekte waren in dieser Testreihe nicht zu erkennen, aber bei den 
Linien- und Flächen-Objekten war der EinfluB von BildmaBstab und Pixelgrófen deutlich zu erkennen. 
1. INTRODUCTION (b) the accepted trade off with field completion. 
Digital photogrammetry can offer new approaches to topo- Different from digital satellite images, which offer a fixed spa- 
graphic data collection and updating for mapping as well as for tial resolution, using aerial photographs allows to influence the 
spatial databases. A key issue in finding cost-effective solutions, information content of the images through photoscale and scan- 
however, is to assure the required degree of interpretability of ning pixel size. In addition to those factors which influence the 
the digital images to be used. This is important for both the image quality of the analogue photograph, the scanner properties 
presently practicable manual and semi-automatic feature and most decisively the pixel size determine the interpretability 
extraction techniques and the future automated change detection of digital images, which are the input to digital photogrammetric 
and object recognition processes. solutions to geo-data collection and revision. 
There is a whole series of factors influencing interpretability. Several investigations have shown that the digital off-springs can 
Obviously, the degree of interpretability that is required is compete with their analogue ancestors in geometric accuracy. 
determined by two aspects: The interpretability aspect, however, has received little attention 
(a) the sizes and nature of the objects to be included (this is yet. We may expect lower interpretational quality due to scan- 
strongly influenced by the scale of the map respectively the ning, certainly when using pixel sizes that lead to manageable 
spatial and thematic resolution of the target database), data volumes on present systems. A reduction in interpretability 
305 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996