a) b) 
  
  
Figure 3. a) Fiducial marks visible on the used historical images, partially visible and with different forms. b) The coordinate system 
is defined on the historical photos assuming the geometrical centre of the fiducial marks as principal point: the same point was used 
to crop the images to the same fixed size. C) and d) Comparison of the radiometric and geometric resolution between historical (c) 
and actual (d) images. The actual image (2009) was resampled to have the same GSD of the oldest photo (1945). 
3. PROCESSING OF THE HISTORICAL IMAGES 
The conventional photogrammetric pipeline for orthoimage 
generation requires digital photos to be rectified using a suitable 
DSM in order to remove the perspective effects in the images. 
The DSM can be either generated from the same images or 
obtained from other sources (e.g., LiDAR data, previous aerial 
or satellite images) if the terrain in the area of interest is not 
significantly changed. In both cases, however, interior and 
exterior orientation parameters of the images are required. 
The processing of historical images is generally done with 
manual procedures, in particular the image triangulation and 
segmentation/classification steps. Redecker (2008) highlights 
the main reasons that affect the photogrammetric processing of 
historical aerial photos: (i) inaccuracy or total lack of meta- 
information about inner orientation (focal length and 
coordinates of fiducial marks) and additional (i.e., distortion) 
parameters; (ii) missing specifications about the flight mission 
(especially flying height); (iii) poor radiometric image quality 
(haze, image darkness, non-uniform luminosity); (iv) distortions 
caused by roll and pitch due to sudden movements of the plane; 
(v) improper transport or storage procedures of the film 
(humidity, temperature, etc.) and (vi) inaccurate processing of 
original films or hardcopies in field laboratories. 
Prior to the 1940's, there were no standard calibration 
procedures for aerial cameras and consequently calibration 
certificates are generally not available for the oldest historical 
aerial imagery (Luman et al, 1997). Moreover usable fiducial 
marks are often difficult to be identified in the photographs: 
identifiable points or features appear as slightly different shapes 
or are not visible at all (Fig. 3a). Therefore assumptions are 
often necessary, introducing some errors in the processing 
pipeline. Additionally, the image triangulation step necessitates 
a sufficient number of ground control points (GCPs). This 
operation often poses a significant problem as it can be difficult 
to identify present-day "stable" and unchanged points or 
features in a landscape (e.g., building corners, bridges, junctions 
of land parcels), that can be matched to the historic photos. 
Another challenge posed by historical imagery is that the 
automatic extraction of DSMs with image matching algorithms 
can deliver unsatisfactory results (spiky surface models) 
resulting from un-modelled distortions, errors in the interior 
orientation parameters and/or low radiometric quality. If this is 
84 
the case, the production of digital orthoimages from historical 
aerial imagery can be achieved employing a contemporary DTM 
instead of generating the DSM using the historical images. 
A faster, but less rigorous method, to geometrically correct 
historical photographs and produce orthoimages can be done by 
applying a polynomial transformation (Luman et al, 1997; 
Merler et al, 2005). This procedure can lead to inaccurate 
registration (i.c, measurement errors up to a few meters); 
however, it may be sufficient for revealing landscape changes at 
macro and medium scales (e.g., identifying land use changes 
such as forests to farms or rural areas to urban arcas). 
3.1 Recovering approximate calibration information 
As specific information was unavailable, it was necessary to use 
proxy data to recover the approximate calibration information. 
After the USA entered into WWII, many reconnaissance aircraft 
were equipped with the Fairchild camera (Redweik et al., 2009). 
The dimensions of the available prints and forms of the fiducial 
marks suggest that the original film format was 9x9 inches and 
the camera employed was the K-17. The fiducial marks consist 
of four half-arrows in the middle of the image sides and appear 
as two different sizes. The pair of bigger half-arrows indicates 
the flight direction (Redweik et al., 2009). This reconnaissance 
and mapping camera for vertical and oblique aerial photos 
could be fitted with Bausch and Lomb Metrogon lens of 6", 12" 
and 24" focal length (Redecker, 2008). 
3.2 Digitizing the historical repository 
Considering the poor resolution of historical photographs, high- 
end desktop scanners can be used for digitizing the hardcopies 
(Redecker, 2008). It was proved that with a suitable calibration 
procedure, desktop scanners can be successfully used for 
cartographic applications and orthophoto production 
(Baltsavias, 1994; Mitrovic et al., 2004). For our task, the 
Epson Expression 1640XL desktop scanner was employed. To 
preserve as many details as possible in the imagery, the prints 
were scanned at 1600dpi geometric resolution (i.e., 16 micron 
pixel size) and a 16 bit radiometric resolution. To evaluate and 
reduce any possible distortions introduced by the scanning 
operation, a reference image, available in both digital (scanned 
with a photogrammetric scanner) and hardcopy (contact print) 
forms, was used. 
Fc 
vi 
co 
fic 
TI 
ca 
op 
sir 
OV 
of