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
