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Pitout, Cedric 
  
After scanning, an image has only a relative referencing and can not be overlaid with other geo-referenced objects. For 
Basquiat (1998), the principle of geo-referencing is to attribute geographic coordinates for each pixel of a raster picture. 
Thus, localization data are coded into a cartesian coordinates system (Star & Dickinson, 1996). In order to geo- 
reference scanned photographs, it is necessary to use an already geo-referenced document. 
For this purpose, two kinds of document have been used. The first one is a numerical geo-referenced orthophoto 
(resolution = 50 cm x 50 cm) from a private authority. For the second site, no support was available so all the geo- 
referencing was performed using DGPS. The reference marks must be visible on the aerial view like surfaces (e.g. 
fields, stadium, etc.) but especially cross-lines (e.g. cross-roads, bridges, etc.). 
The studied-sites have a small surface and a very flat topography consequently deformations are minimized except for 
waste dumps. Thereby, for geographical corrections, "register" and "rectify" ArcInfoO tools are used with good results. 
43 Aerial photograph working 
4.3. Evolution of factory building during time. Although the first aerial photographs appeared about thirty years 
after the beginning of the industrial activity, they allow to follow the evolution of factory building (Figure 2) and to 
locate the production places with the help of general site plan. 
  
    
| 
: 
i 
| 
i 
i 
Figure 2. Building evolution history 
  
  
  
  
From each photograph, the outline of 
buildings have been redrawn. At the end, 
the overlay of these limits shows that 
there differences in size and overlap. The 
main causes are the different conditions of 
aerial surveys (scale, angle of incidence, 
brightness, etc.) To minimize this 
problem, center coordinates (X, Y), 
perimeter (P), surface (S) are extracted for 
each building, then the mean X, Y, P, S 
are computed. From these data, the mean 
building implantation is created (Figure 
3). To check accuracy, we note that the 
digitizing corresponds exactly with the 
existing substructure referenced by 
DGPS. 
4.3.2 Evolution of decanting zones 
during time. First, it is possible to follow 
the history of decanting zones during 
industrial activities. Second, aerial photographs are used to study the brown field itself. Indeed, even if the factory have 
stopped theirs activities since twenty years ago, site managers use them to excavate or to stock contaminated soils. 
  
  
  
  
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 1159