king processes, The 
thern and southern 
enables us to check 
ht lines as wel] gg 
g gives us a clearer 
indling radiometric 
  
> 
J Enghsh — _ 
zs Sm &8 
  
}) are essential for 
hotography (DAP), 
tal elevation models 
istry are capable of 
it of such scanners 
it the geometric and 
are generally in 
ly, there are very 
and it is also quite 
ns on a scanner. In 
of an appropriate 
> project’s variables 
t was decided to do 
eiss PhotoScan PS 
any responsible for 
es) so as to be sure 
the optimal density 
ink or grease spots, 
ulsion would be on 
dium. 
| of 15 um, which 
tre on the ground. 
it possible to obtain 
s provided on an 
e Tag Image File 
was to capture all 
  
the details contained in the aerial photograph digitally. The 
second reason was to provide scanned data appropriate for 
two projects. The first project is the one discussed in this 
paper and the second one is to explore the potential 
application of the high resolution soft colour orthophoto for 
the revision of the forest inventory. 
  
  
PROJECT PHASES 
nned Digital 
BE. |B || = 
  
  
  
  
  
  
  
+ 
| | 
v 
i Control Point Selection ] 
  
  
[Processing of the Soft Orthophoto] 
i 
  
Soft 
Color 
Orthophoto 
Radiometric Corrections 
[ —. Mosakking | 
Neat Line Generation 
Soft Orthophoto 
  
  
  
   
   
    
  
  
  
  
  
  
  
  
  
Da nis Surround 
Data : Preparation 
Interactive Updating 
Figure 5-3 
The Geographic Information Corporation provided the 
contractors with the digital topographic data and the digital 
elevation models. To ensure proper rectification control, 
nine DTD and DEM files, a 3x3 file matrix, were provided 
to the contractors in the CARIS NTX format. 
5.6.2 Preprocessing 
The files produced by scanning at a resolution of 15 um 
were approximately 700 MB. The SUN SPARC station 
IPX assigned to the project cannot use the CARIS software 
in its 24-bit version. The scanned data had to be 
preprocessed in order to convert the data from 24 to 8 bits. 
The preprocessing reduced the size of each photo's files 
from 700 MB (theoretical size) to about 235 MB (actual 
size). 
The scanned data received were in accordance with the 
Red, Green and Blue (RGB) model, which makes it 
possible to define over 16.7 million colours. However, the 
data was transferred from the RGB model to the Intensity, 
Hue and Saturation (IHS) or pseudo-colour model. The 
change in model imposed severe restrictions on colour 
manipulation. 
5.6.3 Selection of Control Points 
The first step consisted of displaying the digital topographic 
data and the uncorrected scanned photo, using the CARIS's 
editor module (CARED). 
The second step was to digitize the coordinates of four 
fiducials marks (corner or middle) using the zoom function. 
The last step involved identifying common points on the 
313 
scanned photo and in the digital topographic data base by 
displaying the two data sets in adjacent windows. Five 
common control points (one near each corner and one near 
the centre of the photo) are sufficient for the orthophoto 
transformation. Choosing ten, fifteen, or twenty control 
points does not improve the results appreciably. 
5.6.4 Parameters of the Photogrammetric Camera 
Certain data, such as the focal length and the principal 
point (centre point) of the photogrammetric camera, are 
also necessary. They were recorded in a file used by the 
software to resolve the orthophoto transformation. 
5.6.5 Production of the Soft Orthophoto 
The transformation module requires several data entries and 
lends itself well to batch processing. The module asks for 
the names, list of files needed, and the names of the files to 
be created. 
Input Files: 
- Name of scanned photo 
- Name of DEM data file 
- Name of control points file (Recorded from the 
topographic data base) 
- Name of control points file (Recorded from 
scanned aerial photography) 
- Name of fiducial marks file 
- Name of photogrammetric camera parameters file 
Output File: 
- Name of orthophoto data file 
Other parameters required: 
- Resolution of orthophoto data 
- Resolution of output files must be specified. In 
order to limit the size of the output files a 1.0 m 
resolution was chosen 
- Type of transformation (e.g. orthophoto) 
- Type of pixel selection (e.g., nearest neighbour) 
- Sector of the photo to be transformed; that is a 
selected portion or the whole photo 
5.6.6 Radiometric Corrections 
The purpose of radiometric corrections is to make the 
appearance of the image in one photo uniform with that of 
another in the direction of the flight line and between the 
flight lines. Reduction of the data from 24 bits to 8 bits 
and the change from the RGB model to the IHS model 
imposed significant restrictions on the radiometric 
corrections during the project. The only corrections 
possible were choosing a palette of colours for one photo 
and adjusting the other photos to this palette. 
5.6.7 Mosaicking 
Mosaicking consisted of assembling overlapping photos 
utilizing the various methods available in the software. The 
method chosen was to digitize a line on the monitor, 
digitizing on it as many points as are needed to reduce 
evidence of the seam as much as possible. As in 
conventional mosaics, the choice of a line along an 
uninterrupted line feature, such as a road, a river, or a 
power line, produced better results. It is possible to create