king processes, The
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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