International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
are then geometrically corrected for lens distortion based on a
calibration of the individual camera heads and are subsequently
combined to form the image composite (see Dórstel et al. 2003
and below for details). The final output images are written to
the output RAID storage designated for the final images on the
postprocessing server. The final output image data can then be
transferred to a data management and distribution system, such
as Z/l's TerraShare, where it can be archived or distributed to
the destination defined by the operator.
Processin | Processing Details In Step
g Site Step
Office Preprocess- | Mission planning
ing
Airplane | Photo Flight | Navigation and flight manage-
ment system
Camera software control
module
Quickview (inflight quality
check)
Data storage
Office Postprocess- | Connection of flight data
ing Storage (FDS) or Copy Station
disks
Postprocessing
e Radiometric correction
e Geometric correction
e Mosaicking (generation
of virtual images)
Color image generation
Data distribution
Office Data Data processing via
Exploitation | photogrammetric and/or GIS
tools
Table 1. Typical DMC Workflow
2. IMAGE COMPOSITE GENERATION
The aim of generating image composites from the different raw
panchromatic and multispectral images is to generate a central
perspective image in such a way that, in subsequent processing
steps, standard photogrammetric procedures and software can
be used.
First of all, each of the four camera heads needs a precise
geometric calibration. This step is also known as ‘single-head-
camera geometric calibration’ and applied at manufacturing
site. The information generated during this procedure is
delivered on a calibration CD to the customer together with the
complete DMC System. During system installation the
calibration data is stored at the postprocessing workstation.
When the aircraft lands, the mission data is transferred to the
postprocessing workstation.
Within one exposure, the raw images are captured from slightly
different positions (Figure 4, upper part). Relative to the tilted
panchromatic images, the image composite is placed in an
average position and can be thought of as a perfectly nadir-
looking image. In Figure 4 the relation between two individual
camera heads is depicted. In order to generate the image
composite, the geometric relations between the four individual
398
images and the image composite must be given. Assuming
known values for this relative orientation, e.g. from a prior
calibration, the individual images are merged together and
projected to a horizontal reference plane (see Figure 4) to form
one perspective image (see Tang et al. 2000 for the related
equations). In this step, the focal length of the image composite
can be chosen freely. In order to avoid under- or over-sampling,
a value close to the actual focal length of the individual camera
heads should be used. Also, lens distortions of the individual
camera heads can be respected in this step. It should be noted,
however, that the image positions need to be corrected prior to
the merge.
projection center
p n er * projection center
ackward view
forward view
|
| reference plane
for composition
CAOW X Ge 3 RR REDEN MN Gl aan EM N N NN
Figure 4. Principle of the combination of convergent sub-
images to an image composite
Above: cross-section Below: 3-D situation
3. ERROR ANALYSIS OF IMAGE COMPOSITE
GENERATION |
In terms of geometric accuracy of the image composite, there
are three issues which need to be discussed: (a) all raw images
must be acquired synchronously in time, (b) the systematic
effect resulting from the different perspective centres must be
negligible, and (c) the relative orientation of the camera heads
must be stable.
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