etail)
up to now the scanner
er resolution than it is
ns. Although there are
trix cameras with high
line-scanning systems
of their extended spec-
apabilities.
irameters of the above
AOSS consists of clear
clear as well as 4 colour
at FOV | Number of
)0m | (deg) | pixel/line
rad
25 42.96 716
30 78 512
20 34.2 512
27 80 5184
.04 12 5184
aracteristics
lg sensors
inna 1996
It becomes obvious that for the geometrical processing of
these image data with pixel- or subpixel-accuracy sensor
position and pointing has to be provided with a quality and
density which can only be realized by modern techniques
like INS, GPS and DGPS.
2. FIELDS OF APPLICATION
Image data of airborne scanners are widely used and
processed with respect to environmental studies and the-
matic investigations. Due to the lack of sophisticated tools
for accurate geometric processing, an enormous amount
of time and cost consuming interactive operations, in par-
ticular the selection and definition of ground control
points, are necessary up to now.
Nevertheless, there have been many investigations de-
monstrating the typical capabilities of this type of image
data. The applications cover a broad variety of different
disciplines as there are hydrology, agriculture, climato-
logy, forestry, urban planning, etc.
Without appropiate geometrical correction the results of
such investigations can be regarded as helpful if geome-
trical concerns are supposed to be of minor importance.
This implies that the relations to other types of information
which are commonly provided in geoinformation systems
(GIS) are not relevant. However, the general trend to
merge information from different sources and to analyse
dynamic changes by means of multitemporal data impose
a strong need for the provision of geometrically corrected
scanner imagery.
3. GEOMETRIC CHARACTERISTICS
OF AIRBORNE SCANNER IMAGERY
The principal idea of using a line-scanner is to image the
Earth's surface by the combination of scanning operations
and the sensor movement above the ground. Because it
is a time dependent procedure it becomes obvious that
x
line-scanner system
ER
| Flight path -
analogue aerial system
Du
“| Flight path _
> -
Figure 2: Simplification of the projection geometry of a
photographic and a line-scanner system
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
sensor velocity and scan rate must be synchronized. De-
viations cause underscans (gaps in image information) or
overscans (redundant image information). Another reason
for such effects can be changes in terrain heights. There-
fore scan lines should adjoin each other in an optimal way
for maximum altitude of the actual flight area. This is how
underscans can be avoided.
Because of the basic principles of line-scanner techniques
the geometric characteristics of the imagery acquired
differ generally from that of aerial photography. Whereas
photographs show central perspectivity the scanner
image data are mixed projections. The terrain is imaged in
parallel projection in the direction of the flight and in cen-
tral projection across the flight line. This effect is schema-
tically sketched in Fig. 2. It is obvious, that this is valid
only if we assume an ideal trajectory without any distor-
tions.
However, the operational conditions are much more com-
plicated. The real trajectory of an airplane shows con-
siderable deviations from an ideal one. Atmospheric tur-
bulences and other influences result in rather irregular
distortions of higher or lower frequencies. This is why the
trajectory cannot be assumed to be a straight line. Fur-
thermore sensor orientation varies due to changes of the
flight attitude parameters, and therefore sensor pointing
differs from nadir viewing, introducing roll, pitch and yaw
angles.
The geometric correction of scanner image data requires
a full reconstruction of the dynamic changes of the sensor
orientation during the flight. This can not be achieved if
the reconstruction is based only on ground control points.
Therefore orientation parameters for each scan line are
necessary. Due to the high frequency components of the
orientation changes it is necessary to record flight attitude
parameters with the same density as the scan frequency.
This is possible with a combination comprising DGPS and
INS systems. With DGPS precise scanner positions with
an appropriate absolute accuracy up to some centimetres
can be provided.
Some systems for the measurement of the flight attitude
parameters which are in use do not fulfil these require-
ments. While angular variations are recorded mostly with
lower density and accuracy, the sensor positions can only
be estimated using secondary information like height
above ground or above sea level, velocity, drift and scan
rate. Also the standard GPS, which is part of nearly every
aerial navigation equipment, normally does not meet the
requirements.
4. THE SOFTWARE SYSTEM »GASIS«
»GASIS« is the abbreviation for General Airborne Scan-
ner Imaging System and was developed at the Technical
University of Berlin. The system was originally designed
for processing data acquired through the opto-mechanical
scanner DAEDALUS AADS 1268, but its principles are ap-
plicable or can be adapted also to any other type of line-
scanner imagery.
The transformation of the image data to the ground is
based on the well-known equations of collinearity. The
system uses adjusted orientation parameters that are de-
rived from INS, velocity, drift and scan rate measure-
ments. The local heights of the terrain must be provided in
