International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004
locations like block corners where the access to the ground may
be difficult must not be used for their location. It is not a
problem to map areas where the access is difficult or dangerous.
The processing time can be reduced against traditional image
orientation.
On the other side unreliable results are not accepted, so a
procedure which is taking care about the reported problems has
to. be used. It became a common strategy to determine the
boresight misalignment every flight day. Some companies have
done it also before and after the photo flight. The reference
flights should be done at least over a small test area with control
points with opposite flight directions to control the principal
point location. If the reference flight will be done with the same
flying height above ground like used over the project area, the
determination of the focal length is not required because the
main effect is covered by the shift parameters. If different
flying heights are used, a complete system calibration including
also the focal length has to be made with two different flying
heights. The influence of the map projection has to be taken
into account — this can be made with a determination in an
orthogonal coordinate system, like geocentric or tangential or
with respecting the local net scale which also can be made by a
local change of the focal length, but of course the refraction
correction and in the case of a direct handling in the national
coordinate system, the earth curvature correction has to be
respected.
If the problems of the net projection and the focal length are
respected, the reference area for the determination of the
boresight misalignment must not be located in the area of the
project. Of course the required GPS reference must be
available, but this can be made also with worldwide differential
GPS services like OmniStar, Skyquest Aviation or NavCom,
reaching sub-meter accuracy. Still most companies are
determining the boresight misalignment within the project area
and with the same flying height, making the handling easier, but
this may not be an economic solution for small projects.
In general it has to be mentioned that some experience and
sufficient education is required for the handling of the direct
sensor orientation. Often the first test fails because of some
missing details. This may be the missing required flight figure
for the initialisation of the inertial system or the use of very
long straight flight lines which may be affected by inertial
drifts. Extreme long flight lines should be interrupted by flight
figures like a circle for avoiding problems with the inertial drift.
Of course the handling has to be done more rigorous, respecting
the geoid undulation and the characteristics of the used
coordinate system. All these aspects are not reducing the large
economic potential of the direct sensor orientation which may
be improved by an integrated sensor orientation.
Some aspects of the limited stability of the boresight
misalignment are caused by problems of the used analogue film
cameras which have not been constructed for the mount of an
IMU-system. This may be different for the new digital cameras
with a stable imaging plane and a foreseen optimal IMU-mount,
but up to now this has not been analysed in a sufficient long
term manner.
The direct sensor orientation allows solutions different from
standard applications with a high flexibility. The use of small
format digital cameras is not economic if the orientation has to
be determined by standard block adjustment. With the direct
sensor orientation an economic use has been enabled.
834
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