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February 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
that are regularly meeting a certain map accuracy
standard using the direct sensor orientation method.
2. IMU to AidAerial Triangulation: This is themost common
use of IMU in photogrammetric mapping. Like Airborne
GPS, IMU can be used in the aerial triangulation process
to provide better estimation for the initial values of the
input parameters and to reduce the number of unknowns,
therefore decreasing the reliance on ground control
points. Since the GPS-derived camera positions, the
IMU-derived orientations, ground controls points, and all
other measurements and parameters are considered as
observations with certain errors budgets, i.e. weighted,
in the mathematical model of the aerial triangulation,
the need for perfect boresight misalignment diminishes
as such less-than-perfect values will be refined and
adjusted in the aerial triangulation process anyway.
Based on the last statement, you do not need to do a new
IMU boresighting with each mission as long as you use
the IMU-derived exterior orientation within the aerial
triangulation and not for direct sensor orientation. As
for how often you need to redo the IMU boresighting, I
can safely say that you do not need to redo it if you did
it right the first time and if the IMU was not physically
disturbed during operations or removed from the camera.
Again, if you are using the IMU-derived orientations to
aid aerial triangulation, then you can relax or ignore the
experts’ advice, as you do not need a new IMU boresighting
for each mission. In fact, you can achieve excellent results
from frame-based aerial triangulation using only airborne
GPS and ground controls, i.e. without data from IMU. That
is not the case if you are using a push broom type of camera,
where the IMU input is integral for processing the imagery.
However, even if you are using a push broom sensor, the
IMU-derived orientation is always updated during the aerial
triangulation adjustment. Therefore, a slight problem in the
boresight values is not going to cause a catastrophe.
Finally, even if you are using the IMU-derived orientations
for direct sensor orientation, performing a new IMU
boresighting does not always guarantee parallax-free stereo
models setup. Experience taught us that you may experience
severe parallax on some stereo pairs in the block despite
the fact that you performed new IMU boresighting for that
mission. Such behavior could be caused by different system
components other than the IMU. High-end IMU is great
technology and has excellent performance and accuracy. To
be fair to the IMU manufacturers, less-than perfect IMU-
derived exterior orientation performance does not always
mean that the IMU was not accurate. The aerial camera of
today is a complex system that contains imaging sensor or
camera, GPS, IMU, complicated wiring and cables, many
electronic signals and signals synchronization, and IMU-to-
camera mount. The IMU could perform perfectly, but other
problems could be happening during flight due to one or more
of the following:
-
Faulty design or fabrication of the camera housing that
holds the lens cone
-
Defect in the IMU-to-camera mount or in its performance
in accurately transferring the motion dynamics of the
lens to the IMU box, especially in severe turbulence or
vibration caused by a weather front
-
Excessive thermal and mechanical stresses that the
imaging system is subjected to during rough aerial
operations and/or vast change between the temperature
on the ground at the hanger and the atmospheric
temperature at a flying altitude of 4,000 meter or higher
-
Signals synchronization or timing problems between
GPS, IMU, and the camera shutter opening signals
All these sources of errors could result in less accurate
performance of the IMU-derived exterior orientation despite
the high accuracy of the IMU itself. I hope this addresses
your concerns and contributes to a more efficient production
mapping process.
**Dr. Abdullah is Senior Geospatial Scientist and Associate at
Woolpert, Inc. He is the 2010 recipient of the ASPRS Photogrammet-
ric (Fairchild) Award.
The contents of this column reflect the views of the author,
who is responsible for the facts and accuracy of the data pre-
sented herein. The contents do not necessarily reflect the offi-
cial views or policies of the American Society for Photogram-
metry and Remote Sensing and/or Woolpert, Inc.
“You do not need to do a new IMU
boresighting with each mission as
long as you use the IMU-derived
exterior orientation within the aerial
triangulation and not for direct sensor
orientation”
“You can achieve excellent results
from frame-based aerial triangulation
using only airborne GPS and ground
controls, i.e. without data from IMU”
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