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DIRECT / INTEGRATED SENSOR ORIENTATION - PROS AND CONS 
K. Jacobsen 
Institute of Photogrammetry and Geoinformation, 
University of Hannover, Germany 
jacobsen(@ipi.uni-hannover.de 
TS ThS 16 — Direct Georeferencing 
Invited paper 
KEY WORDS: Calibration, Orientation, Direct, Georeferencing, Status 
ABSTRACT: 
The direct geo-referencing of sensors based on a combination of relative kinematic GPS-positioning and inertial measurement units 
(IMU) has reached a high accuracy level and growing application. It includes the advantage of a very flexible use, independent upon 
control points which are only required for the system calibration and independent upon block or strip configurations. It is in use in 
areas with difficult access, as well as for standard applications. The direct georeferencing is a prerequisite for the economic use of 
small format digital images instead of standard aerial photos. 
Projects with standard accuracy requirements can be handled without problems. Only some limitations may appear for the model 
setup; disturbing y-parallaxes cannot be avoided with the today dominating application of standard photographic aerial cameras. But 
an improvement of individual models or a whole block by a combined adjustment together with image coordinates of tie points can 
solve this problem. For large scale projects with higher accuracy requirements an integrated sensor orientation by a bundle block 
adjustment with the orientations as observations and a minimal number of control points is proposed. 
An overview of the status of direct and integrated sensor orientation will be given. 
1. INTRODUCTION 
A basic geometric problem in photogrammetry is the 
determination of the sensor orientation. For analogue and digital 
frame cameras this can be made by resection for single images 
or relative and absolute orientation for a stereo model. In aerial 
applications we usually do have an image block and the 
common determination of the exterior orientation by block 
adjustment is more economic. The expensive and time 
consuming control point measurement can be reduced by a 
common bundle block adjustment with projection centres from 
relative kinematic GPS-positioning. This method of combined 
adjustment is today a standard solution, but it is economic only 
for larger blocks and it requires in addition to a small number of 
control points, the photo measurement of tie points and also a 
satisfying block configuration with usually additional crossing 
flight lines. For individual flight lines the use of projection 
centres for the block adjustment has only a limited advantage, it 
cannot control the lateral tilt. In addition an extrapolation out of 
the area of the control points should be avoided. 
The direct and the integrated sensor orientation are able to solve 
several up to now existing problems of the sensor orientation 
and can speed up the projects. With a combination of relative 
kinematic GPS-positioning and an Inertial Measurement Unit 
(IMU), the projection centre position and the attitudes can be 
determined. This gives a wide range of flexibility like for 
example in coastal regions where only a small part of the 
Images is covering land and the traditional tie of images are 
failing. Also no problems exist in forest, desert and 
mountainous areas where the automatic aerotriangulation has 
problems. 
829 
Under direct sensor orientation we do understand the 
determination of the exterior orientation just by the combination 
of IMU and relative kinematic GPS-positioning. Of course the 
determination of the attitude and shift relation of the IMU to the 
sensor system, the boresight misalignment, or a complete 
system calibration has to be made over a controlled reference 
area. A disadvantage of the direct sensor orientation is the 
missing reliability and also some problems with y-parallaxes of 
the model set up. This can be improved by a common 
adjustment of the directly determined exterior orientation 
together with image coordinates, which is named as integrated 
sensor orientation. With the integrated sensor orientation we 
still do have the advantage of an orientation without control 
points and also problems of the image tie do not lead to missing 
solutions. 
The usual block adjustment is in general an interpolation within 
the area of the control points. This is different for the direct 
georeferencing which is an extrapolation from the projection 
centres to the ground. By this reason, the steps of computation 
have to be handled with more care. 
The accuracy and also the reliability of the direct sensor 
orientation are depending upon the relation of the IMU to the 
sensor, the so called boresight misalignment. This has to be 
determined and respected. Of course it is an additional effort 
influencing the economic aspects. By this reason the required 
methods are critical for the wide acceptance of the direct 
georeferencing. For the determination of the boresight 
misalignment control points are required. Of course it is easier 
if always the same reference area, located close to the airport of 
the survey aircraft, will be used. But this requires a correct 
handling of the coordinate systems and also a system