Michael Cramer 
  
DIRECT GEOREFERENCING USING GPS/INERTIAL EXTERIOR ORIENTATIONS FOR 
PHOTOGRAMMETRIC APPLICATIONS 
Michael Cramer, Dirk Stallmann and Norbert Haala 
Institute for Photogrammetry (ifp) 
University of Stuttgart, Germany 
michael.cramer@ifp.uni-stuttgart.de 
Working Group III/1 
KEY WORDS: direct georeferencing, GPS, INS, aerial triangulation, photogrammetry 
ABSTRACT 
One crucial point during the application of direct georeferencing is the accuracy and reliability of directly measured 
orientation parameters using integrated GPS/inertial systems in an operational photogrammetric airborne environment. 
In order to investigate the potential of direct georeferencing for standard photogrammetric applications the accuracy of 
a commercially available GPS/inertial system (Applanix POS/DG 310) is evaluated. For that purpose a well controlled 
airborne test comparing the results from standard bundle adjustment and point determination by direct georeferencing is 
described. Within this test special focus is given on the quality of the GPS/inertial exterior orientation using several 
master stations with varying baseline length up to 380km. Additionally, the influence of variable image overlap on the 
resulting object coordinates is investigated. Although very consistent and high accuracy in object space is proved using 
direct georeferencing, the tests show the great importance of proper calibration between GPS/inertial and camera 
components. From this point of view, the integration of the GPS/inertial exterior orientations in a combined aerial 
triangulation provides the most flexible approach and is recommended for highest photogrammetric accuracy demands. 
1 INTRODUCTION 
The determination of the exterior orientation parameters (e.g. position Xo, Yo, Zo and attitude ®, «p, x of an image at the 
time of exposure) is an essential pre-requisite for the evaluation of imagery based on any type of data from terrestrial, 
airborne or satellite platforms. Traditionally, in photogrammetry this orientation task is solved indirectly using the well- 
known method of aerial triangulation (AT). Although aerial triangulation was essentially improved and expanded to so- 
called automated aerial triangulation (AAT) in the last years (e.g. Schenk (1997)), the orientation process still suffers 
from a large amount of interactive editing and supervision of highly skilled operators. This is especially due to the high 
computational effort that is necessary for automatic tie point measurement. A reliable matching of tie points is 
necessary to determine the exterior orientation of each image correctly. With the availability of integrated GPS/inertial 
systems this situation changes. GPS offers high absolute accuracy position and velocity information. The short term 
noise is dependent on the data quality and observation approach. In contrast to this, inertial systems provide very high 
relative accuracy for position, velocity and attitude information, but the absolute accuracy decreases dependent on run- 
time if the system is working in stand-alone mode and no external update measurements are available. Since GPS and 
inertial systems are of complementary error behaviour, their optimal integration allows fully exterior orientation 
determination with improved overall accuracy and at higher reliability compared to the stand-alone units. Hence, this 
integration has already been proposed since a couple of years. Meanwhile integrated GPS/inertial systems are 
commercially available and commonly used for the operational processing of digital airborne line sensor data. 
Nevertheless, a GPS/inertial component is advantageous for the orientation of standard frame sensors like 
photogrammetric cameras, too. The potential of integrated GPS/inertial systems for photogrammetric applications is 
investigated in this paper in more detail. Within the following section the two different approaches of georeferencing of 
image data are described. Although the process of standard AT is quite familiar, some time is spend on the revisitation 
of its main characteristics, which are important for the later direct georeferencing. For the estimation of the empirical 
accuracy potential of direct georeferencing the commercially available POS/DG310 GPS/inertial system developed by 
Applanix Corp. Markham/ON Canada (Lithopoulos (1999)) was tested in conjunction with a standard photogrammetric 
aerial camera. The results of this photo flight carried out over a well-surveyed test field close to Stuttgart, Germany are 
given in Section 3. Finally, the results are summarized and a short outlook on the potential use of direct georeferencing 
for photogrammetric applications is given. 
  
198 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.