Remko Wicherson 
  
Such a reduction can be achieved by using an integrated GPS/Inertial system on board of an aircraft, by which the 
camera attitude and position can be measured accurately. In principle this means that the directly measured exterior 
orientation is accurate enough to make the use of ground control points (GCPs) and aerial triangulation obsolete. For 
a highly accurate and reliable product a low number of GCPs and an triangulation are still necessary. 
Gathering dGPS co-ordinates of the camera can already bring a significant reduction in the amount of GCPs 
(Haala et aL, 1998). Because the SD mainly maps elongated objects like highways, the stability of the strip-wise 
configuration (figure 1) is poor and asks for attitude information of the camera. Inertial systems provide this 
information. 
  
  
  
  
  
Figure 1: Strip-wise configuration 
2. GPS/INERTIAL INTEGRATION 
The Global Positioning System (GPS) is used in photogrammetry for determining the camera position for years 
already (Ackermann et al., 1993). Due to recent improvements of the Inertial Measurement Unit (IMU) direct 
determination of the full exterior orientation of the camera is feasible. After integration of both systems, the position 
and attitude data are even better (Cramer, 1999). 
21 GPS 
The Global Positioning System (GPS) is a system that measures the position of mobile GPS receivers using the 
signals broadcasted by the GPS-satellites. If such a system is mounted on an aircraft, it delivers position data of the 
camera indirectly. Differential GPS (dGPS) means that close to mobile GPS receivers, a master GPS receiver measures 
simultaneously. Using phase observations, dGPS delivers relative an accuracy at the centimetre level (Fritz et al., 
1997). 
2.2 Inertial systems 
Inertial systems mounted on an aerial camera provide both position and attitude of the camera. 
Mostly, strapdown-measurement units are used. The axes of these systems are fixed to the platform they are 
mounted on, for example the camera. An Inertial Measurements Unit (IMU) delivers seven output parameters: time, 
three spatial angular and three velocity components. These data can be transformed to position, velocity and 
attitude. The transformed data is called 'Inertial navigation data' or 'INS data'. 
The IMU gathers orientation data which is used for repositioning the camera during the triangulation. Therefore, the 
IMU has to be mounted close to the camera. Normally, the IMU bodyframe is mounted on the camera frame directly, 
or on a bracket close to the camera. The offset between the IMU and camera projection centre (initial alignment) can 
be measured tacheometrically. 
Besides that, the misalignment, the deviation in orientation, between the IMU bodyframe and the camera system has 
to be determined by carrying out a calibration flight (see paragraph 4.2). 
The IMU axes are defined by the earth gravitation and the earth angular velocity. To get an aligned IMU, an 
alignment procedure is carried out before the recording. This procedure consists of two steps. First, roll and pitch can 
be retrieved by flying constantly in a straight line for sufficient time. Second, the heading is defined after making a 
360? curve (Fritz et al., 1997). 
23  GPS/Inertial integration 
A typical IMU records with a frequency of 200 Hz changes in angle and velocity. Out of these data, the position can 
be extracted accurately. However, IMU drifts cause deviations on the long term. Therefore, an IMU is not able to 
determine an accurate position of the camera on its own. 
  
972 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.