Figure 1: The Hawkeye M-3 high-resolution, digital camera 
system. 
For coarse positioning of the airplane and navigation 
purposes we installed a Navstar real-time differential GPS 
system. This unit sends the base-station corrections via a 
radio link to the airplane's receiver, so that an improved 
position is immediately available. As pseudo-ranges are 
measured the accuracy of this method is in the range of 1-5 
meters, which is sufficient for many remote sensing 
purposes and tagging the images in a data-base. 
Finally, the system is being controlled by a board level 
PC, which is installed with most other components in a 
portable box. We do not use a hard-disk in the airplane, as 
the storage requirements during operation are minimal; 
instead a small RAM-disk was included. The PC software 
receives time signals from the GPS receiver, it computes the 
corresponding exposure time of the camera and activates the 
shutter. The user interface consists of a flat-screen with a 
touch panel overlay. It displays information from all 
sensors, as well as a small video image of the area covered 
by the most recent exposure. As there is no keyboard the 
user simply selects functions by hitting buttons on the touch- 
Screen. 
A diagram of the full MapCam system is shown in figure 
2. Notice that the GPS units are exchangeable dependent on 
the application and its accuracy requirements. This 
equipment was installed a Cessna 207 aircraft owned by the 
Ohio Department of Natural Resources. 
  
  
- frame capture board 
digital input mmm] 
16 MB 
ALU (enhancement) 
    
   
   
  
  
  
  
image bus 
  
  
Ee, 
data 
compression 
JPEG 
  
  
  
  
  
SCSI 
du 
Storage 
digital 
tape 
  
  
time tag 
  
   
  
—C7o""7"BoQn."9£gtwg. 0 
  
  
  
  
PC- AT 
Control 
GPS multi-tasking 
base-station 
  
  
Figure 2: Layout of the complete MapCam system. 
3. SYSTEM CALIBRATION 
Before MapCam can be used for aerial mapping all 
sensors (Hawkeye M-3, GPS) must be calibrated. We need 
to determine the geometry and distortions of the camera, the 
offset of the GPS antenna's phase center from the 
perspective center of the digital camera, as well as any time 
delay between the shuttering of the camera from the PC and 
the actual exposure of the image. 
3.1 Camera Geometry 
As the Hawkeye M-3 uses major components of an 
amateur camera (Nikon F-3), its interior orientation is 
unknown. We assume that the CCD sensor itself is free of 
distortions, such as unflatness or irregularity of pixel- 
spacing. The manufacturer of the sensor (Kodak) specifies 
the size of its square pixels as 16 microns. This results in a 
sensor size of 20.48 mm x 16.38 mm, which is smaller than 
regular 35 mm film. Therefore, we have to use a fairly short 
focal length to get wide angle coverage. We chose a 20 mm 
lens which was focused to infinity. The focusing ring was 
taped at that position to avoid changes of the interior 
orientation. 
Using a 3-dimensional test-field, which was established 
on a building at The Ohio State University, we performed a 
pre-calibration of the camera. Its major purpose was to 
determine a good approximation of focal len gth and principai 
point, as well as to estimate lens-distortions. The pre- 
calibration was computed by standard analytical techniques: 
the bundle-solution with self-calibration and additional 
parameters. The results of this adjustment show the great 
potential of a high-resolution digital camera; the bundle 
solution also considered radial distortions which can 
considerably improve the results when corrected (Table 1). 
We will apply these parameters for aerial triangulation to 
find out, if self-calibration is necessary for each block of 
images. 
3.2 Antenna Offset 
The vector between the perspective center and the phase- 
center of the GPS antenna must be derived. This offset can 
be included in the aerotriangulation as control information. 
The offset vector must be determined in the image coordinate 
system (figure 3). It is applied to bundle adjustment by 
constraint (2) which relates a perspective center Oj with the 
corresponding GPS position Gj. The offset vector AQ is 
transformed into the ground coordinate system by the 
rotation matrix R;. 
Gi - Oi * Ri (AR) AO (2) 
with: Gi... GPS position for exposure station i, 
Öfen perspective center of image i, 
Biocon. rotation matrix of image i, 
AR; correction for camera leveling if recorded 
during the flight, 
a... offset vector in the image coordinate 
system. 
The offset vector is tied to the motion of the aircraft. If 
the camera is attached to a mount which can be leveled 
during the flight, which means that the direction of the vector 
changes relative to the image coordinate System, it must be 
multiplied by another rotation matrix AR; that considers 
these attitude changes. AR; can only be computed, if the 
angular changes caused by leveling the camera mount are 
automatically recorded during the flight. In our experiments 
we did not change the camera mount during operations, so 
that the initial calibration was maintained and AR; could be 
omitted.