of 10 W halogen lamps which can be easily attached to 
the net by cable clips. 
The whole pen structure consists of different parts, 
which can move slightly with respect to each other while 
the pen is floating in the water. It is impossible to have 
totally fixed control points on the object. Some points 
(lights) are attached to vertical steel pipes which are fixed 
to the pen structure. During the photogrammetric image 
capturing nobody is allowed to be on the pen so that the 
movement will be reduced to a minimum and these 
points can be used as fixed points. Distances between 
the fixed points are measured to obtain a correct scale. 
Distances from the fixed points to the water surface are 
measured to level the coordinate system. The x- and y 
directions of the coordinate system are chosen arbitrarily. 
2.2 Hardware System 
The underwater imaging system must provide high reso- 
lution and sensitivity. Due to the fact that the target 
points are not labelled, a continuous video is appropriate 
to track the points through the different images. 
The system is composed of the following components: 
e Underwater CCD video camera with good low-light 
sensitivity (Micro SeaCam 1050, focal length 2.8 mm 
(air), fixed focus, wide angle, depth of field 10 cm to 
infinity, 1/3 inch CCD sensor (537x595 pixels), 8bit 
grayscale). 
e Cable to carry power and video signals between cam- 
era and surface (2 x 40 m). 
e S-VHS Video recorder unit (Panasonic AG-4700E). 
e Video frame-grabber (Cameron CamDrive, PAL Video 
Signal, 800x600 pixels, PCMCIA). 
e Portable computer (Design-Notebook, 486DX4-100). 
2.3 Camera Calibration 
The focal length of the camera in water is different to the 
focal length in air because of the different optical densi- 
ties. The optical density varies due to the influence of 
salinity, temperature and water depth. Therefore an on- 
site camera calibration in seawater has to be performed. 
There are parameters for linear (scale difference as and 
non-orthogonality a4) and radial distortion (a1 and az) as 
well as the principal point (xo and yo) and the focal length 
(c) (Krzystek, 1995). 
A flat test field with a large number of points (cf. Figure 
1) is used for the calibration. The point measurement can 
be done automatically by least squares template match- 
ing for most of the points. Some are disturbed by reflec- 
tions and had to be measured manually. In the following 
bundle adjustment the calibration parameters are ap- 
plied. The calibration results are shown in Table 1; the 
distortion vectors are shown in Figure 2. 
The calibration results lead to corrected image coordi- 
nates with an accuracy of 3 um (1/3 pixel). 
526 
2.4 Procedure 
Depending on the pen setup, the recording device is 
operated from the pen platform or from a boat. The cam- 
era is connected to the recorder by a 40 m or 80 m video 
and power cable. Since we used an off-the-shelf video 
recorder without special housing etc. this equipment has 
to be protected from sea water and bad weather condi- 
tions. 
The camera is operated by a diver who swims around the 
pen at different depths and distances, panning the cam- 
era left and right as well as up and down. He cannot see 
the image contents since there is no viewer at the cam- 
era. The recording has to be supervised from above, 
where it can be checked using a TV-monitor. This leads 
to rather long recording times and should be improved. 
The recorded tapes are reviewed and appropriate images 
are selected by the opeator. The images are transferred 
to the notebook hard disk as 800x600 pixel TIFF-Files 
(cf. Figure 3) using the PCMCIA frame grabber. With 
these images the standard photogrammetric procedures 
(interior orientation, point measurement, bundle adjust- 
ment) are performed. 
2.5 Digital Photogrammetric System 
The photogrammetric aspects of the CoSMoLUP system 
are based on the PICTRAN system's modules for digital 
photogrammetry. Both systems run on standard PCs 
under Windows 3.1. 
For CCD-cameras the interior orientation is constant; so 
the transformation from the pixel coordinate system to 
the image coordinate system can be written to the im- 
ages directly without using any fiducial marks. 
The PICTRAN modules offer the opportunity to measure 
image coordinates of signalised points manually or by 
using least squares template matching. For most of the 
points the semi-automatic measurement could be used, 
others had to be measured manually because of poor 
visibility (contrast and size). 
The integrated bundle adjustment does not need any 
approximate values for projection centres, image rota- 
tions or object coordinates. In a first step the images are 
oriented by using only a few of the points which can be 
identified properly (in general the fixed points). Since the 
points all look very similar and there is no background 
information in the images it is sometimes very difficult to 
identify the points. In the second step this preliminary 
orientation information is used to display the epipolar 
lines of the measured image points in all of the other 
images. This helps to identify the same object points in 
the different images (cf. Figure 5). After having measured 
all image points the bundle adjustment was performed 
again and the object coordinates of all points determined. 
2.6 Results 
In order to check the internal accuracy measures, two 
different operators determined the object coordinates 
independently - from selecting the images to the object 
coordinate determination. In this test three different pen 
types and sizes where measured, but only with a mini- 
mum number of points on two sides of the nets (not 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996 
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