2.2 UDPS Configuration
The system consists of four components: data acquisition, data
transmission, data recording and data control (figure 2).
[Cala video camera] Data Transmition Data Recording
SVS wie 2
Figure 2. UDPS configuration
2.2.1 Data Acquisition
One color video camera and two color CCD digital cameras are
mounted on the ROV, each with a housing cover. The video
camera is used to navigate the ROV. The CCD cameras are
used to capture the desired objects for measurements which are
two Sony ultra-small interline CCD color cameras with an
image size of 768 x 494 (picture elements). The sensing area is
6.4mm x 4.8mm. The external synchronization frequency in the
vertical direction is 59.94+0.0009Hz, and that in the horizontal
direction is 15734.264+0.016Hz. Four electronic shutter
speeds are available: 1/60s, 1/50s, 1/100s and 1/1000s. Focal
lenses from 3.5mm to 12mm can be chosen. Each camera 1s
housed and covered with a flat front cover lens for waterproof.
An artificial illuminating source using two Halogen lamps
(USHIO, Japan) is also provided.
2.2.2 Data Transmission
Currently, the tethered transmission mode is applied. The
fiber-optic transmission will be a highly recommended option
for the future use which supplies a maximum signal quality for
real time transmission. The assessment of the degradation of
signal quality during the transmission and the effect of signal
compression remain as two important topics to be researched.
2.2.3 Data Recording and Retrieving
Two Sony CVD-100 computer controlled Hi-8 video decks are
applied to record image sequences from the two CCD cameras,
respectively. The exposure time of the two cameras 1s
synchronized and the synchronized time-tags are
simultaneously recorded. An image recording and retrieving
system, called IOS2-EYE, is implemented. It has utilities
especially designed for retrieving the large amount of
underwater stereo image sequences, such as image selection
for measuring a specific object. Individual digital image pairs
can be grabbed during the survey in real-time and can also be
converted from the analog images.
2.2.4 Data Control
This control component serves as a center unit. Two monitors
are connected to the video camera and CCD cameras. The
operator can control the ROV to capture the images of the
desired objects, with the help of viewing the color images from
the video camera. The ROV can be controlled manually by
operating a control panel.
3. UNDERWATER PHOTOGRAMMETRIC MODEL
In underwater photogrammetry, the propagation of the light ray
from an object point to its image point is refracted at different
media surfaces. Thus, the central perspective imaging
geometry does not hold anymore. As shown in Figure 3, the
systematic displacement of the ray on the image plane caused
by the medium refraction is significant and not negligible. In
the following, two models addressing the medium refraction
problem are described. A medium refraction correction formula
for our imaging system is derived.
3.1 Reduced Central Perspective Model
The objective of this model, called reduced central perspective
model, is to compensate the refraction by corrections applied
on the image plane so that photogrammetric principles using
central perspective geometry can be used. Illustrated in Figure
3, if the correction Ar is obtained (Ar —pp "), a pseudo central
perspective light ray can then be determined. Based on the
imaging system, the image correction for the medium refraction
is derived.
Let Ar=pp” and r=Cp. As shown in figure 3, we have
Ar/r - PP"/P"N — (PP'«P'P"y P"N
PP’ = p2p2’ = d (tan0;-tan 0,
P’P” = H (tan0;-tan 63) (1)
À TN Object poit P ir pS
| 1 : v.
|
H 7
/
0 4
3 =
ny(water)-1.34 i j
O. / js
2 d P; É
; = P
02, 7I +
n;(glass)-1.65 d ;
; Cover lens
O, / P
n,(air)-1.00 8
Exposure center
Image plane
Figure 3. Underwater photogrammetric model
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
Furtherm
The corre
where D
point pos
The incic
and the
refractioi
Incorpor:
equation
where,
and ky |
distortio
media re
It shouk
correct
needed.
32 3D
Within
arbitrar
propaga
by suc
characte
trigonor
chosen
derive 1
can det:
or inde
In Figui
P; (X,
surface
