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Proceedings International Workshop on Mobile Mapping Technology
Li, Rongxing

iris was fixed and the automatic exposure function set as iris
Figure 2. The zoom CCD camera
The frame grabber used for this project was the MRT PCMCIA I
imaging card. This card can be used directly with a notebook
computer or with a desktop computer via an adaptor. It digitised
both fields of the composite video input and generated a digital
image of 920(H)* 574(V) pixels. Only the luminance signal,
which was quantised into 8 bits, was used for processing.
It is intended that the focus and the iris are fixed for both
calibration and application in order to reduce the possible
combinations to a manageable level. The focus distance is then
needed to be determined before calibration. This is obviously
related to the depth of field, the distance range the system is to be
used for and the object resolution to be achieved. For this
calibration the object resolution was specified as that 1 pixel was
equivalent to 3mm in object. The working distance range was
from 2.5 m to 30 m, which means the zoom was able to be
adjusted to achieve a 3 mm object resolution within this distance
range. To ensure sufficient depth of field throughout for a fixed
iris of f/22 and 0.2 pixel circle of confusion, the focus distance
was calculated to be 25 m. The camera was then fixed at this
focus distance and the f-stop fixed at 22 throughout the course of
calibration. The detail of this derivation is beyond the scope of
this paper. This scenario was designed only for the convenience
of fitting the experiments in the laboratory space. More relevant
scenarios to city survey will be used in future where longer
distances will be assumed.
The calibration was required to determine the interior orientation
parameters, the lens distortion parameters and the exterior
orientation parameters to the telescope system at various focal
settings within the zoom range. It was also required to assess the
accuracy of calibration and the repeatability of the calibrated
values against zoom action.
The camera-on-theodolite calibration method was used for this
calibration. Two targets were set 16 m and 27m away from the
camera theodolite station respectively and almost aligned with
the station, as shown in Figure 3.
Figure 3. The calibration set-up
The telescope of the theodolite was rotated to 25 directions so
that each target was imaged at 25 different positions evenly
spread in the image frame. For each image/telescope position, an
image was captured and the horizontal and vertical angle readings
were noted. For each of both targets, the 25 target images were
located by image processing. The corresponding 25 sets of three
dimensional co-ordinates of the targets with respect to the
rotating telescope system were calculated using the following
- cosv u sin(/i - h u ) N
= D
sin v sin v u + cos v cos v„ cos{h - h a )
cos v sin v o - sin v cos v H cos(/j - h n ) y
In the equations, D is the distance between the target and the
theodolite; h Q , v Q are the horizontal and vertical readings while,
initially, the theodolite is sighting at the target; h and v are these
readings an image is captured. With these corresponding 2D and
3D co-ordinates for 50 points (for both targets), the analytical
space resection with lens distortion models was performed to
determine the interior and exterior parameters. The exterior
parameters here were the camera to theodolite parameters. For the
detail of the camera-on-theodolite calibration method, see the
author's previous papers (Huang and Harley, 1989)
The feature of this calibration method of needing no control array
greatly reduce the cost of the calibration. It does not suffer from
the problem of having too few targets at the zoom in setting as
the test field methods do (Wiley and Wong, 1995). The camera to
theodolite parameters are determined as the natural and direct
outcome of the method. If the control field method had been used
instead, those parameters would have had to be determined by
sighting the theodolite at the targets in the control field. In short,
this method is most suitable for this particular calibration though
it is a general camera calibration method.
A ring shaped target is used as shown in Figure 4. The outer and
inner circles of the ring were used to locate the target centre
independently then the mean was taken. For each circle, the edge
is detected first using automatic thresholding, refined to subpixel
by interpolation secondly and fitted with an ellipse finally
(Huang & Harley, 1990). The discrepancies between the outer
and inner circle determinations are systematic and thought to be
attributed largely to the biased lighting condition. They were
hoped to cancel out each other to a great extent in their mean
value, which was always used as the final target image centre.
Figure 4. The original target image (a) and its edge image (b)
The accuracy of the mean was assessed using a more objective
method - the subpixel shift method. A series of images were
captured. Between two captures, the target was shifted by an
amount equivalent to 0.1 pixel at the image scale. The detected
image positions and the positions-should-be were subjected to a
regression analysis. The standard deviation estimated forms the
accuracy measure for the image co-ordinates. This accuracy
measure reflects the pixel phase effect.