environment, with limited access and severe instability, the
design and testing of a maintainable co-ordinate system is of
critical importance. In order to obtain the desired sub-
millimetre accuracy, a complex configuration of co-ordinate
reference points and camera stations is required. The
photogrammetric design relies on two types of control points
to maintain the reference co-ordinate system throughout the
entire photogrammetric model, these are co-ordinated points
on the reference frame and tie points (points required for the
transfer of co-ordinate control from one stereo-image set to
the next). These points must be configured to provide an
adequate geometry to ensure accurate determination of the
camera station positions and orientations and thus must be
able to withstand the impact of the pre-conditioning blast.
In order to realise this objective, a stable reference co-ordinate
system is required. The use of a pre-constructed reference
frame, consisting of steel tubes and a steel back plate with
attached circular retro-reflective targets to define the co-
ordinate system, provides a means of achieving this. Due to
the limitations of the mining environment, the maximum
reference frame size tolerated is 0.5m x 0.5m x 0.2m.
The only stable structures within the stope (the working area
at the rock face in the mine) which are capable of maintaining
stability during the pre-conditioning blast, within the required
accuracy, are the wooden packs which support the overhead
"hanging wall". This poses a substantial problem as it restricts
the reference co-ordinate frame, attached to a pack, to being
placed at right angles to the rock face being measured. In
order to compensate for the poor geometry and diverging
imagery resulting from this, a complex configuration of
control points is required for the establishment of accurate
point co-ordinate on the rock face itself. Figure 1, below
shows a diagrammatic description of the configuration
required for transferring the co-ordinate system from the
reference frame to the rock face.
forward facing image
---: backward facing image
8 camera stations
® target points
Figure 1 - Camera and Target Pole Configuration
The adopted design is based on the acquisition of images at
two height levels. During the image acquisition, the camera
is moved sequentially from station to station, beginning with
the reference frame and following the wall until the entire
deformation area is covered. Multi-images at each station are
taken while transferring the co-ordinate system from the
reference frame to the face, through an angle of 90 degrees.
During image acquisition the operator is faced with the
difficult task of guaranteeing full multi-image coverage of the
729
face, while hand-holding the camera in the physically very
demanding and constrained environment.
3. IMAGE ACQUISITION
In the interest of maximum portability of the measuring
system, it was decided to rely on the use of a digital still
camera. This decision was taken after first attempts with a
CCD video camera, linked to a frame grabber in the docking
station of a laptop, proved extremely cumbersome and
impractical. The Kodak DCS420m was found to be a suitable
image capture device, both portable and able to withstand the
harsh environment of the deep-level mine, while satisfying all
safety condition for electronic equipment. The DCS420m has
a solid state CCD sensor with a resolution of 1524 x 1012
pixels on a 14mm x 9.3mm chip (9um x 9um per pixel). This
camera stores up to 65 black and white images on a single
PCMCIA Type III hard drive (105Mb capacity); the drives
are interchangeable and manufacturers claim the capture of
up to 1000 images is possible on one battery charge.
Due to the constrained space available in the stope, a very
wide angle lens is required to obtain the maximum field of
view. For the photography, a 14mm lens was employed,
Which is the approximate equivalent of a 28mm lens with a
conventional 35mm camera. As the CCD sensor only
samples the centre part of the image created by the lens, the
fish-eye distortion typical for a 14mm lens, at the image
edges, has little negative effect and can be well modelled with
the lens distortion model.
As there is no readily available power source in the stope,
image lighting needs to be flash generated. For this purpose,
the Nikon SB-20 Speedlight provided a more than adequate
solution.
3.1 Pre-Site Preparations
Before entering the stope, it is necessary to pre-calibrate the
camera and determine the co-ordinates of the reference
control frames to be used.
3.1.1 Camera calibration must be carried out prior to
entering the mine. Throughout the process of image capture
the camera must remain at the fixed and pre-calibrated focal
length. This is vital to the analysis of the data after image
capture, as no self-calibration procedures are possible.
For the image capturing process, the camera must be focused
to a suitable distance for the stope conditions. This typically
varied between 1.5m and 2.5m depending on the closure
within the stope. Once the focal length is set, the lens
focusing ring is taped into position to prevent movement
during the image capture process.
Calibration of the interior orientation parameters of the
camera, including principal distance, principal point and lens
distortion parameters, is achieved by the capture of multiple
images of a calibration control frame, from differing
perspectives. Semi-automated analysis of the calibration
images by target centring and target identification algorithms
(described below) and a constrained bundle adjustment
provides the desired parameters.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
