RATA OF 
close range 
ons showed 
atability of 
> matching. 
le projector 
nera model 
comparable 
placed over 
ce area and 
  
Optimised pillar size 
1 (plan and 
ent in poor 
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(Figure 3) 
| size, light 
ffective way 
ring targets 
e laser dots 
  
Figure 2: Cross section of deforming sandstone roof strata. 
can be used during three dimensional measurement. Further 
this system of projection provides sufficient depth of field to 
accommodate the range variation during this measurement. 
The centroid of the large number of distinct targets (laser 
dots) can also be located precisely through different suitable 
algorithms and so the accuracy problem of line projection or 
shadow profilometry does not exist. The laser projector was 
  
Figure 3: A laser dot matrix projector, overall 8 cm long and 
1.7 cm of diameter. 
rigidly attached with the telescope of a geodimeter (Figure 4) 
for accurate calibration of interdot angles among the laser 
dots and other measurement purposes. 
  
Figure 4: Placement of the laser dot matrix alongside the 
telescope of geodimeter. 
1.3 Design constraints 
A detailed consideration of the impact of the mining environ- 
ment revealed that a CCD based system can be deployed for 
data acquisition during partial extraction of coal under mas- 
sive sandstone strata. The design of a close range digital pho- 
togrammetric system for industrial measurements involves a 
541 
number of factors including imaging geometry configuration, 
storage of digital data and field of view/resolution problems 
due to the small format of the commercially available CCDs. 
A detailed discussion about network configuration task is pre- 
sented by Mason (1995). The analogue signal acquired dur- 
ing industrial measurement by a solid state camera creates 
large amounts of data and a host computer is required in the 
vicinity of the camera for frame-grabbing and storage. Al- 
though a number of alternatives like PCMCIA and DCS 420 
(Kodak) cameras are coming up as a solution to this prob- 
lem the readily available camcorder may also be considered 
as an Intermediate data storage device in the industrial en- 
vironment. Performance testing of a camcorder as a data 
storage device [Singh et al., 1995] provided subpixel image 
co-ordinate accuracy with the help of a suitable image co- 
ordinate measurement algorithm. The dot matrix projector 
fixed rigidly with the telescope of a geodimeter is combined 
with another arrangement (Figure 5) in which a CCD camera 
is placed over a motor driven turning and tilting system to 
overcome the field of view problem and automatic derivation 
of three dimensional information of the object space. 
  
  
Figure 5: Turning and tilting based scanning system. 
CCD based remote inspection and measurement have enor- 
mous scope in hazardous areas [Chapman et al., 1992] but the 
limitations of the stereomatching techniques during identifi- 
cation of corresponding points in stereopairs at close range 
provide a way for active triangulation techniques for such 
measurements. Light projections have been practiced a num- 
ber of times for different automatic measurement purposes 
like robotic vision [El-Hakim, 1985] and measurement of 
dynamic textureless objects [Ethrog, 1991]. Unfortunately, 
these systems involve inconvenient calibration procedures for 
a photogrammetrist. Furthermore, until recently, no suitable 
projection method was available for remote placement of high 
density spatially stable measuring points. In this work the 
projector is treated as a virtual camera and a camera model 
for the projector is developed using bundle adjustment. Pre- 
cise knowledge of the orientation parameters of the camera 
and the projector is used for three dimensional object space 
information by intersection of the known orientation of each 
dot of the projector and their respective measured image co- 
ordinates. 
2 CENTROID LOCATION 
To meet the level of accuracy of close range industrial survey- 
ing/inspection from low resolution images of a commercially 
available CCD camera, subpixel image co-ordinate measure- 
ment becomes important. Most widely used algorithms to 
measure centroid location of a target point are based on a 
thresholding technique. Targets placed over the object sur- 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996