CIPA 2003 XIX"' International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
carrying out a calibration procedure after mounting the
camera. This calibration routine is described in detail
below. As soon as the scan position is registered in the
project coordinate system the image information can be
completely utilized.
The mounting calibration information specifies the 6
degrees of freedom of the camera's coordinate system
with respect to the scanner's coordinate system, i.e., 3
rotational parameters and 3 translation parameters.
Within RiSCAN PRO the mounting calibration is
stored as a 4 x 4 matrix for convenience of definition
and of application. The mounting calibration matrix
transforms from the scanner coordinate system into the
camera system in case the rotational part of the scanner
is at the position of phi equal to 0 deg. The additional
rotation at an other angle is modelled by the COP
matrix (camera’s orientation and position matrix)
stored with every image of Type 1 reflecting the phi
angle.
The three translation parameters are well defined and
are determined for a specific camera model and lens
after final calibration at the manufacturer's premises.
Mounting and de-mounting the camera do not change
these three parameters. The three parameters describe
simply the coordinates of the center of the scanner's
coordinate system at phi equal 0 deg within the
camera's coordinate system.
4 Calibration tasks
4.1 Laser sensor calibration
The laser sensor itself has not to be calibrated by the
user. As the sensor accurately measures all geometrical
information, i.e., range and the angles, no calibration
tasks with respect to the laser data have to be carried
out.
4.2 Calibration of internal camera
parameters
For a camera with a wide-angle lens calibration of the
camera is done by taking images of a flat regular
structure with well-defined dimensions, for example a
flat black-and-white image of a check pattern. A series
of images are taken covering in total the complete
field-of-view of the camera.
For a camera with a telephoto lens the method with the
check pattern becomes impractical. In order to get an
image in focus the distance of the camera to the check
pattern has to be quite large. In this case it is possible
to base the camera calibration on identifying tie points
in the calibration images which are arbitrarily
distributed in 3D with known 3D coordinates. Again,
by taking a series of images covering in total the
complete field-of-view with a nore or less uniform
distribution, the parameters can be determined
accurately.
4.3 Mounting calibration
The three rotational parameters of the mounting
calibration have to be optimised after each mounting of
the camera. This can be done directly after mounting
the camera and taking both scan data and an image
sequence, but can also be done off-line after taking all
data of the data acquisition campaign. This calibration
task is based on identifying at least two tie points in the
images taken at one scan position with well-known 3D
coordinates in the scanner's own coordinate system
(SOCS).
As the mounting calibration has to be carried out after
each mounting of the camera RiSCAN PRO manages a
large number of mounting calibrations within a single
project, although most projects can be handled with a
single mounting calibration.
5 Example project
Data have been acquired with a RIEGL LMS-Z360 3D
Imaging Sensor combined with a Nikon D100 camera
with a Nikkor 14 mm lens. The key features of the
instrument are summarized in Table 1.
3D Imaging Laser Sensor RIEGL LMS-Z360
Measuring range
2m up to 200m
Range measurement
accuracy
12 mm
Laser
0.9pm/Class 1 (eye safe)/3 mrad
beam divergence
Measurement rate
8000 -24000 points/s
Scanner performance
Scan range
Up to 90 deg x up to 360 deg
Minimum scan step width
0.004 deg
Angular resolution
0.0025 deg
Physical data
Main dimensions (Lx0)
490 mm x 210 mm
Weight
approx. 13 kg
Power supply
12-28 V DC, 4 A @15 V DC
Temperature range
Operation: -10°C to +50°C,
Storage: -20°C to +60°C
High-resolution camera system
Camera type
Nikon D100
Resolution
3008 x 2000 pixel, pixel size 7.8 pm
Lens
Nikkor 20 mm
Table 1: Key parameters of the data acquisition system used in the
example. For a more detailed description of the laser measurement
system see www.riegl.com and (ULLRICH 2001).
The object is the arena in Verona, Italy (compare
Figure5). Data have been acquired from only four
positions. Data acquisition has been done in
cooperation with Instiuto Universitario di Architettura
di Venezia, Prof. F. Guerra. At every scan position at
least 9 images have been taken with fixed focus and
fixed aperture. For merging the data numerous retro-
reflecting targets have been posed in the scene
