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Close-range imaging, long-range vision

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3.1 Testfield camera calibration
Usually a testfield calibration is applied as the standard method
for a photogrammetric accuracy investigation. The IAPG
laboratory is equipped with a number of different testfields
consisting of retro-reflective and diffuse-reflective targets.
Plane 2-D testfields are available as well as 3-D testfields with
spatially distributed points. In order to get a first impression of
the photogrammetric accuracy potential of the camera a 3-D
testfield has been imaged by 76 photos according to well-
established imaging configurations. The camera was equipped
with a 35mm wide-angle lens (a 24mm lens is also available
but not yet tested). The testfield was illuminated by a ring flash
that could not perfectly be mounted with respect to the viewing
direction of the camera due to mechanical limitations.
The test set-up (FIG) consists of two independent 3-D
testfields. While the outer frame (2m x 2m x 2m) serves
primarily as a mounting device for reference scale bars and tie
points, the inner frame (0.8m x 0.8m x 0.8m) consists of
calibrated 3-D target points measured on a high-precision
CMM. The overall dimension was designed according to the
German guideline for acceptance and reverification tests of
optical 3-D imaging systems VDI 2634 that has been developed
over the past years (Luhmann & Wendt, 2000).
Image point measurement has been carried out by two different
programs. Firstly, all points have been measured automatically
by the AICON system DPA-Win. Although the measured
points provide sub-pixel accuracy down to 1/20” of a pixel, all
points are remeasured using the AXIOS Ax.Ima package that
allows for the measurement of problematic points based on a
sophisticated ellipse operator.
Initial bundle adjustment with self-calibration has been
performed by DPA-Win. Due to significant changes in
geometry of the camera over the series of images the program
(as well as other standard bundle adjustments) could not
process the data successfully. It has been investigated that the
sensor back was subject to a severe geometric shift in x'-
direction that could not be modelled by a standard set of
parameters of interior orientation. Although this is a rather bad
result it should be pointed out that the camera has been used as
delivered with no additional fixtures of the camera back, hence
the x'-shift is probably caused by uncareful camera handling. It
is recommended to add suitable mechanical aids to fix the
sensor back with respect to the camera body if no extended
camera model is available in the bundle adjustment.
3.2 Image-variant camera modelling
An extended model for camera calibration has been developed
at IAPG (Tecklenburg et al 2001, Hastedt et al. 2002). Briefly
described, the model does not assume a bundle-invariant
interior orientation, but uses additional parameters for image-
variant shifts of the perspective centre with respect to the
image. In addition, a finite element correction grid is estimated
within the bundle adjustment in order to compensate for
remaining imaging errors and sensor unflatness as well. The
effect of lens distortion is kept fixed for the whole set of images
since it is assumed that distortion is mainly caused by optical
refraction inside the lens, hence should be independent of the
sensor related parameters. Due to the grid correction the
distortion parameters B1,B2 (decentring) and C1,C2 (affinity,
sheering) must not be determined.

principal distance





deviation in mm
62 j
74 5“


-0,02 :

a) variation of principal distance

principal point x
deviation in mm

b) variation of principal point in x'

principal point y
deviation in mm


c) variation of principal point in y'
Fig. 3: Image-variant shift of perspective center

number of images 76
image observations 2984
object points 209
additional observations 1
finite element grid spacing 2mm
grid stability a priori 2um
sigma 0 0.33pm
principal distance c * 35.4377mm
principal point x'o * -0.0256mm
principal point y'o * 0.2942mm
distortion parameter Al -9,80E-05
distortion parameter A2 7,01E-08
distortion parameter A3 -1,18E-11
RMS X 0.008mm
RMS Y 0.008mm
RMS Z 0.011mm

Table 1: Result of bundle adjustment with image-variant
interior orientation and finite element sensor grid
* variant values, here given for image 1