art BS. Istanbul 2004
ging system.
s were such that one
] measurement task
ngles between wall
o illumination. The
s approximately 40
he experiment took
accuracy from the
ence datum the tar-
ymeter. Tachymeter
yrizontal/vertical an-
1easurements was a
no special instrumen-
surements there oc-
tected by tachymeter.
nalysis it was some-
incies between pho-
we a consequence of
and which is due to
spot as tachymeter
aging differed only
coordinate system.
ordinate system con-
with computer con-
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
trolled system with 32 photos per image block and camera
was attached to supporting rod approximately 45cm apart
from the rotation center with perpendicular viewing angle.
The camera settings were fixed to same focus and aperture
values as used in camera calibration. The exposure time
as allowed to be determined by automatic function of the
camera. The camera used in experiment was an Olym-
pus E-10 (1680x2240pix cz2350pix) digital still camera.
The imaging was carried out under artificial illumination
conditions and there was no control how fluorescent lamps
took place on image in an individual shot. For this reason
some images were over exposured and some under expo-
sured. Still, targets were able to be measured on images
reasonably well. The scale bar with length of 2m pro-
vided the scale for the 3-D measurements.
The targeted points were measured on images by applying
image correlation and image LSQ-techniques. On chosen
source image template was extracted in size selected by
the operator. The selection was made in sub-pixel accu-
racy. The best position of template on next image on se-
quence was resolved with largest cross-correlation inside
the search area. The final position was then estimated with
LSQ-estimation with sub-pixel accuracy. This resolved
best position of the template on target image was then used
to extract a new template on this image for matching on
next image. So the template image and target image were
always from subsequent images. This way we could be
sure that viewing angle on these camera poses did not dif-
fer much. This semiautomatic matching continued until
the point was out of sight or occluded. For matching stop-
ping criteria some limit correlation value was assigned. In
order to get same target points measured on the second
block, the previously measured image points were used as
templates in matching on the image of the second block.
The selection and locating the good initial position on sec-
ond block image was given by operator. The best image
pair for this point transfer was evaluated according to in-
itial orientation. Images with nearly identical orientation
angles were preferred.
4 ACCURACY ANALYSIS
The initial values for o-angle parameters were received
from computer controlled imaging system as an output.
The initial values for radius r was measured by simple tape
measure and camera was supposed to be attached to rod
with orientation angels wy = 0, ¢; = O0 and &4 = 0 for
the first block and wy = 0,00» = 180 and x, = 0 for the
second.
At the first stage camera calibration values were fixed. Af-
ter an adjustment the size of residuals and standard de-
viation of unit weight were reasonable. Investigating the
residuals more closely a clear systematic error was visible.
On some images main direction of residuals was upward
and on other images it was downward. At first, the incon-
sistency was suspected to be result of incorrect calibration
values. Therefore, the next step was to to take calibration
parameters with in the adjustment. Although as a result of
this action the size of residuals and standard deviation of
unit weight were substantially reduced, still on some im-
ages the similar systematic pattern was present. Especially
on images where object point distance was larger. In or-
der to observe results as 3-D point coordinates, photogram-
metric measurements were transformed to same coordinate
system with reference data with rigid 3-D transformation.
Big discrepancy between point sets was evident, especially
on points far away. The previous residual inspection indi-
cated clear inconsistency in image information. So this
kind of occurrence of differences was quite presumable.
Based on these adjustment computations a suspicion of
camera after all not lying on one plane during imaging
arose. More support for this theory came when discovering
that similar phenomenon had been reported to appear with
panoramic cameras (Parian and Grün, 2004). The “tum-
bling" motion was explained to be caused by an incom-
plete shape of ball bearing and the contacting surfaces. To
get evidence of existence of this type of phenomenon, the
combined image block was calculated as photogrammet-
ric free network adjustment. In this adjustment the num-
ber of parameters was n * 6 + m * 3 where n denotes the
total number of images in block and m the number of ob-
ject points. The convergence was achieved after few hun-
dred iteration rounds and size of residuals and sigma was
essentially smaller than on previous attempts. We have
to remember now that this image block was free of con-
straints between camera orientation parameters. So param-
eter values were free to adjust according to image infor-
mation. Looking more closely at precision values and re-
liability of parameters it was obvious that system was not
capable of finding parameter values reliably with such an
imaging geometry and there was clear evidence of over-
parametrization. However, by examining the height values
of projection centers we can notice clear fluctuation from
the nominal height (Figure 5).
1.37 (77-0609 Am A AN TET II EE Ts TEEN
LAG Blockl ^ 77
ze Block I...
1.35 =
— * IN;
g 5 f
= x = / 3
e pLME/ A A x 1
2 / e z Ness
= x ns z " > P —
1.33 1 N . / NF
\ vi soni
\ /
1.32 + = 1
1.318: ———— leen eite been E late anie este eren aee SN
0 50 100 150 200 250 300 350
G— angle (Deg)
Figure 5: The fluctuation. of projection center y-
coordinates after free net adjustment. Values are depicted
according to two separate image blocks.
This was convincing enough to improve the mathematical
model to include also this kind of variation on camera height
values. So an additional parameter 0 was added to the
model to describe the height difference from the nominal
plane of rotation on each camera pose. The number of par-
ameters was increased by n — 1 from the original set of par-
ameters in Equations 1 and 2. Only first camera of the first
block was supposed to have a fixed 0-value defining the
