CRACK MONITORING USING A DIGITAL CAMERA AND AN IMAGE
TOTAL STATIONJong Hyeok JEONG and Masataka TAKAGIKochi University of Technology

ABSTRACT: There are many bridges in Japan due to geographic characteristic. Japanese government
spends much budget for the maintenance of them. If the crack monitoring of bridges is possible, maintenancecycle of bridges can be expected with high accuracy. Therefore accurate maintenance cycle of bridge
structures can be calculated based on the crack monitoring data. Kochi University of Technology isdeveloping crack monitoring system using an eight mega pixel SLR digital camera. To survey cracks on abridge structure, it is necessary to attach control points on the bridge structure. In some cases, it may beimpossible to attach control points, if bridge structure height is too high. An image total station was used tosolve the problem because image total station can acquire coordinate of a target with an image which
describes the measured point. The images which have measured positions were used as control points for theexterior orientation of the digital camera. The corresponding points of image control points in stereo images
were matched by least square matching method. The space intersection of the stereo image to calculate three
dimensional coordinates of cracks was carried out by the least square matching. The accuracy of three
dimensional measurement was evaluated by the image total station measurement. Millimeter accuracy could
be obtained in the three dimensional crack measurement using the digital camera and image total station.
KEYWORDS: Close range monitoring, GIS, Spatial database,
1. INTRODUCTION

An image total station was used in this study tosolve the problem because image total station canMany bridges are exiting in Kochi prefecture due toacquire coordinate of a target with an image which

the geographic characteristic of Shikoku Island,

describes the measured point.

Japan. The bridge structures located near the sea are

The images taken by image total station were usedmuch suffered from Nacl from the sea. Japan is

as control points (ICP: image control point) for thespending much budget for the maintenance ofexterior orientation of the digital camera. After that

infrastructures. Therefore deciding maintenancethe orientation results was evaluated by comparing

cycle of infrastructure is very important.

their three dimensional measurement accuracy withimage total station measurements.

To survey cracks on a bridge structure using a
digital camera, it is necessary to attach control

The objective of this study is to evaluate thepoints on the bridge structure. In some cases, it may

photogrammetric measurement accuracy of crack

be impossible to attach control points if bridge

measurement using image control points taken by

structure’s height is too high.

an image total station.

2. MEASUREMENT DEVICES

Figure 1. Radial lens distortion

2.1 Digital Camera
In this study, eight mega pixel resolution SingleLens Reflex (SLR) camera which produced bycannon was used for photogrammetric measurementof a concrete structure. Table 1 shows thespecification of the camera used.

Figure 2. After the correction of lens distortion

Table 1. The specification of cameras used

A pair of stereo images was taken by the camera.

CCD size
22.7×15.1mm

Focal lengthResolution

Figure 3 shows left hand side image. Figure 425mm

3072 × 2048

shows right hand side image.

2.2 Image Total Station
Total station is a combination of theodolite (anglemeasurement) and an electronic distance measuring
device (EDM). Therefore, total station enables us tomeasure angle and distance at the same time.
Image total station can measure distance and angle
with a picture which describes the measured point.
Figure 3. Left hand side image
3. CAMERA CALIBRATION
Camera calibration is seriously necessary process
for using a camera in a measurement. The camera
calibration was carried out by using Photo modeler
4.0. the coefficientsof lens distortions werecalculated by the Photo modeler.
Most of commercial camera shows serious radiallens distortion. Figure 1 shows an example of lensFigure 4. Right hand side image

distortion. Figure 2 shows how the lens distortion
corrected.

4. THE CALIBRATION OF IMAGE TOTALSTATION
4.1 Noise reduction
Photos produced by the image total station contains
stripe noise, therefore the noise should beeliminated. Figure 5 shows an example of stripe

taken, then the amount of brightness offset wasnoisecalculated. Figure 7 shows the brightness offset
The stripe noise could be eliminated by averaging

image.

pictures by redundant measurements (Equation 1).
N∑ image
Averaged image=ii =1

N

Equation 1. Calculation for averaged image
where N is number of images.

Figure 7. Brightness offset image
In adjustment, pixel value of brightness offset
image was added to a target image. Figure 8 showsa target image to be adjusted. Brightness values offigure 7 and 8 were combined. Figure 9 shows theresult of bright adjustment of Figure 8.
Figure 5. Photo taken by the image total station

Figure 6 shows the noise reduced image. The stripe
noise could be removed by averaging the images
taken by the image total station.
Figure 8. A target image to be adjusted by thebrightness offset image.

Figure 6. Noise reduced image
4.2 Brightness adjustment

Figure 9. After brightness adjustment of the Figure

The corners of the pictures taken by the image total

8station are darker than inside. A white object was5.

THREE

DIMENSIONALCRACK

MEASUREMENT

gt: target window
gs: template window
e: error vector

Accurate three dimensional measurement depends

h0,,h1:on 1)

adjustment

the accuracy of image matching and 2) theaccuracy of exterior orientation of a camera.

unknowncoefficientsfor

brightness

a0,a1,a2,b0,b1,b2: unknown coefficient for geometrictransformation

Totally four image control points were taken by theimage total station. Figure 10 shows the four image

This equation is a nonlinear equation; therefore thiscontrol points and an image taken by the digital

equation was linearized by Taylor series. Initialcamera.

values for unknowns were input to the linearized
equation, then iterative calculation carried out until

5.1 Least squares matching

the correction of the initial input minimized to befall in global optimization.

For exterior orientation of a camera, control points
are required. The control points were measured by

Minimum four control points were required for theimage total station. Corresponding points ofexterior orientation of a digital camera, leastmeasured control points in photos taken by a digitalsquares image matching of four image control

camera were matched by least square image

points were carried out.

matching method.
5.2 Exterior camera orientation
For accurate three dimensional measurement, theaccurate position and attitude of camera is required.
The position and attitude of camera is calculated bythe collinearity equation (Equation 3).
The collinearity equation means that the object inimage coordinate space and the object in object
Figure 10. Image control points taken by the image

coordinate space are existing in a ray.

total stationx + Δx = − fLeast square matching method searches thea11 ( X − X o ) + a12 (Y − Yo ) + a13 ( Z − Z o )a31 ( X − X o ) + a32 (Y − Yo ) + a33 ( Z − Z o )corresponding points by changing the shape andbrightness of template image. Equation 2 shows they + Δy = − fmathematical description of least squares matching.

g t ( x, y ) =a21 ( X − X o ) + a22 (Y − Yo ) + a23 ( Z − Z o )a31 ( X − X o ) + a32 (Y − Yo ) + a33 ( Z − Z o )Equation 3 collinearity equation

h0 + h1 × g s [(a 0 + a1 x + a 2 y ), (b0 + b1 x + b2 y )]Equation 2 least squares matching equation

wherex and y: photo coordinate;f: focal length

6.

Δx, Δy : the correction of lens distortions;

RECONSTRUCTIONTHREE

DIMENSIONAL

MODEL

X, Y and Z : the coordinate in object space;
Xo, Yo and Zo : the coordinate of a camera; i isAfter exterior orientation, three dimensional model

point number;reconstruction carried out. The three dimensional

a11 = cos ω cos φ ;coordinate of a target object could be obtained by

a12 = cos ω cos κ + sin ω sin φ cos κ ;intersecting normal vectors from the two exterior

a13 = sin ω sin κ − cos ω sin φ cos κ ;oriented camera stations (Figure 11).

a21 = − cos φ sin κ ;a22 = cos ω cos κ − sin ω sin φ cos κ ;a23 = sin ω cos κ + cos ω sin φ sin κ ;a31 = sin φ ;a32 = − sin ω cos φ and a33 = cos ω cos φ .

Three dimensional crack measurement was carriedout using the digital camera, then the result wascompared with the measurement result by the image
total station.

The knowns are image coordinate and object spacecoordinate of a target object, and unknowns are theposition and attitude of camera.
This equation is nonlinear problem; therefore theequation should be linearized in Taylor series. The
optimum

initials

wereinput,

theniterative

calculation was carried out.
Figure 11. Three dimensional model reconstruction
Table 2 and table 3 show the orientation results of

by intersecting normal vectors from the tworight image and left image.

oriented camera station.

Table 2 The orientation result of right image

Figure 12 shows measured point of the first crack

X(m)target by the image total station. Table 4 shows the0.010657
Omega(degree)
72.13485

Y(m)

Z(m)

-0.887418
Phi (degree)-7.799178

0.455425
Kappa

maximum 0.48mm measurement error was shown(degree)in this measurement.

0.379806

Table 3 The orientation result of left image
X(m)-0.219093
Omega(degree)
73.451863Y(m)measurement evaluation of the first crack target:

1Z(m)-0.92756

Phi (degree)-16.433997

0.45729
Kappa(degree)-4.582743
Figure 12 Crack target 1 measured by the image

total station.
Table 4. Evaluation of crack 1 measurement by thedigital camera
TRUE
By image control
Error

X (m)
Y (m)
Z (m)
0.122390.40169
0.145390.123630.397010.14054-0.00124
0.00468
0.00485Figure 14 Crack target 3 measured by the image
total station.

Figure 13 shows measured point of the second
crack target by the image total station. Table 5

Table 6 evaluation of crack 3 measurement by theshows the measurement evaluation of the first crack

digital camera

target: maximum 0.56mm error was shown in thismeasurement.

TRUE
By image control
Error

X (m)
Y (m)
Z (m)
0.16358
0.39723
0.12178
0.16487
0.39845
0.11614-0.00129 -0.00122
0.00564

7. CONCLUSIONSIn this study, crack measurement method using adigital camera and an image total station wasintroduced.

Crackmeasurement

using

imagecontrol point taken by an image total station
Figure 13 Crack target 2 measured by the image

showed millimeters accuracy.

total station.
Table 5 Evaluation of crack 2 measurement by the8. REFERENCES

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X (m)
Y (m)
Z (m)
TRUE
0.179880.397770.12704
By image control 0.18251
0.405810.12043
Error-0.00262-0.00804
0.00661

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Figure 14 shows measured point of the third crack

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