Accuracy Evaluation of Laser Scanner Data Depending onLocation of GCPs for Monitoring Landslide
Kazu KINOSHITA*, Tomonori MIYAZAKI** and Masataka TAKAGI***
Kochi University of Technology
Kami-shi,Kochi,782-8502,Japan*105506p@gs.kochi-tech.ac.jp** 115120t@gs.kochi-tech.ac.jp***takagi.masataka@kochi-tech.ac.jp
KEYWORDS: Ground Control Point, Laser scanner, Geometric transformation
ABSTRACT: Laser scanner is expected as a useful measurement device for monitoring
landslide. Laser scanner can acquire three dimensional data in a short time, in widearea. The data of the time series are needed to measure the movement phenomenonsuch as landslides. Therefore, the geometric transformation that makes the same
coordinate system for change detection becomes very important. Accuracy of geometrictransform is depending on the types of control point and location of control points. The
prism, the reflector sheet, and the multi surface measurement were used as control
points in this study. The multi surface measurement showed accurate in in-door
experiment. However, accuracy of the multi surface measurement is no good in the out
side experiment. The reflector sheet and multi surface measurement showed including
systematic error. When the systematic error is eliminated, stable accuracy will beexpected. Method to cancel the systematic error must be developed.

1 BACKGROUND
1.1 Observation of landslide

A landslide is a phenomenon of mass movement on the ground, which moves 0.01mm to10mm/day in the wide area. Current monitoring systems are used expansion gauge,
inclinometer or GPS. Those monitoring systems for landslide displacement canmeasure at some points or along lines. It is difficult to measure whole landslide
because of very wide area. Laser scanner is expected as a useful measurement device
for monitoring landslide. Laser scanner can acquire three dimensional data in a shorttime, in wide area. For the extraction of landslide displacement by using laser scanner,
millimeter accuracy is required.

1.2 Used Laser scannerIn this study, LMS-Z210 produced by Riegl was used as laser scanner. Maximum
measurement range by the laser scanner is 350m. Accuracy is about 2.5 centimeter instandard deviation. Table 1-1 shows Performances of measurement distance.

Table 1-1 Performances of measurement
Range

≦350m
(reflectance≧80% of objects)
Range
≦150m
(reflectance≧10% of objects)
Shortest Distance 2mDistance
Measurement±2.5cm
Accuracy(Standard Deviation)
Laser Wavelength 0.9μm
Wavelength(Infrared)

1.3 Control Point for Geometric Transform

Time series data must be prepared for monitoring landslide. The each data should begeometrically transformed in order to become same coordinate system for comparing
each other. Therefore control point must be set up in the test field. Figure 1-1 shows thekinds of the control point for laser scanner. Currently, there are three kinds of control
point for laser scanner such as prism, reflector sheet and multi surface measurement.

Figure 1-1 kind of the control point1.3.1 Prism
The prism is often used as a control point. The prism can be strongly reflected laser
light that can be set up very far point from laser scanner such as 300m. However,coordinates of the control points in laser scanner data cannot be accurately extracted.
The coordinates of the same control point have 10 cm error in maximum when the data
were acquired several times in same condition. It was very big error comparing withlaser range accuracy (2.5cm).
1.3.2 Reflector Sheet
It is difficult to use a lot of prisms because it was expensive. Therefore, a lot ofreflectors sheets are expected to use because of cheap. However, reflection strength islower than the prism, so the reflector must be set up in 100m distance. from laserscanner.
1.3.3 Multi Surface Measurement
Authors suggested multi surface measurement to keep accurate control point last year.

The accurate measurement of control points can be improved by measuring some flatsurfaces. When three flat surfaces are measured, coordinates of the intersection inequation of three planes can be calculated. This intersection can be used as a control
point of the laser scanner data. Geometric transformation by using multi surface
measurement was high accuracy. However, there are limited spaces to set up thesurface in the landslide field to keep enough accuracy. When the flat surface set up in thelong distance, very wide board must be prepared.

2 OBJECTIVESThis study is focusing on accuracy of geometric transform. Accuracy of geometrictransform is depending on the types of control points and the location. Three types ofcontrol points are compared in this study. And special distribution of control point andthe accuracy will be discussed.

3 METHODOLOGY
3.1 Ground Control Point
The ground coordinates of control point are measure by the total station. ( xi, yi, zi ) isrepresented as ground coordinate. i means number of control point.
3.2 Laser Coordinate of Control Point
Corresponded laser coordinates of the prism and the reflector sheets are extracted fromreflection strength of the laser. Coordinates of the intersection are calculated by solving
equations of three planes which were derived from many laser points data. (ui, vi, wi )is represented as laser coordinate.
3.3 Geometric Transform
Three dimensional affine transformation was applied for geometric transformation asfollows;⎛ x ⎞ ⎛ p0
⎜ ⎟ ⎜⎜ y ⎟ = ⎜ p3⎜z⎟ ⎜ p⎝ ⎠ ⎝ 6p1
p4
p7

p2 ⎞⎛ ui ⎞ ⎛ X 0 ⎞⎟⎜ ⎟ ⎜ ⎟p5 ⎟⎜ vi ⎟ + ⎜ Y0 ⎟p8 ⎟⎠⎜⎝ wi ⎟⎠ ⎜⎝ Z 0 ⎟⎠x,y,z: Ground coordinate
u,v,w: Laser scanner coordinate
X0,Y0,Z0:Coordinate of laser scanner point
p0…p8:Transform coefficients

i :number of control point
Coefficients of transform are calculated by least square method using control point data which are( xi , yi , zi ) and (ui , vi , wi ) . Transform coefficients ( p0 … p 8 ) and coordinate of laser scanner point
must be derived to establish geometric transform by using control point. At least 4 control points
are needed.
3.4 Evaluation
The validation point ( x p , y p , z p ) was established. The coordinates of the validation point
was measured by the total station. The coordinate was assumed as true. The error oftransformation can be evaluated by the root mean square error(RMSE) which means
distance between transformed coordinate and validation coordinate. RMSE can becalculated following equation.

(X − X ) + (Y − Y ) + (Z − Z )Error = ∑2pj

2pj

n2pj

j :number of measurement

4 IN DOOR EXPERIMENTFirstly, in-door experiment was carried out in a gymnasium. Figure 4-1 shows locationin test area. The prism was set up at the four corners in the gymnasium. Four reflectorsheets were put on one board. Totally 48 reflector sheet were used. The boards for multisurface measurement were set up in four corners. Validation point was established in
the center. Laser scanner measurements were repeated 8 times in same condition.

Figure 4-1 Location in test area

Table 4-1 shows R.M.S.Error of geometric transformations by using prism, reflector
sheets and by using the intersection of three planes, Figure 4-2 shows coordinates ofthe verification point in each measurement after geometric transformation.
Table 4-1 R.M.S.Error of geometric transformation(m)measurement1
measurement2
measurement3
measurement4
measurement5
measurement6
measurement7
measurement8
average
S.D

Prism
Reflect sheet Multi Surface Mearsurement
3.882E-02
5.809E-02
5.033E-02
4.223E-02
5.188E-02
4.553E-02
6.807E-02
5.182E-02
4.527E-02
1.276E-01
5.363E-02
4.526E-02
7.794E-02
5.656E-02
4.842E-02
8.626E-02
5.664E-02
4.609E-02
8.224E-02
5.853E-02
4.607E-02
1.122E-01
6.033E-02
5.015E-02
7.942E-02
5.594E-02
4.714E-02
3.073E-02
3.168E-03
2.162E-03

28.2428.23
28.22

Y(m)

28.21
28.2
28.19
28.18
validation point
Prism
Multi surface measurement
Reflector sfeets

28.17
28.16
28.15
-0.02 -0.01
0Validation
0.01

0.02

0.03
X(m)

0.04

0.05

0.06

0.07

0.08

Figure 4-2 coordinates of validation point by each geometric transform

Geometric transformation by multi surface measurement was higher accuracy than geometrictransformation by a prism. However, a systematic error is included in the each control point. The
systematic error might be come from spatial distribution of control point.

5 OUTDOOREXPERIMENTOut door experiment was carried out in landslide area. Figure 5-1 shows the location of control
points. The 6 prisms were widely set up on the landslide site. 36 Reflector sheets were put theboards. 3 sets ofthe boards for multi surface measurement were set up around the laser scanner tokeep the accuracy within 20m distance. Laser scanner measurements were repeated 10 times insame condition.

Figure 5-1 Location in Land slide

Table 5-1 shows R.M.S.Error of geometric transformations by each control point. Figure 5-2 shows
coordinates of the verification point by each geometric transformation.

Table 5-1 R.M.S.Error of geometric transformation(m)prismreflect sheet Muti Surface Measurement
measurement1 1.975E-02
7.302E-02
2.859E-01
measurement2 6.591E-02
8.167E-02
2.779E-01
measurement3 3.942E-01
7.718E-02
2.622E-01
measurement4 1.045E-01
7.794E-02
2.773E-01
measurement5 3.515E-02
9.392E-02
2.365E-01
measurement6 1.209E-01
1.050E-01
3.077E-01
measurement7 8.329E-02
1.059E-01
2.598E-01
measurement8 2.507E-02
8.473E-02
2.486E-01
measurement9 9.738E-02
8.521E-02
1.724E-01
measurement10 5.829E-02
8.505E-02
2.488E-01
average
1.149E-01
8.937E-02
2.517E-01
S.D
1.088E-01
1.127E-02
3.652E-02
55497.35

Y(m)55497.3

55497.25

Validation
55497.2
Validation poin
Prism
Multi surface measurement
Refrect sheets-34220.8-34220.75-34220.7 X(m) -34220.65-34220.655497.15-34220.55

Figure 5-2 coordinates of verification point by each geometric transform

Geometric transformation by prism was higher accuracy than other control point. Theerror of the geometric transformation of the multi surface measurement was large. A
systematic error is included in the reflector sheet and multi surface measurement. However,standard deviation of reflector sheets and multi surface measurement showed very small.

6 CONCLUSIONS

The accuracy of geometric transform is influenced by spatial distribution of control
points. Prism can be set up in very wide range, then the average became close tovalidation point in the landslide area. However, standard deviation was very big.
Therefore, stable accuracy cannot be expected. On the other hand, standard deviationof reflector sheets and multi surface measurement showed very small. When thesystematic error is eliminated, stable accuracy will be expected.

7 REFERENCES
Tomoya SAKAI, Jong Hyeok JEONG and Masataka TAKAGI, Laser Scanner
Measurement for Monitoring Landslide Dispacement, Proceedings of the 26th Asian
Conference on Remote Sensing, Hanoi VIETNAM, pp.LID2-2, 2005