Accurate Geometric Transformation ofLaser Scanner Data for Landslide Monitoring
Tomonori MIYAZAKI*, Kazu KINOSHITA** and Masataka TAKAGIInfrastructure Systems Engineering, Kochi University of Technology
Tosayamada-city, Kami-shi, Kochi, 782-8502, Japan*115120t@gs.kochi-tech.ac.jp**105506p@gs.kochi-tech.ac.jp***takagi.masataka@kochi-tech.ac.jp
KEYWORDS: Landslide, Laser Scanner, Geometric transformation
ABSTRACT: Landslide is a phenomenon of mass slowly movement in wide area. It is difficult to measurewhole landslide area by using existing monitoring systems. Nowadays, ground based portable the laser
scanner is expected to monitor landslide, because the laser scanner can acquire three-dimensional data ofwide area in a short time. The acquired data by the laser scanner has own coordinates. The coordinate iscalled laser coordinate in this study. The laser coordinate must be transformed into the ground coordinates
using Ground Control Point(GCP). The prism or reflector sheet is usually used as GCP. GCPs of groundcoordinate can be precisely measured by total station. The laser coordinates of GCPs can be acquired
automatically by image processing software of laser scanner. The image processing is based on reflection
strength of the laser around the prism or reflector sheet. The reflection strength of the prism is higher thanreflector sheet. Then prism can be set up widely. However, cost of the prism is so expensive that manyprisms cannot be set up. On the other hand, cost of reflector sheet is so cheap that many reflector sheets canbe set up. Therefore, the prism and the reflector sheet should be compared in order to establish accuratetransformation method. In this study, 6 prisms and 16 reflector sheets were used for geometric transformation.
The accuracy of geometric transformation was discussed. The required accuracy is 10mm of Root Mean
Square Error(RMSE) for land slide monitoring. In the case of prisms, result showed 95.9mm RMSE. In thecase of reflector sheets, result showed 25.0mm RMSE. Many reflector sheets were effective for accuratetransformation. Repeat measurement will make accurate transformation by average calculation. The averagecalculation of 10 times measurement showed 5.4mm error. Therefore, this measurement method will beeffective to monitor land slide.

1. BACKGROUND

measure at some points or along lines. It is difficult
to measure whole landslide. Laser scanner is

A landslide is a phenomenon of mass movement on

expected as a useful measurement equipment for

the ground, which moves 0.01mm to 10mm a day inmonitoring landslide. Laser scanner can acquire

the wide area. Current monitoring systems are used

three dimensional data in a short time, in wide area.

expansion gauge, inclinometer or GPS. Those

For the extraction of landslide displacement by using

monitoring systems for landslide displacement canlaser scanner, 10mm accuracy is required at least.

However, used laser scanner has 25mm error instandard deviation. And laser scanner data must betransformed geometrically to extract displacement oflandform using multi temporal data. Therefore,
accurate geometric transformation is needed.

2. OBJECTIVES
Objective of this study is establishment at ofaccurate geometric transformation of laser scannerdata. Accuracy of geometric transformation isdepending on the type of ground control point (GCP).

Table 3.1 Performances of measurement
Range≦350m(reflectance≧80% of objects)

Range≦150m(reflectance≧10% of objects)

Shortest

2m

Distance
Measurement±2.5cm

Accuracy(Standard Deviation)

Laser

0.9μm

Wavelength(Infrared)

In this study, prism and reflector sheet were used as
GCP. Two types of GCP were compared in this study.

Table 3.2 Specification of laser scanner

And spatial distribution of the GCP and the accuracy

Frame scan

was evaluated. The required accuracy of in this study
is 10mm root mean square error for landslide
monitoring.

3. USED LASER SCANNER

Line scan

Pixel(max)1106 pixels

4621 pixels

Angle Range±40°

0°～333°

Pixel(max)

1106

Angle

0.036°

0.018°

resolution(actual 0.072°)(actual 0.072°)

pixels4621

pixelsIn this study, LMS-Z210 produced by Riegl wasused as laser scanner. Maximum measurement range

4. CONTROL POINT

by this the laser scanner is 350m. Measurement
accuracy is about 25mm in standard deviation. Laser

The acquired data by the laser scanner has own

scanner can acquire color information, reflection

coordinates. The coordinate is called laser coordinate

strength and 3 dimensional coordinate of the object.

in this study. Laser scanner data must be transformed

Figure 1 shows picture of laser scanner. Table 1

into the ground coordinates using GCP. The prism orshows Performances of measurement distance. Tablereflector sheet is usually used for GCP. GCPs of

2 shows specification of laser scanner.

ground coordinate can be precisely measured byusing total station. The laser coordinates of GCPs
can be acquired automatically by image processing
software of laser scanner. The image processing isbased on reflection strength of the laser around theprism or reflector sheet.

4.1 Prism
The prism is often used as a GCP. Figure 4.1.1
Figure 3.1 Used laser scannershows a picture of the prism. The prism can bereflected laser strongly. Then the prism can be set up

in maximum range of 300m. Cost of the prism isStandard deviation of the laser coordinate showed

expensive with 1,500 US dollars. The cost of the20mm by 8 times measurement when the reflector

prism is so expensive that many prisms cannot be set

sheet set up at 60m range from laser scanner.

up. In this study 6 prisms were used. Firstly accuracyStandard deviation of reflector sheet is smaller thanof laser coordinate of prism was validated. By 8

the prism very much.

times measurement in condition, laser coordinate of
the prism were extracted by image processing.

5. TEST AREA
The test area was selected CHOJA landslide in theNIYODO, KCOHI, JAPAN. This landslide ismoving 10mm to 30mm a year. Figure 5-1 shows thelocation of GCPs. 6 prisms were widely set up
around the landslide site. 16 Reflector sheets wereset up in about 80m range from laser scanner. Onevalidation point of the geometric transformation wasFigure 4.1.1 Prism

set on the edge of landslide.

Standard deviation of the laser coordinate showed
150mm when the prism set up at 60m range fromlaser scanner. The coordinates of the control points
in laser scanner data cannot be accurately extracted.

4.2 Reflector sheet
Reflector sheet can be also used as GCP. Figure
4.2.1 shows a picture of the reflector sheet. Reflector
sheet can not reflect laser strongly comparing withthe prism. Reflector sheet can be set up in maximum
range of 60m. Cost of the reflector sheet is cheapwith 5 US dollars. The cost of reflector sheet is socheap that many reflector sheets can be set up. InFigure 5.1 Location of GCPs

this study 16 reflector sheets were used.

6. EVALATION METHOD
The geometric transformation was applied 3D Affine
transformation. The conversion equation shows asfollows.

Figure 4.2.1 Reflector sheet⎛ x i ⎞ ⎛ P0
⎜ ⎟ ⎜⎜ y i ⎟ = ⎜ P3
⎜ z ⎟ ⎜P
⎝ i⎠ ⎝ 6P1
P4P7P2 ⎞ ⎛ u i ⎞ ⎛ X 0 ⎞⎟⎜ ⎟ ⎜⎟
P5 ⎟ ⎜ v i ⎟ + ⎜ Y0 ⎟P8 ⎟⎠ ⎜⎝ w i ⎟⎠ ⎜⎝ Z 0 ⎟⎠7. RESULT(6.1)

Laser scanner measurements were repeated 10 times
in same condition. Coordinates of validation point
were converted by each GCP. Figure 7.1 shows Error

xi,yi,zi: Ground coordinates

on validation point after transformation.

ui,vi,wi: Laser coordinates
X0,Y0,Z0:Coordinate of laser scanner
p0…p8:Coefficients of transformation

Coefficients of transformation are calculated by leastsquare method using GCPs. GCPs have groundcoordinates(xi,yi,zi) and laser coordinates(ui,vi,wi). At
least 4 GCPs are needed for solution. Establishedtransformation using GCPs is evaluated usingvalidation point. Ground coordinate of validation
point can be measured accurately by total station.
The coordinate can be assumed true. When laser
coordinate of the validation point input to theestablished transformation, ground coordinate of thevalidation point can be calculated. The error can be

Figure 7.1 Validation point coordinates after

calculated comparing with true ground coordinate.

geometric transformation by each type of GCP.

By repeat measurement, distribution of the error canbe plotted. In this study, root mean square error

Origin is a coordinate of validation point which(RMSE) is calculated by following equation.

measured by total station. The prism and reflector
sheet are distributed around validation point. RMSEand Standard deviation of the validation point ofeach GCP are shown in table 7.1.

nΣ

( X v − xi ) 2 + (Yv − yi ) 2 + ( Z v − zi ) 2

Error =i =1n(6.2)

Table 7.1 RMSE and standard deviation of thevalidation point

Xv,Yv, Zv: True coordinates
xi,yi,zi: Transformed coordinates

RM SE(m m )standard deviation(m m )prism
95.938.1reflector sheet
25.0
7.9

n: Scanning number

RMSE is used for evaluating accuracy of geometricIn the case of prisms, RMSE of the validation pointtransformation.

after the geometric transformation showed 95.9mm.
In the case of reflector sheets, RMSE of thevalidation point after the geometric transformation
showed 25.0mm. The error using the reflector sheet

has smaller than the prism. Standard deviation using
the reflector sheet showed also smaller than theprism. Therefore, many reflector sheets can expect
accurate transformation. However, this accuracy
cannot be used to monitor landslide. If the error in
the validation point showed a random error, the error
margin can be decreased by average calculation ofrepeat measurement. The average coordinates of thecontrol point that had been obtained by 10 times
measurement. The following table showed the resultof the average calculation.
Table 7.2 Average coordinates of
10 times measurements

error(m m )

prism
28.1

reflector sheet
5.4

Figure 8.2 Relation between number ofmeasurement and error
In the case of prisms, error of the validation point

A line was drawn predicted error by the low of error

after the geometric transformation showed 28.1mm.

propagation, and a point was plotted

In the case of reflector sheets, error of the validation

margin in an actual measurement. The result showed

point after the geometric transformation showed

at least 6 times measurement is necessary to satisfy

5.4.mm. The average calculation made satisfy

required accuracy which is 10mm.

requiredaccuracy

forlandslide

an errormonitoring.

Equation 7.1 is showed predicted error by average

9. CONCLUIONS

calculation the repeat measurement based on low oferror propagation.

Required accuracy for measuring the landslide wasable to be achieved by repeat measurement that uses

E=σreflector sheet for control point. By using reflector(7.1)

nsheets, average calculation of 10 times measurement
showed 5.4mm error. measurements that used thereflector sheet. As further study, it is necessary to

E : Predicted error

σ: Each measurement error
n: Number of repeat measurement

decide the optimum number of the GCP and thespatialdistribution.

transformation

isMorenotover,

accurate

3D

Affinemodel.

MoreAccuracy geometric transformation can be expected
Number of repeat measurement to satisfy 10mm
accuracy can be calculated using the equation.
Relation between number of measurement and theerror is shown in the following Figure 8.2.

by using rotation matrix

10. REFERENCE
Tomoya SAKAI ・ Jong Hyeok JEONG andMasataka TAKAGI, Laser Scanner Measurement forMonitoring Landslide Dispacement, Proceedings ofthe 26th Asian Conference on Remote Sensing,
Hanoi VIETNAM, pp.LID2-2, 2005
Kazu KINOSHITA・Tomonori MIYAZAKI

andMasataka TAKAGI, Accurate Evaluation of LaserScanner Data Depending on Location of GCPs forMonitoring Landslide, Proceedings of the 26th AsianConference on Remote Sensing 2007
RIEGL Japan. Ltd., ”LMS-Z210 owner’s manual”