Study on the Accuracy of Tree’s Diameter Measurement by Laser Range
Finder System
M.R. Yaacob
Faculty of Electrical Engineering
Universiti Teknikal Malaysia Melaka,
Malaysia
rusdy@utem.edu.my
M.F. Miskon
Faculty of Electrical Engineering Universiti Teknikal Malaysia Melaka
Malaysia
C.X. Zhi
Faculty of Electrical Engineering
Universiti Teknikal Malaysia Melaka,
Malaysia
chan_xin_zhi@hotmail.com
Keywords: Laser, Diameter, Tree
Abstract
This paper presents a study on the accuracy of laser range finder system for the measurement of tree’s diameter. The study has been conducted by using Hokuyo URG-04LX-UG01 Laser Range Finder. The measurement was done in different distances from the measured tree at a fix elevation in two different modes; sensor held in hand and sensor mounted on a tripod. An algorithm was developed to derive the formula for calculating the diameter by applying the principle of circle tangent theory. The results obtained are the values of accuracy for the diameter measurement of a tree for different distances between the sensor and the tree measured which are presented in graphs. Finally the accuracy levels between the two modes are compared to find the most preferable mode for the measurement in real application.
1. Introduction
Contact and non-contact measurement devices have been developed for decades to measure the diameter of a tree. These devices are still considered to be time consuming and low in accuracy while forestry and plantation workers are still using one of these methods for diameter measurement. Fast responding and accurate non-contact measurement device is in need for this kind of measurement in order to reduce the time for measurement of the whole plantation. Thus an accurate measurement device is required to determine the correct time for fertilization and time to tap off the trees measured.
Available methods used to measure diameter of tree do not consist of Graphic User Interface (GUI), time consuming and low in accuracy. This causes difficulty for forestry workers or researchers as well as affecting the result of measurement values. Rubber Industry Smallholders Development Authority (RISDA) is one of the estate developers in Malaysia who faces difficulty when dealing with problem in measuring circumference of tree trunk. This is caused by massive amount of rubber trees to be measured using difficult and low accuracy measurement methods such as tape measuring, calipers and Biltmore Sticks. In order to eliminate these problems, alternative method is researched to measure tree diameter together with the study of its performance in term of accuracy.
Previous researches have been done which involves measurement of tree’s diameter and circumference. Researchers from the US Department of Agriculture have come out with Criterion Laser Instrument which employs laser beam to measure tree diameter [1]. Despite considering its high cost, this device requires user to manually entering the tick marks across the tree and distance from the tree into the keypad to calculate the diameter of the tree which is then required for further analysis.
Another research was previously done by researchers from Universiti Teknikal Malaysia Melaka regarding the linear and rotary infrared scan system for the circumference measurement of trees [2]. This measurement system resulted in poor performance due to the step angle resolution of stepper motor used. Larger angular resolution results in bigger approximation for analog linear scan method. There was also an issue related to the limitation of IR sensors used as these sensors gave unstable results. Therefore another sensor that employs different light source, which is laser, is much preferable to replace the infrared as the measurement medium. In this paper, we will further discuss on the measuring principle in Section 2. Section 3 describes the experimental setup used for the diameter measurement and finally in Section 4, we will thoroughly discuss the results obtained from the experiments.2. Formatting your paper
2.1. Hokuyo URG-04LX-UG01 Laser Range Finder
Hokuyo LRF is a laser sensor with infrared light source as shown in Fig. 1. Its scanning area is about 240° at accuracy until 4000 mm as clearly illustrated in Fig. 2 [3]. Object’s color has minimum effect upon the Laser Range Finder. The specifications of URG-04LX-UG01 are summarized in Table 1.
Fig. 1 Hokuyo URG-04LX Laser Range Finder
Fig. 2 Detection Angle and Maximum Distance of Hokoyo LRF
Table 1: URG-04LX-UG01
Power Source 5V DC (from USB) Detection Distance 0.02 – 4.00m
Accuracy Distance : 0.02mm – 1.00m (±0.03m)Distance : 0.02m – 4.00m (±3% of measurement) Total Scan Angle 240
°
Weight Approximately 160g Life Span Approximately 5 years Light Source Laser Class 1
surface of detection because its data approaches 1500 mm. Wood material is considered very good to be the surface used for LRF detection since its data is closer to 1500 mm compare to gray sheet, steel and electrical steel.
Fig. 3 Material Properties
2.2. Tree Diameter Measurement Algorithm
The basic principle behind the tree diameter measurement algorithm is based on the circle tangent theory. Cross section of tree is considered as circle and can be calculated using circle equations. From circle tangent theory if a line is tangent to a circle, the straight line drawn from circle’s radius to the point of contact between circle’s radius and the tangent line is perpendicular with both the tangent line and circle’s radius as illustrated in Fig. 4.
Fig. 4 Line Tangent to Circle
Now consider in Fig. 5 that consist of two lines tangent to the circle located at different contact points but both of them are extended to a same point. A straight line is drawn from the contact point up to the circle’s radius for both the tangent lines. The intersection point of both lines drawn from the two tangent lines will meet at centre point of the circle.
Fig. 5 Two Lines Tangent to Circle
Fig. 6 Finding the Length c
From Fig. 6, length c can be calculated using law of cosines formula provided that length a, length b and
θ
are known:c
2=
a
2+
b
2−
2
abcos
(
θ
)
(1)c
=
√
a
2+
b
2−
2
abcos
(
θ
)
(2)Meanwhile radius of the circle, R can be calculated if length c is found and thus yield to the determination the circle’s diameter, D:
θ
2=
180
°
−
θ
(II)c
2=
R
2+
R
2−
2
× R × R ×
cos
(
θ
2)
(4)c
2=
2
R
2−
2
R
2cos
(
θ
2)
c
2=
2
R
2[
1
−
cos
(
θ
2)
]
2
R
2[
1
−
cos
(
θ
2)
]
=
c
2R
2=
c
2
2
[
1
−
cos
(
θ
2)
]
R
=
√
c
2
2
[
1
−
cos
(
θ
2)
]
(3)
Diameter of the circle, D is:
D
¿
2
R
(4)3. Experimental Setup
There were two experiments conducted to test the performance of the proposed method. The objective of the experiments is to test the accuracy of the device when the device is held on hand and when the device is fixed to the ground with tripod. The purpose of carrying out two experiments is because the reliability of data collected with human’s hand holding the device difference from the reliability of data collected when the device is fixed to the ground. Human’s hand tends to move around when holding measurement device while tripod can help to fix the device to the ground.
The first experiment was done with data collecting by measuring a tree with LRF held on hand as depicted in Fig. 7. Meanwhile the second experiment was done through data collecting by fixing LRF above the ground with tripod as depicted in Fig. 8.The apparatus used for the experiments are:
i. Hokuyo URG-04LX-UG01 Laser Range Finder (LRF) ii. Algorithm
iii. Lenovo G475 Notebook with Microsoft Windows 7 Professional iv. Tree with actual diameter of 191mm and circumference of 600mm
v. Tree with actual diameter of 284mm and circumference of 892mm vi. Tripod
The experiment was conducted on a field during daylight condition at temperature of approximately 30. The temperature and lighting condition do not affect the performance of the laser. Therefore it is not critical to control these variables. The experiment was conducted at different distance. 100 data of diameter and circumference measurement were taken for each distance. The result was recorded. The experiment was repeated by changing the distance between the Laser metric and the tree within range of 0.5m to 3m at an interval of 0.1m.
Fig. 8 LRF fixed to the ground with tripod
4. Results and Analysis
Fig. 9 shows the result for diameter measurement with LRF held in hand. Accuracy decreases when distance increases where the highest accuracy achieved is 98.63% at diameter of 0.6m while the lowest one obtained is 93.76% at diameter of 2.9m. The accuracy seems to be fluctuating and this is agreed with previous research in which distance data obtained from Hokuyo LRF URG-04LX has been proven to be fluctuating along its measurement [4]. Fluctuating measurement may be due to heat dissipation of the LRF during its activation time. Heat dissipation of motor in the LRF may also be the cause of fluctuation data produced by LRF because small vibration produced when motor rotates to spin the mirror used for LRF scan.
Fig. 10 Accuracy versus Distance (LRF mounted to Tripod)
Fig. 10 shows the result for measurement with LRF mounted to tripod. Accuracy also fluctuates within the range of 96% to 98% from distance of 1.3m to distance of 2.6m. The highest accuracy obtained 99% at distance of 1.1m while the lowest accuracy obtained is 85.6782% at distance of 3m. Accuracy begins to drop below 96% at 2.7m and drops significantly at distance 3m. However, when compared to the measurement with hand held, results in Fig. 10 shows that the accuracy level is maintained at high level up to the distance of 2.6m.
5. Conclusions
Stability of holding the Laser Metric affects the accuracy as well. Holding the LRF with hand has inconsistent data compared to fixing it to the ground. This may be due to human tiredness when holding the device to measure tree diameter and circumference. Therefore it is more preferable to use the LRF with tripod in taking the diameter measurement in the real application for more accurate results. Future work could be done by investigating the effect of tilt angle when measuring tree and size of the tree on the accuracy of the measurement. It would be interesting to find out how these variables affect the accuracy of the measurement
6. References
[1] Instrument for Measuring Stem Diameters. S. Garrett, J.J. Kempf, R.L. Copstead. Washington : United States Department of Agriculture, 1997.
[2] Linear and Rotary Infrared Scan System for Measuring Circumference, M.F. Miskon, A. M. Deraman, A.Y.B. Ahmad, M. A. Ibrahim, A. Ahmad, Z. Sani, M.H. Jamaluddin, Universiti Teknikal Malaysia Melaka, 2011.
[3] Hokuyo Automatic Co, LTD. Scanning Laser Range Finder URG-04LX-UG01(Simple URG) Specifications. Hokuyo URG Series Download Page. [Online] [Cited: Septermber 15, 2011.] http://www.hokuyo-aut.jp/02sensor/07scanner/download/products/urg-04lx/data/URG-04LX_spec_en.pdf
