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AUTOMATIC TARGET IDENTIFICATION FOR LASER SCANNERS 
Valanis A., Tsakiri M. 
National Technical University of Athens, School of Rural and Surveying Engineering 
> 
9 Polytechniou Street, Zographos Campus, Athens 15780, Greece 
(artvalanis(@yahoo.gr, mtsakiri@central.ntua.gr) 
WG V/1 
KEY WORDS: Laser scanning, automation, recognition, algorithms, close-range, metrology 
ABSTRACT: 
Terrestrial laser scanners are becoming increasingly important for many fields of imaging applications, providing a great amount of 
3D positional information in a fast and efficient way. This information is always expressed by means of coordinates in a somewhat 
random 3D space defined by the scanner orientation, which changes whenever the scanner is moved. Therefore, targets are usually 
employed either for registration (i.e. for the referencing of the data in a common 3D space) or for referencing of the data into a local 
coordinate system. 
The use of targets for these purposes is a standardized process, which is invariably carried out by proprictary software. However, the 
algorithms used for the identification of targets (i.e. automated definition of the centre of the target) are not described by the 
software vendors. In this paper, methods for automating target identification which are based on fuzzy classification, gridding and 
averaging techniques are presented. Experiments are conducted using a Cyrax 2500 terrestrial laser scanner in laboratory conditions. 
The performance of the proposed methods is compared and assessed with reported methods from published literature. Furthermore, 
given the fact that due to reflectance topographic artefacts are observed on the surface of the reflective targets, experiments are also 
conducted for different scan angles and distances. 
1. INTRODUCTION 
Terrestrial laser scanning allows for detailed and precise 
documentation of objects of interest. In practice, collection and 
processing procedures are adapted to the type of application 
(e.g. use of different resolutions, acquisition of multiple, 
overlapping scans from different distances, points of view). 
However, regardless of the application (e.g. conducting 
metrological experiments, registering multiple ^ scans, 
referencing the position of the data in a given coordinate system 
etc.), automatic target identification is a matter of great 
significance. Therefore, the need for a reliable and precise 
algorithm that identifies targets automatically is importànt. In 
this paper, the capabilities of a current commercial laser scanner 
system (Cyrax 2500) regarding target identification are 
explored and several new methods for automatic target 
identification are presented. 
The second section of the paper gives a brief overview of the 
Cyrax 2500 system and presents two experiments conducted for 
evaluation of the repeatability of collected data from multiple 
scans. In the third section, the way that target centres are 
determined using the Cyclone software is described along with 
several methods for target identification proposed in published 
literature. The properties of the reflective targets are thoroughly 
examined and new algorithms for target identification are 
described. In the final section, experiments conducted to 
evaluate the stability, reliability and accuracy of the proposed 
methods are described and comparative results are presented. 
2. SYSTEM OVERVIEW AND REPEATABILITY 
CHECK 
The experiments presented in this paper were all conducted 
using a Cyrax 2500 laser scanner. The instrument has a field of 
view of 40° by 40°, and operates with a green laser beam of 
532nm. The spot size is less than 6mm for distances up to 50m, 
distances are measured with an accuracy of = 4mm and the 
angles are measured with an accuracy of + 60micro-radians. 
The accuracy in the position of single points is, according to the 
manufacturer, approximately + 6mm for distances that range 
between 1.5m — 50m. The scan rate is very high, namely 1000 
pts/second. The system is operated using a laptop and the 
processing of the data can be carried out using the Cyclone 
software suite (www.cyrax.com). 
Measurement repeatability is a very important property for a 
laser scanner system. In order to evaluate this property for the 
Cyrax 2500 svstem, two experiments were conducted. The 
former involved scanning four targets mounted on four pillars 
of the internal EDM calibration baseline of NTUA. The latter 
involved the scanning of five targets placed on a wall. 
For both cases, nine scans were collected for each one of the 
targets. The collected data were exported into an ASCII format 
which contains the cartesian coordinates in the scanner's system 
along with the signal strength (reflectivity) for each point in the 
scan. The selection of the target image from each point cloud 
was performed through the proprietary Cyclone software.