Since the School of Surveying and Spatial Information System's purchase of a Cyrax 2400 terrestrial laser scanner in January 2001, a number of students have studied terrestrial laser scanning. The aim of this thesis was to continue the study of terrestrial laser scanning with respect to the Cyrax 2400 system. Most terrestrial laser scanning systems use distance measurement by laser to collect spatial information. A laser scanner creates a model consisting of a large number of points with x y z coordinates. The model is a representation of the 'real world'. The x y z coordinates relate the points, measured on 'real world' objects, to the origin of the scanner. The ability of a laser scanner to capture large amounts of data quickly, and with a fine resolution, means that the 'real world' can be accurately modelled.
The Cyrax 2400 (see Fig. 1) laser scanning system comprises of a scanner head (incorporating the laser and scanner hardware), a power unit and laptop running the Cyclone software. This software manages the scanner operation, data storage and data processing. The pulsed laser can measure to almost any surface using, in principle, a 'time-of-flight' measuring system. It is scanned from left to right, bottom to top across a window of 40° by 40° by high speed rotating mirrors within the scanner head. Terrestrial laser scanners have been used to gather spatial data for construction and engineering purposes as well as architectural and archaeological applications. Many articles have been written in trade magazines, documenting their use and the end results of a variety of projects.
Fig. 1: Cyrax 2400 Terrestrial Laser Scanning System at UNSW
Some interesting aspects of the scanner were examined in the course of this thesis. These included: testing the ability of the scanner to recognise special targets (designed for use with the Cyrax 2400), day and night scans over distances up to 250 m, testing the repeatability of the scanner in locating targets on a pole of calibrated length, modelling of a scanned object to calculate a volume and looking at various aspects of the Cyclone software. By placing the special targets at various angles of incidence to the laser beam, the ability of the scanning system to locate the target accurately was tested. It was found that, when the target was placed at an angle greater the 60° from the line perpendicular to the laser beam, the solution was significantly degraded. The test was only carried out at short range.
A number of scans were carried out during daylight and at night to cardboard targets placed at approximately 100 m, 150 m, 200 m, and 250 m from the scanner. The purpose of this project was to determine how far the scanner could scan (its range is quoted as 100 m) and if scanning at night results in more data being collected. The scanner was able to measure distances up to 250 m, although few points were captured at this distance (partly due to the low resolution set). Scanning at night resulted in more data being collected. However, it is recommend more scans be carried out to test the significance of the difference.
Fig.2: Left: cardboard targets at distances up to 250 m from scanner; Right: scanning at night
A test to determine the ability of the scanner to repeatedly locate special Cyrax targets was carried out by Jason Raic in 2001. This test was repeated. The targets, positioned on a calibrated pole exactly 2 m apart, were scanned ten times to determine the accuracy of the scanner. The distance between the targets determined by the scanner was 2.000 m with a standard deviation of ±0.59 mm.
Often, more than one scan is required to accurately model the subject. This is known as multi-site scanning. The Cyrax 2400 scanner is not set up over a known mark, levelled or back sighted to another known mark; every time the scanner is moved, the coordinate system of the scan changes. The Cyclone software enables the 'merging' (coordinate transformation) of multi-site scans. This is achieved by either placing the special targets mentioned above in overlapping areas of the scans or by using common features in each scan, that can be defined by simple geometric shapes. The shapes or targets are labelled as common to relevant scans, thus enabling merging. The term for this process is 'registration'. Three projects completed for this thesis required registration using these methods. Fig. 3 shows Closebourne House at Morpeth, NSW. This model is a registration of two scans. Also registered were the scans of a yacht and of a water tower.
Fig. 3: Closebourne House, colours represent intensity of reflectance of the laser
Once scans have been registered they can be 'modelled'. The result of modelling in the Cyclone software includes, among others, output to CAD software and extraction of measurements from data. For example, using the registered scan of a water tower, a cone was 'fitted' to the main part of the tower using the Cyclone software. The cone could then be used to determine the approximate volume of the water tower.
The ability of terrestrial laser scanners to capture large amounts of spatial data quickly and accurately makes them an important tool for those requiring and working with spatial information. The release of the HDS3000 by Leica Geosystems on the 24 September 2003 marks another step in the evolution of laser scanning technology. A thorough understanding of this technology will enhance its use as a tool for those working in Surveying and Spatial Information Systems.
For more information, please contact:
Dr. B. R. Harvey (Supervisor)
School of Surveying and Spatial Information Systems
University of New South Wales
UNSW SYDNEY NSW 2052