School of Surveying and Spatial Information Systems

The University of New South Wales


Comparison of Industrial Measurement Techniques

by Glenn Egan

Supervised by B. Donnelly

Edited by J. M. Rüeger

October 2002


Aim

The term Industrial Measurement Techniques can be applied to a wide variety of image acquisition and image analysis methods. The two techniques researched were the Cyrax 2400 Laser Scanner and Digital Close Range Photogrammetry. The aim of this project was to examine which of these two technologies is the more appropriate for the measurement of industrial objects.  To compare these two technologies, a small boat was measured with both techniques.

Terrestrial close range photogrammetry has been used to measure large irregularly shaped objects such as ships and aircraft for many years.  The new technology of 3D laser scanning can also be used to measure such objects.  For this project, a comparison was made between these two technologies using literature sources and a practical measurement using the School of Surveying and Spatial Information System's Cyrax 2400 3D laser scanner and digital photogrammetry with a Kodak DC260 digital camera.  Incorporated into the learning of the two measurement techniques were their associated software.  For the laser scanner the Cyclone software was used and for the close range digital photogrammetry the Australis software package.  

 

Figure 1: Kodak DC260 Zoom Camera

Several key aspects of the equipment and post-processing steps, that are used in combination to produce 3D data for the exportation to CAD software, were compared for these two image-capturing devices.  The most important aspects to consider are the following:

Equipment

For the experiment, a Kodak DC260 zoom digital camera, pictured in Figure 1, was used.  It has a resolution of 1536 x 1024 pixels per image when used in best quality mode.

The Cyrax 2400 laser scanner is a high speed, 'high accuracy' laser radar scanner.  The scanner is operated from a laptop PC and features an integrated video camera. Its point position accuracy at maximum range (100 m) is 6 mm (1s).  The Cyrax 2400 Laser Scanner uses a pulsed green laser with a central wavelength of 532 nm.  The angular field of view of the Cyrax 2400 is 400 in the vertical ( 200 from the horizontal) and 400 in the horizontal.  Figure 2 shows the Cyrax 2400 Laser Scanner in operation.

 

Figure 2: Cyrax 2400 Laser Scanner

  

Digital Close Range Photogrammetry Experiment

Eighteen digital images were taken with a Kodak DC260 zoom camera for the digital close range photogrammetry experiment.  The digital images were imported into the Australis processing software and a least squares bundle adjustment was performed.  The Australis bundle adjustment gives a list of the XYZ coordinates of the retro-reflective targets on the boat.  Figure 3 is a sample of the images gathered in the experiment.   

Cyrax Laser Scanning Experiment

A total of three scanner locations were used for the scanning of the boat. This resulted in three 3D point clouds that could be used to produce a 3D model of the boat.  It was possible to carry out the merging ('registration') of the three point clouds in the field. It took only thirty minutes to merge the three clouds and to produce the final 3D point cloud model.  Figure 4 below shows a screen capture of the 3D model of the boat.

 

 

Figure 3: Sample image from digital close range photogrammetry experiment.

Results

The field time required for the acquisition of the boat data was compared between the techniques.  The laser scanning of the boat took four hours, compared to two and a half hours for the photogrammetric measurements. For the post-processing of the acquired data, half an hour was spent on the laser scanning and 10 hours on the photogrammetry. The Cyrax targets, that were common to both data sets, were used to compare the two results.  Using the coordinates of the targets from both methods, the inter-target distances were calculated.  The differences ranged between two and ten millimetres. Cost is an essential aspect of any comparison between the two techniques.  A close range digital camera, together with the Australis processing software, costs approximately $6 500. A Cyrax 2400 Laser scanner, with Cyclone software, costs approximately $250 000.

Conclusions

This project allowed a comparison between two relatively new techniques of industrial measurement. Both gave accurate results of the measurement of a boat.  When determining the best measurement technique for industrial measurement, a combination of the features of the two techniques must be compared. Time and cost are two major considerations to be looked at.  If cost is no obstacle in a project where accurate XYZ point coordinates are needed then the Cyrax 2400 Laser Scanner is far superior to digital close range photogrammetry.  However, when using the (relatively) cheaper digital camera (with a bundle adjustment software), a better accuracy can be obtained, at the expense of lengthier post-processing. 

One of the most important distinctions to be made between the two techniques considered is the amount of 3D data acquired by each technique.  Laser scanners acquire thousands of points over the field of view in a very short period of time.  When several scans are merged, the resulting 3D point cloud gives a detailed, useable 3D model of the object's surface.  A major difference with digital close range photogrammetry is the number of 3D points that can be acquired.  Digital photogrammetry can only acquire 3D coordinates of points on an object that are marked with retro-reflective targets.  This puts a limit on the number of points acquired on an object, and the detail that can be displayed in 3D models.

 

Figure 4: 3D point cloud model of the boat

 Further Information

For more information, please contact:

Brian Donnelly (Supervisor)

Email:  B.Donnelly@unsw.edu.au

 

Mail:

School of Surveying and Spatial Information Systems

University of New South Wales

UNSW SYDNEY NSW 2052

Australia

 

Phone: +61-2-9385-4202

Fax: +61-2-9313-7493

WWW: http://www.gmat.unsw.edu.au