Summary
Geospatial Survey Solutions and Deri Jones & Associates (GSS/DJA) and FARO UK were asked to perform a demonstration of the Faro LS880 3D laser scanning technology on a typical road junction that the crash investigation team would be involved with. Caldra junction on the M4 in South Wales is one of the busiest junctions in Wales and the scanning was carried out without affecting the flow of the traffic. The 3D data was compiled in to a Navisworks 3D file that could be viewed on any reasonably modern PC using their free viewer. Data was also compiled for the crash investigation team to test the point cloud import capabilities of their existing 3D animation system, Relmo.
Method
A set of four full colour scans were carried out to demonstrate the ease of use and speed of the instrument. The ability to select a small area and scan that at a higher resolution was also demonstrated. The scans were registered together using the reference spheres and total station survey data, demonstrating the fact that the scan data can be integrated with the team's existing survey equipment.
The data was then loaded and filtered using the FARO Scene software, the reference spheres identified and registered with the 3D total station data and the colour data from the photographs applied. This was exported to Navisworks for creation as a viewable file on other machines.
Results
Accuracy of scan location
The reference spheres were located using a total station reflective target and the resulting 3D point file used as a reference for the manipulation of the scans. Identification and alignment of the reference spheres in FARO Scene software gave the following result:
|
Scan No |
Orthogonal misalignment |
Angular misalignment |
|
01 |
7.5mm |
0.07o |
|
02 |
5.2mm |
0.05o |
|
03 |
6.4mm |
0.05o |
|
04 |
4.5mm |
0.09o |
Reflection scan data
The scanner picks up any objects within range during a scan - this can result in false readings from moving traffic which obscure areas of road. These can be removed in the scanner software, but to ensure full coverage of a section, more than one scan must be made. The scanner also picks up 100% reflections from retro reflective clothing, signs and markings on emergency vehicles. Areas covered with retro material cannot be measured accurately unless covered, but having retro materials within a scan does not affect the accuracy of the other areas of scan.
Colour data
The colour data is collected on a second pass using a standard digital SLR camera with a fisheye lens. There is a slight area above the scanner that is not collected by the camera and there are issues with the camera picking up vehicles that were not visible during the scan cycle. This results in the projection of vehicle images on to the road surface or bridge walls and we would recommend that the reflective data is used for accurate analysis of what geometry the scanner has recorded. There appears to be some camera shake visible in the images applied to the scans (although the scans appear to have little vibration effects) - we believe this is due to the exposure times required.
Filtering and smoothing
The data has had the standard FARO filters to remove atmospheric feedback and dust. No additional filters were applied.
Exporting of scan data to Navisworks
To reduce the size of the Navisworks file, a subsampling ration of 1:4 is used for the colour data and 1:2 for the reflective data (ie half the recorded data is used in the reflective, ¼ in the colour). Using the full data set would generally slow the software down to an unusable rate when more than one set of data is loaded.
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