During the 2010 and 2011 earthquake sequence in Canterbury, a series of LiDAR (Light Detection and Ranging) surveys was commissioned to enable analysis with a pre -earthquake baseline LiDAR dataset collected in 2003.
As a result of the ongoing seismic activity, five LiDAR surveys were carried out by the end of 2011. These surveys followed each major earthquake greater than magnitude 6.0.
The effective use of LiDAR datasets is dependent on an understanding of the inherent errors. Consideration of these possible errors when analysing datasets enables levels of confidence and limitations to be applied for specific uses.
Preliminary comparisons by the client between the LiDAR datasets and the published levels of official control marks suggested that the supplied LiDAR dataset was outside its stated accuracy tolerance.
Inherent problems arise when comparing the levels of control marks to LiDAR points, as the LiDAR points may not have been secured on the same ground surface as the control mark. Control marks are often buried under trees or vegetation and therefore not measureable by LiDAR.
The number of official control marks available after earthquake events reduced from 3500 to just 30 marks.
Computer upgrades to 64-bit operating system and 32GB RAM were necessary to allow the import and processing of 2 billion points.
12d automated macros were developed to complement 12d Model’s TIN reporting functions to compare and report differences between LiDAR TINs and known topographical control points.
The height difference between LiDAR and topographical points was presented on plan using height range files to represent and illustrate the variance in colour.
“We used 12d Model to not only achieve good results from the analysis, [which was] made possible by the existing reporting and macros we developed. We were able to impress the client in our ability to respond to their needs and provide a solution to a problem with unique timeframes and [a unique] audience...
“Our results have been peer reviewed by an independent party and one dataset has also been re-tested (in another package). The results from the peer review indicated 12d Model was the right choice on the software package used to undertake this analysis and confirmed our results match those of the GIS software.”
Sam Cech, Eliot Sinclair and Partners
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