Laser Scanning and Conventional Surveying
Laser scanning involves the collection of millions of accurate points from a single scan position. By ‘conventional surveying’ we refer to total station (TS) or GPS surveys where typically one or two operators are required to record individual points. Some of the advantages – from our point of view – of the scanning technique (also referred to as Terrestrial LiDAR) are:
- total coverage of all visible features meaning the risk of omissions is eliminated
- the ability to analyse subtle or complex geometries of targets such as wall structure
- simple comparison of datasets acquired on different dates for monitoring movement or subsidence
- true colouring of the points can give life-like representations of the data (good for planning illustrations for example)
- the scan data point cloud can be sliced to give quick, detailed cross sections or plans of a building
- the point cloud can be animated to create impressive fly-throughs
- unique 3D viewpoints and ortho elevation views can be rendered from the points to give unique views of the scanned target
- Speed – scan times from one setup can be a matter of minutes. Typical site times are 1-2 days.
Conventional surveying of course has its place and we routinely find that it compliments laser scanning surveys. For example, a total station survey to produce CAD building detail can be cross-checked against a scan point cloud. Laser scan setup positions can be coordinated using the familiar occupation and backlight method, using a special target that can be detected by the scanner. Depending on the accuracies needed, total station traversing or GPS observations are used to tie point cloud data into real world coordinates.
The example below is from a refurbishment project where a total station was used to produce linework for building features such as window openings, vents, pipes etc. This was cross-checked using the point cloud data, however one further deliverable was generated that could not be derived easily using conventional data: an analysis of the verticality of the walls of the building was made by comparing the measured point cloud surface to a vertical model plane. Bespoke software is used to carry out deviation analysis of the actual surface versus the theoretical model. This information is essential for projects such as building refurbishment or analysing movement due to subsidence.
The instrument used for this survey was a Faro Focus 3D. The software used to process the data was Faro Scene (registration and export), Pointools View Pro (ortho rendering) and Topcon Scanmaster (analysis and QA checks)