LIDAR

What is LiDAR

Airborne laser scanners use Light Detection and Ranging (LiDAR) technology for measuring terrain points in horizontal and vertical, from an airborne platform such as an aircraft or a helicopter. Airborne LiDAR is primarily a height acquisition system. The laser scanner sends pulses of laser light towards the ground and records the range of the signal as it hits a surface and returns. The laser pulse produces up to four echoes as it hits different targets on its way towards the ground. This allows the system to receive signals through vegetation and tree cover where the light hits multiple targets such as leaves and branches on its way towards the ground. LiDAR is an active light system and can operate at night or in poor light conditions.

Airborne laser scanning provides high density, high accuracy spatial data. The density and accuracy of the point cloud produced depends primarily on the flying height. Point densities typically range from 2-3 points per square metre for regional datasets to 12-16 points per square metre for detailed corridor mapping.

How does it work

Airborne laser scanners use GPS and IMU to determine the scanner position and trajectory during flight. In addition the GPS receiver on the airborne platform ground based GPS receivers are also used to record control points to help geo-reference the point cloud data. The geo-referencing and quality of the data can be checked by analysing the difference in height between overlapping areas of adjacent data strips.

The point cloud data produced from airborne laser scanning requires classification to group points reflected from different surfaces into classes such as ground points, building, high vegetation and low vegetation. The classification process is undertaken in LiDAR processing software such as TerraScan. Various algorithms are used to group points into the different classes. Classification is not a fully automated process. Automatic point classification will have varying degrees of success depending on the characteristics of the terrain and will require manual intervention to inspect classification results and identify point classifications by visually interpreting the data. To assist this process it is useful to use orthophotography to help interpret and classify the point cloud.

Once the point cloud data has been classified, the groups of points can be used to generate “bare earth” Digital Elevation Models (DEM) produced from only the ground points and Digital Surface Models (DSM) which include buildings, roads, vegetation, ground and other surface features. The DEM and DSM models are first produced as Triangular Irregular Network (TIN) models and then statistically interpolated to produce regular grid models with height posting at a regular spacing appropriate to the surface accuracy requirements of the project.

Range of LiDAR Outputs

• 3D point cloud - xyz data
• “Bare earth” Digital Elevation models
• Digital Surface models
• Contour models
• Vector mapping
• Cross sections
• 3D models / fly through presentation
• High resolution orthophoto
• Video imagery

Applications

• Corridor mapping for road, rail and utility applications
• Flood plain mapping and monitoring
• Watershed modelling
• Forestry
• Extracting GIS Data
• Noise propagation modelling
• Land slide risk assessment
• 3D modelling of terrain and structures
• City modelling
• High resolution orthophotography
• High resolution archaeological landscape modelling

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