What does LiDAR stand for?
LiDAR stands for Light Detection And Ranging. LiDAR uses light waves from a laser fired from a sensor or scanner. A LiDAR system calculates how long it takes for the laser pulses to hit an object or surface and then return back to the sensor. This gives an accurate positioning point of where the laser has hit.
Each of these measurements, or returns, can then be processed into a 3D visualisation known as a point cloud, along with other deliverables such as a digital surface and terrain models and their corresponding contours.
In order to capture ground levels, the LiDAR scanner exploits gaps in the foliage in order to reach the ground. Typical LiDAR scanners used in the commercial surveying sector do not pass through the vegetation itself.
What are the advantages of LiDAR flights?
Photogrammetry techniques that are commonly used by drone pilots will only result in the creation of a Digital Surface Model (DSM). The DSM incorporates elevations from natural and built surfaces, such as buildings, tree canopy, and power lines. Essentially, a DSM represents the earth's surface and all objects on it. The image below is of a DSM.
The use of a LiDAR scanner allows data to be captured in areas where photogrammetry would not be able to reach. Typically this would be underneath vegetation where the laser can find gaps in the vegetation to reach the ground.
This allows for the creation of a Digital Terrain Model (DTM) which is a 3D representation of a terrain surface. DTMs include features such as rivers, ridge lines, etc. It does not include vegetation and built elements, as illustrated below.
Where can LiDAR be used?
Can I capture ground levels in a woodland with LiDAR?
LiDAR scanning can be very efficient in capturing ground levels in areas of woodland when compared to traditional survey techniques, where a surveyor would need to work their way through the woodland on foot. Resulting in slow data capture using total station set ups.
However, as mentioned above the laser of a LiDAR flight does not physically penetrate vegetation, the laser from the LiDAR scanner is able to get through small gaps in between leaves, branches etc, which therefore allows it to ‘see under’ the vegetation.
If a LiDAR flight is carried out in the middle of Summer when the tree canopy is dense, then the laser will struggle to find gaps to pass through. Compared to a deciduous woodland in winter, where the laser would have plenty of gaps to pass through and reach the ground. It should be remember though in Autumn the leaves will have fallen on to the ground and the laser will not pass through a carpet of leaves.
What about grassland and agricultural land?
If the vegetation is short and dense the laser can struggle to find gaps to penetrate, therefore the number of point so the ground itself will become limited. Sometimes it is a case that no ground levels can be detected over grassland / agricultural land in the middle of summer when crops are at their maximum growth.
Ideally if agricultural land is to be flown, it should be done so once crops have been harvested or in early spring whilst they are still small/short and not dense.
What Accuracy can I expect from LiDAR flights?
LiDAR flights are typically carried out using Ground Control Points or RTK survey techniques in order to gain high absolute accuracy. The data will therefore be georeferenced to within 5cm horizontally to the true world coordinate location of the site.
The vertical error of LiDAR flights can be impacted by numerous factors which include:
Climatic conditions (high dew point, dust, etc) - all of which can cause the laser to scatter and create noise in the final dataset.
Vegetation - dense vegetation will reduce the number of points at ground level.
Surface Reflectance - reflective surfaces like solar panels, metal roofs and vehicles can cause scatter of the laser and create noise in the data captured.
The biggest influencer of vertical accuracy is the density of vegetation. This is explained in further detail below:
Tall vegetation
Typically found in woodland locations, the dense tree canopy will restrict the number of LiDAR returns that reach the ground. As a consequence the resulting digital terrain model (DTM) will have some inaccuracies in it as the resulting point cloud of the ground will contain very few data points. However the data points that have been captured will have a vertical error in the region of 5cm-10cm.
Short dense vegetation
Where the vegetation is short and dense this will stop nearly all LiDAR returns reaching the ground, as a consequence the DTM can only be created using computer algorithms and the vertical error can anywhere between 20cm - 50cm - 100cm.
Care should therefore be exercised when considering the use of LiDAR for grassland and/or agricultural land when vegetation is present.
What does a LiDAR scan look like?
The LiDAR return does not contain RGB information. Therefore LiDAR scans are by default colourless.
The example data below is shown with gradient shading, otherwise it would just appear as a single colour.
It is possible to colourise the LiDAR scan in RGB, however Sitemark needs to be informed prior to any flight being carried out if this is a requirement. Below is an example of an RGB colourised LiDAR scan.
How do I see the ground under the vegetation in my data?
A LiDAR flight includes a full laser scan of the selected site, the resulting point cloud will therefore include vegetation, buildings and other above ground features/structures.
The LiDAR scan will classified. Therefore you need to download the point cloud from the platform, which will be in an las file format. Using point cloud software you will be able to view just the points captured of the ground, this can be achieved by selecting only the ‘Ground’ classification in the las file.
The Sitemark platform also creates a DTM tiff and contours based solely on the Ground classification and these can be downloaded directly from the operation.