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LIDAR Surveys of Cliffs and Saltmarsh Systems

Light detection and ranging (LiDAR) is an airborne remote sensing technique which uses lasers in the same way that other surveying techniques such as sonar use sound or radar uses radio waves. The technique obtains information on the location and height of features on the ground using lasers and a GPS system in order to produce terrain maps from millions of georeferenced XYZ points collected during a single survey.

A laser is fired from aircraft flown at a constant height of roughly 800m and navigated using a GPS system corrected to known ground reference points for greater positional accuracy. The distance from the aircraft to the ground is derived by firing the laser and calculating the time taken for the pulse emitted to be reflected back to the instrument onboard the aircraft. The range of each pulse is calculated as the laser scans rapidly across the surface allowing the height of a series of points to be calculated by each pulse. Elevation accuracy is typically +/- 15cm.

 

LiDAR survey of part of the Lynher River

The LiDAR is being flown at 1m resolution for this programme. The density of the points generated is sufficient to create a highly resolved digital terrain model.

The uses of LiDAR are not solely limited to the production of terrain models; Height contour plots and 3D views are also produced from LiDAR data and provide impressive visualisations of topographic data.

3D View of Lyme Regis Harbour Created from Aerial Photography Overlain on a LiDAR Survey

One of the greatest benefits of LiDAR is its ability to survey areas which would otherwise be inaccessible or dangerous to survey, allowing data to be collected for areas such as cliff faces and saltmarshes. It is also more cost effective and less time consuming than some more traditional techniques when used to collect data over large areas. Additionally it allows the development of DTMs for large areas; these can be easily understood and interpreted within GIS to produce graphical representation of relief and changes in relief. Relief shading makes visual interpretation of the data very simple. Profile data can also be extracted from the data set, if required.

LiDAR DTM of Plymouth Hoe

The main drawback of using LiDAR in topographic surveying is its dependence on the weather. Cloud cover limits the ability of the system to collect data by obstructing the laser pulse's path to the ground surface. Vegetation cover can also hinder the collection of topographic data as it causes the laser pulse to reflect back to the aircraft before hitting the ground, resulting in inaccurate data. For the purposes of the programme the LiDAR survey has to reach a depth of MLWS on the shore. Surveys can therefore only be carried out at low water on spring tides, reducing the windows for data collection.

The programme used LiDAR to survey the topography of the whole of the coastline between Portland Bill and Beachley Point, with repeat surveys being carried out after five years or more frequently depending on the vulnerability and accessibility of the site. The data will be invaluable in areas which are inaccessible to topographic surveyors and will help to reinforce the baseline topographic data collected during the first year of the programme.