LiDAR is the buzz word in surveying and drones, but is it the best tool to use in all circumstances? In this article we look at the application of LiDAR surveys versus drone Photogrammetry surveys. We will discuss what works best, where and when. There are more in-depth and technical articles on our website however this article is intended to simplify the topic as a quick reference.
LiDAR
Drone based LiDAR is becoming increasingly popular within a variety of industries. With the ability to penetrate thick vegetation, lasers omitted and then received can provide valuable insight to the terrain under canopies. See the below video below for insight into what a LiDAR survey can deliver.
LiDAR is an effective tool when measuring elevation data beneath vegetation – even in instances where the vegetation canopy may appear to completely block all light to the ground. LiDAR relies on the smallest of gaps to capture the terrain data. LiDAR can pentrate through canopy cover and thicker vegetation to extract details about the bare ground underneath.
Point cloud from a LiDAR survey – Drone Scotland
LiDAR’s primary output is a point cloud. After verifying and cleaning the data, a user can produce even more detailed outputs including rasterised grids, contour lines and 3d meshes. With the data easily exportable to CAD, we can even provide same day turnarounds from data capture to the deliverables.
LiDAR does require more than a single return to achieve this penetration. We recommend a sensor with 3 returns for areas with thick vegetation. This lets each laser beam capture 3 separate returns at 3 different elevations which, in turn, increases the likelihood of a return from the base terrain.
Photogrammetry
Photogrammetry is the art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring, and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena (Wolf and Dewitt, 2000; McGlone, 2004)
Photogrammetry makes precise measurements of three-dimensional objects and terrain features from two-dimensional photographs. Applications in construction include the quantification of distances, heights, areas, and volumes; the measuring of coordinates; the preparation of topographic maps; and the generation of digital elevation models and orthophotographs.
At Drone Scotland we utilise aerial photogrammetry to allow our clients to make accurate measurements, prepare topographic maps and create digital elevation models. Our key sectors include environmental consultants, housebuilders and construction companies
LiDAR surveys v Photogrammetry Example
From looking at the images below, it is clear to see that the LiDAR survey was substantially more effective than photogrammetry. This is because the LiDAR can identify a single xyz point even if it only sees the point once. Photogrammetry, on the other hand, needs to see each xyz point from several different images so that it can cross triangulate the location. This becomes substantially more difficult with vegetation. Note the large gaps in the data of the photogrammetry ground points and rasterised grids. This occurred in photogrammetry at all heights (50m, 80m and 120m).
LiDAR survey – Rasterised 1m grid at 50m
rasterised grid – Photogrammetry survey
LiDAR wasn’t just more effective at creating a point cloud but it also returned more accurate data. The photogrammetry flight was unable to produce an accurate digital terrain model and produced many false z values in areas where there was vegetation. This is to be expected as photogrammetry performs better in areas with no or minimal vegetation.
LiDAR isn’t perfect and it also had gaps in the data. This is because LiDAR is not Xray. In other words, it can’t see through objects. Areas with extremely thick vegetation, that prevent the laser beams reaching the ground, produce no data. In our opinion, the LiDAR has been very effective at capturing the terrain data at this site.
Summary
To summarise, Photogrammetry is an excellent tool for capturing precise measurements of objects and the environment, particularly volumetrics and topographical surveys. Photogrammetry is however limited when it comes to areas of thick vegetation as the sensor can only capture what it can see. In these instances we would use a LiDAR sensor to capture the ground through the canopy.
To find out more about our LiDAR and Photogrammetry services contact Drone Scotland on (0141) 302 4685 or (0131) 203 3077 office@dronescotland.com
