Before introducing LIDAR technology in forestry in the last decade, forest management personnel struggled to figure out ways of determining the height of trees and the general structure of a forest. Today, modern aircrafts fitted with this technology have made it easy to gather even more detailed information on vegetation distribution and surface features that impact the survival of trees.
LIDAR systems use a beam of light cast towards a surface or a desired feature, and then the travel time taken for it to return to the sensor is used to determine the distance. A product of the constant speed of light and half the travel time of this light gives us the exact distance or height of a canopy. Therefore, this article will attempt to explore how LIDAR remote sensing technology has been crucial in determining forest characteristics.
LIDAR technology
Even with advanced aerial photography, forest managers couldn’t accurately figure out tree height, tree distribution, forest landscape elevation, and other physical characteristics. Whereas LIDAR technology might be comparatively expensive, it’s solved all these concerns, and the level of precision of data collected is astounding.
With 2D LIDAR technology, only a single beam of light is focused towards the target forest surface, and as the sensor spins, it gathers object data along a horizontal plane. This data processing results in 2-dimensional images of forest features because the returning light collected data only on the X and Y coordinates of target features.
On the other hand, to gather even more detailed forest information, including canopy height, advanced 3D LIDAR technology is used. 3D sensors can capture details on the Y coordinates of target features, thus suitable for estimating tree height. 3D systems are designed to transmit several laser beams as opposed to 2D.
In both cases, the beam’s travel time multiplied by the speed of light gives an accurate distance between the target and the source of light.
Classification of LIDAR systems for forestry applications
Despite the general LIDAR classification in a broad industry spectrum, categorization is quite different in forest management. LIDAR systems in forestry are classified based on:
- The forest surface scanning pattern.
- Recording of the first and last return.
- Footprint size.
Basing on the factors above, LIDAR systems in forestry management can be categorized into:
- Small footprint LIDAR technology
In small footprint LIDAR, the transmitted beam’s diameter is a few centimeters (usually between 5 and 30 cm). Because of beam concentration in a small area of focus, high-density data can be gathered, enhancing data precision. This type of LIDAR technology’s primary limitation is its inability to scan a broader range at once, thus not suitable for forest mapping since the helicopter has to fly extensively.
Moreover, a small footprint LIDAR system is designed to capture either first and last beam returns. In some cases, the beam doesn’t reach the ground, thus jeopardizing precise canopy height determination.
- Large footprint LIDAR technology
An extensive footprint system is generally superior to small footprint LIDAR, thus eliminating its limitations. First, the beam that’s being transmitted has a larger diameter (between 10 to 25 meters), therefore covering a broader forest area scope. This type of LIDAR technology is suitable for mapping forest structure because the beam can sweep a larger forest area as it touches adequate ground and tops of trees. Canopy height determination is accurate with the deployment of large footprint systems.
Moreover, unlike its counterpart technology, large footprint system deployment is cost-effective as it eliminates the expenses involved in mapping a larger forest area. Lastly, the full beam (first and last) that returns are digitized, thus enhancing canopy height precision because of sufficient vertical distribution between the top of trees and the ground surface.
- Profiling LIDAR technology
That was the earliest LIDAR instrument in forestry used for figuring out canopy height, but it has evolved considerably. With this LIDAR system, a beam of light is focused along a single plane of the target object, and the viewpoint is fixed. This mode of canopy height determination is less expensive, though it lacks sufficient forest surface detail.
- Ground-based LIDAR technology
Unlike airborne LIDAR technologies, the ground-based LIDAR sensor is positioned below the trees. The system’s algorithm can detect trees, gather data on forest base level characteristics, et cetera. Its evolution has transformed its role into a tool for forest inventory management.
Digital Elevation Models (DEM)
Data collection by the use of LIDAR systems results in a large volume of point clouds. These data points, the X, Y, and Z coordinates, must be processed to provide meaningful forest surface information. But before analysis, these data points have to be separated to form the digital elevation model that depicts vegetation and ground structure.
The denser a forest is, the lesser the light beams reflected from the ground return to the LIDAR sensors than those reflected from canopy tops. The opposite is true. The need for adequate data for both vegetation and non-ground features in the formation of digital elevation models has made LIDAR technology suitable in forest management. Lastly, the LIDAR technology application in forestry has been embraced due to the high-resolution 3D images that enhance data precision.
Advantages of LIDAR in forestry
- LIDAR’s ability to provide large quantities of data on vegetation distribution, forest topography, and precise canopy height information at higher resolutions makes it a suitable tool in forest management.
- The application of LIDAR systems in forestry is independent of lighting and weather conditions, making it a useful tool.
- Because of LIDAR’s ability to collect data over a vast forest area quickly, it can significantly help minimize data gathering costs.
- LIDAR laser light can penetrate thick forests, thus enhancing data collected over a wider forest area.
Disadvantages of LIDAR in forestry
- Advancement in LIDAR technology has made it more sophisticated to operate than before, which requires highly-skilled personnel.
- LIDAR is a costly approach in forestry, especially inventory management.
- Whereas LIDAR data collected might have high precision, canopy height is usually less than that from photogrammetry.
The bottom line
LIDAR systems have eliminated forest managers’ struggle to collect accurate and sufficient vegetation and ground elevation data. LIDAR technology uses first and last light pulses to determine forest landscape topography, canopy height, et cetera. LIDAR systems used in forestry can be classified into a small footprint, large footprint, profiling, and ground-based LIDAR technologies basing on surface scanning patterns and footprint size.
Lastly, the LIDAR technology application has its pros and cons, which must be evaluated carefully before deciding.
