Points of Light: Trends in Aerial LiDAR.
Trends in aerial LiDAR are driven by the desire for more−more information, more detail and more accuracy.
This fact is particularly evident in the trend to collect increasingly higher point densities and to combine other sensors with LiDAR to collect multiple data sets simultaneously. As LiDAR continues to evolve, it’s time for more surveyors--a group of professionals uniquely qualified to properly apply these technologies--to learn about these technological trends and get in the game of advising clients about the advantages of these services.
Traditional specifications for LiDAR point density call for one pulse per square meter to derive the ubiquitous 1-foot contours as specified by FEMA for floodplain mapping. However, LiDAR customers are increasingly requesting higher point densities for a variety of reasons.
By flying lower and slower to collect higher point densities, it is possible to saturate the ground with five to 20--and even up to 40--points per square meter. When flying at around 400 meters above mean terrain at 120 knots collecting 200,000 measurements per second, an object can be hit by multiple pulses, and the chance of hitting the ground in areas of dense vegetation increases. Since this provides a more accurate and better overall map product, this method is increasingly being used for topographic surveys of areas with vegetation during leaf-on conditions to produce 1-foot contours.
High-density LiDAR data is also proving valuable to forest management. In areas where open foliage is limited, the probability of canopy penetration is vastly increased by optimal ground coverage and a wide-beam, narrow viewing angle. And in addition to providing a much higher level of accuracy for measuring the bare earth, high-density LiDAR enables precise determination of single-tree segmentation, crown-area verification, trunk-diameter measurements and tree-center coordinate values.
Another driver for higher point densities is object identification. The level and precision of identification depends greatly on the density of the laser point cloud. Measurement overlap and optimal ground coverage provide a number of distinct advantages such as the ability to detect small, linear-type objects, like fences, as well as breaklines caused by planimetric features, like retaining walls, ditches, ridges and embankments. Proper data filtering provides the ability to effectively identify buildings and rooflines or drainage patterns and other hydrographic details. This level of detail enables diverse application potential, such as urban planning through the production of 3D city modeling. In these models, cadastral and engineering data can be integrated with rendered building structures to visualize expansion plans, redevelopment projects and infrastructure networks.
The trend toward higher point densities has been driven extensively by the electric-transmission market where the objective is the extraction and vectorization of individual power lines as well as 3D coordinates such as pylon positions, cable fixations, attachment points, pylon center lines, lower cross-arm edges, power-line sag and other details. Because many of these processes are automated, this detailed data can be available quickly.
The cost calculation of flying an aerial LiDAR mission is somewhat comparable to that of flying a photogrammetric survey. Lead time, mobilization cost, etc., are similar. However, there are some significant differences between the applications of the two technologies.
LiDAR is not affected by the sun’s angle as is photogrammetry. Seeing the ground photogrammetrically usually requires leaf-off conditions, which are not necessary for LiDAR. For this reason, photogrammetric surveys often take longer to deliver results than do aerial LiDAR missions. However, photogrammetry does produce a useful visual record of the area of interest.
The information and operational benefits accrued by collecting and combining multispectral imagery and LiDAR far outweigh what is provided by the individual data sets alone. The fusion of data from diverse sensors can provide information for the creation of higher-value products and services such as the development of intelligent 3D urban/natural geospatial databases with far-reaching applications and benefits.
Acquiring imagery simultaneously with the surface data can save up to 50 percent in acquisition costs. But the real value is that each surface point possesses an accurate spectral signature assigned to its location. The simultaneous collection of imagery and LiDAR facilitates the creation of true orthoimages by applying image rectification and georeferencing using intelligently filtered LiDAR data. The correct positioning of all features, such as buildings, bridges, towers and trees, is achieved by incorporating the elevation data from these features together with the terrain data to effectively eliminate object displacement. This strategy allows for the accurate classification of features such as urban terrain, forested terrain, agricultural lands, mountains, cliffs, ravines, pervious/impervious surfaces, vegetation type, wetlands and agricultural classes. Additional elements can be identified, including building footprints, height and structural characteristics; vegetation type, height and density; and natural and cultural land-use and cover information. All of these elements provide a powerful tool kit to enable intelligent geospatial analysis.
The Surveyor’s Role
There are many roles the surveyor can play in the application of remote sensing technologies such as aerial LiDAR and imaging. The surveying profession is already providing its expertise in measurement and positioning because these applications require good ground control--and there is no replacing the surveyor for that capability. Clients also typically turn to surveyors to validate the results of a LiDAR mission. Surveying for quality control following aerial LiDAR involves checking both the absolute and relative accuracies of elevations and positions throughout the area of interest.
Yet perhaps the most important role a surveyor can play in this sphere is the role of an advisor. Clients are becoming more aware of the potential of LiDAR, but exactly how it can be applied to serve their project is seldom clear to them. It is a fairly well-settled principle that as the project size increases, the unit cost for LiDAR decreases. Generally speaking, the more difficult the terrain, the greater the benefit of LiDAR. However, opinions differ on the project size required for the use of aerial LiDAR to begin to make sense. The truth is that the cost benefit of LiDAR can only be defined from individual consideration of each project and an understanding of the uses, applications and evolving trends of the technology. Sorting through these details is exactly where the client may ask the surveyor to step in.
The capabilities of LiDAR are significant. The only question is how far surveyors are willing to delve into this technology to add value to their services.