IntroductionThe need to acquire high density LiDAR data sets is on the rise throughout many industries. This 3D technology provides an invaluable tool for measurement, planning and assessment for projects such as the identification of geographic features, urban infrastructure mapping, and energy and safety assessments. When problem-solving with LiDAR data, it is preferable to use a data set with many survey points because a denser point cloud yields more accurate feature extraction. The native pulse density of a LiDAR data set is the number of pulses emitted by the LiDAR system, and is commonly expressed as Pulses per Square Meter (ppsm). When determining the specifications for any given LiDAR project, the questions often asked include: “What density is applicable to my study area?” In January, 2010, WSI published a white paper that described the need for a minimum of 8 ppsm data for LiDAR-based analysis in general and in the Pacific Northwest, focusing in on applications in geology, urban planning, fire modeling, and forestry. This document set a new industry standard for the quality of LiDAR data. In the time that has passed, the possibilities and applications of LiDAR data have expanded even further, as has the potential for higher quality data at more attainable costs. For modern standards in data usability, WSI recommends a minimum LiDAR density of 15 pulses per square meter (ppsm).
In regions of very thick vegetation, LiDAR sensors produce fewer ground returns because the ability of the laser to penetrate the thick canopies and branches is diminished. Acquisition with a higher pulse density increases potential ground returns, resulting in a more useful and valuable data set for any kind of analysis. In addition, when deriving tree crown polygons, performing vegetation segmentation or extracting canopy heights, there is a direct correlation between the pulse density and the quality and precision of the resulting products.
Terrain ModelsAreas having steep terrain often require higher density LiDAR data, as deep gullies and abrupt topographic features are not often clearly represented by lower density data. Fewer pulses hitting the ground surface equates to greater interpolation between returns, and steep features may not be clearly defined within the LiDAR at 8 ppsm.
Raster SizeHigher resolution LiDAR data allows for a smaller raster pixel size, and therefore a sharper ground model at large scale. A sharp Digital Elevation Model (DEM) is crucial for observing and measuring small changes in ground features. When demonstrating findings of geological studies using LiDAR, a high resolution raster increases confidence in the results.
Feature ExtractionAbove-ground features, such as human-made structures, are best represented with higher density LiDAR data because edges and angles are better defined. Creating building footprints and 3D models is most efficient and precise when derived from a point cloud with at least 15 ppsm. At this resolution, features are illuminated from more angles, creating sharper edges. For the same reason, LiDAR projects involving public utilities are also superior at higher densities. Power line infrastructure modeled for engineering applications depends on a full-bodied data set of laser returns for each transmission tower and conductor.
Percent IlluminationHigher density directly correlates to the percent illumination of features on and above ground level. The table below indicates percent illumination of an object’s surface compared to LiDAR pulse density. At 8 ppsm, only 56% of an object’s surface is illuminated, but at 15 ppsm the percentage doubles. At 30 ppsm, over 200% illumination is achieved.
What Density is Appropriate?
8 ppsm or less
- When the terrain is flat, with little topographic relief
- When there is a separate primary source of information
15 ppsm or greater
- When there is thick vegetation
- When modeling data
- When performing feature extraction
- When deriving planimetric vectors