Light Detection and Ranging (LiDAR) technology offers an efficient way to produce digital elevation models (DEMs) for a variety of large-scale, high-accuracy mapping applications. Innovations such as multiple intensity returns and increased repetition rates have broadened the technology’s usefulness. But deciding how to use LiDAR for an application can be difficult, and several requirements must be considered:
 
• What is the coverage area?
• How much vegetation and relief are in the coverage area?
• What are the accuracy requirements? • What is the time frame for completion?
• Where is the project located, and how does it affect collection?
• How will the LiDAR data be used?
• How much will the project cost?

Determining such requirements during project planning, data acquisition and data processing will help ensure a project’s success. Product Selection To simplify LiDAR selection, consider the three basic types of LiDAR products and determine which is most appropriate for a particular application: basic LiDAR, Federal Emergency Management Agency (FEMA) LiDAR and concentrated LiDAR.
 

Basic LiDAR
Basic LiDAR products provide low-cost, general-use, horizontal and vertically accurate elevation data sets that meet most DEM needs. Typical uses for these datasets include the following:

• Generating bare-earth elevation models in sparse to moderate vegetation terrain
• Large-area DEM applications in which vertical and horizontal accuracies are required
• Watershed and hydrological study areas
• Forestry and tree canopy analysis
• Building and man-made structure detection and 3-D modeling


Basic LiDAR products are used as a cost-effective way to collect large-area projects in a time-
constrained environment. Although collection and processing specifics vary from company to company, such products provide roughly the same results at varying costs.

The density of basic LiDAR varies, but most average 1.4-meter point spacing, and collection occurs within 45 kilometers of a ground control base station. Typically, two base stations are used, and ground control points are set up or tied to National Geodetic Survey control.

Scan frequency and scan angle values are determined by project elevation relief, along with terrain and vegetation criteria, to generate an optimal model based on the specifications of the LiDAR system used. Flight planning is done to optimally cover the project area.

It’s also important to consider Positional Dilution of Precision (PDOP), the geometrical effect on GPS accuracy. The higher the PDOP value the less accurate the position solution. When collecting basic LiDAR data, the PDOP is typically at or below 3.2. Also, a KP index forecast is checked prior to collection missions to determine geomagnetic activity and its effect on Earth. No missions should be performed when the KP index is at or above 4. This activity affects GPS results, which will in turn greatly impact the accuracy of the LiDAR data. For more information on the KP index, go to www.sec.noaa.gov/info/kindex.html.

In addition, data voids should be considered. Typically, data voids aren’t tolerated when they arise during data collection, as they usually occur between collection flight lines as a result of flying conditions and/or bad planning when relief and terrain aren’t considered. Data voids also can arise from system malfunctions. All data voids can be checked during collection, and additional passes can cover these areas without affecting data accuracy or integrity. Basic LiDAR usually is delivered as a bare-earth product, so data voids resulting from vegetation and building removal in the filtering process is accepted.

 
There are many different filtering processes from which to choose, depending on point density, relief, terrain and vegetation. When filtering basic LiDAR, a minimum of 89 percent of all artifacts are removed automatically. A minimum 90 percent of all outliers will be removed without any manual editing. Processes may vary within the project to account for variances in vegetation, features and terrain.

A basic LiDAR product’s quality and accuracy are relatively consistent for most systems. To ensure quality data, most providers routinely calibrate their systems every project or even every mission. Manufacturer specifications, collection parameters and industrial standards determine accuracy requirements. Vertical accuracies are 18.5-centimeter RMSE or better in flat terrain and meet 2-foot contour requirements; for rolling and hilly areas expect 37-centimeter RMSE or better and 4-foot contour requirements. Horizontal accuracy—usually at or less than 1-meter RMSE—is a function of flight altitude and beam divergence.

Concentrated LiDAR specifications and accuracies vary depending on the project, but overall the requirements for this product are more stringent than basic and FEMA products. On average, the point density is 0.7 meters and higher for corridor mapping. Baselines for collection vary between 8 kilometers and 40 kilometers, depending on project requirements. Ground control requirements for point accuracies are consistent with other products, as well as fight planning parameters, including PDOP and KP index. Flight line spacing and number of passes are determined based on the LiDAR system used for the project to achieve the desired point density, which in some cases can be eight points per square meter. Data void consideration and system calibration should follow the same constraints as any other LiDAR product.
 

Data quality and accuracy for this product is more refined than other products, but varies depending on project application. Most often collection of static and kinematic ground control is required. System calibration will be performed for every mission using a post-processing differential kinematic survey in addition to a basic calibration process. Vertical accuracies are 15 cm RMSE or better, while horizontal accuracies are usually at or under .5 meter RMSE. However, higher accuracies have been achieved. Many LiDAR providers base accuracy analysis on several different collection and manufacturer specifications.

Although the products mentioned here meet most project requirements, LiDAR can provide additional tools using intensity returns generated from the LiDAR pulses. LiDAR systems collect 12-bit intensity information for several pulses depending on the system. This information is usually best represented in an 8-bit georeferenced gray shade image representing the intensity values transformed between 0 and 256. The accuracy of such an image is a function of the beam foot-print, point density, scanner encoder and reflective nature of the surface, and it varies depending on the system. Applications for the technology are still growing, but recent uses include 2-D and 3-D break line generation, location and risk studies, and rough image interpretation. For more information on LiDAR’s diverse uses, see “Limitless LiDAR Applications” below.
 

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