Updated: Sep 11, 2018
Flying High with LiDAR
Perhaps you’re planning a county and state mapping project. If you’re collecting data across a wide area like that, odds are you’ll want to use high-altitude LiDAR sensors. Usually this involves a sensor mounted in a fixed-wing aircraft, flying somewhere between 400 to 2,500 meters above ground level, depending on the type of system and project configuration. These sensors can operate at higher altitudes, but most requirements and sensor performance limitations will keep your surveying no higher than 2,500 meters.
In general, the higher you fly, the lower the accuracy of the sensors, particularly compared with lower-flying LiDAR operations. Accuracies for a high-altitude project are in the neighborhood of 9.25 to 18.5 centimeters vertically and 20 centimeters to 1 meter horizontally. As technology continues to improve, expect to be able to fly higher altitudes while still hitting the accuracy requirements you’re seeking.
To do their job, high-altitude systems will require differential ground reference GPS stations. Typically, you’ll need two or more GPS stations during a flight. The GPS control information can come from virtual reference stations (VRS), continuous operating reference stations (CORS), national geodetic survey (NGS) points, or established points that are referenced to a network. Base stations are set within a certain radius of the aircraft at any given time, so several points may be used during a project, depending on how big an area you’re mapping. Typically, the radius will range from 20 to 80 kilometers, depending on accuracy and project requirements.
Even if you’re covering a large territory, the flight line lengths are usually limited because the inertial measurement unit (IMU) tends to drift over time if you’re continuing in a straight line. The IMU needs movement to find itself, so you’ll typically fly the project with opposing directions from flight line to adjacent flight line. Operating this way helps verify the relative accuracy of the data or the relationship of one flight line to another.
It almost goes without saying that as LiDAR technology has developed, the size of LiDAR projects has increased dramatically. Plus, these systems have improved significantly in accuracy, efficiency, and performance.
Flying Under the LiDAR
If your project involves a transportation corridor — such as road surveys, rail line surveys, transmission surveys, and pipe-line surveys — you’re more likely to make use of a low-altitude airborne LiDAR system. Not only are the altitudes much lower than other airborne LiDAR sensors, these devices are typically mounted in helicopters rather than fixed-wing aircraft.
These sensors can be flown at altitudes as low as 50 meters above the target and as high as 800 meters, depending on the application. They tend to operate at much higher repetition rates, and the point sample spacing is much higher than the high-altitude LiDAR projects. The typical point density is between 20 and 100 points per meter, depending on application. Because of all of these differences, the accuracy is higher than the typical high-altitude project.
For corridor mapping projects, several base stations might be set up along the project corridor. Depending on the accuracy requirement, these base stations are often located much closer together than those used for high-altitude surveys. The GPS base station points used for these operations are the same as with the high-altitude systems. More often than not, extensive ground surveys will facilitate the LiDAR collection, to increase and verify accuracies.
The IMUs used in these systems are very similar to the ones used in the high-altitude systems. Typically, these LiDAR systems are coupled with additional sensors to provide more data for the applications.
Young, James. LiDAR for Dummies. Hoboken: Wiley Publishing, Inc., 2011.