Article by Chuck Boyer, Director of Geospatial Solutions, Aerial Services
When discussing LiDAR accuracy, the most important statement a LiDAR consultant can end their sentence with is “with proper ground control”. Without proper ground control, the measurable and definable accuracy of LiDAR point cloud data cannot be known. In capable hands of a professional mapper, accuracy can be inferred, but without ground control it cannot be objectively measured.
Airborne LiDAR sensors fire pulses of light up to 500,000 or more times per second. These pulses then hit the ground, reflect back to the sensor and are recorded as distances. In a typical mapping mission, there are billions and billions of points collected and form what we refer to as a “point cloud”. Each point in the cloud has an associated 3D coordinate. It is a marvel of modern science that an aircraft moving at high rates of speed can emit then capture so many points of reflected light every second and that each point can be accurately determined where it came from.
“Ground Control” is comprised of special points on the ground, visible in the “point cloud” where each point has a very precise measurement of its position in 3D space (XY & Z coordinates). This control is usually established by a surveyor utilizing accurate measuring devices accurate to within hundredths of a foot or better. As the point cloud of LiDAR data is processed, these known surveyed points are used to mathematically adjust the point cloud of data to a more accurate position in the 3D world.
Positional accuracy is defined as “The closeness of results of observations, computations, or estimates of graphic map features to their true value or position“. There are two kinds of accuracy in remote sensed data, including aerial LiDAR: absolute and relative. Relative accuracy is defined as a measure of the accuracy of features relative to other features on the same map. Absolute accuracy defines a features absolute position in the world’s coordinate system.
A number of factors contribute to the achievable accuracy of a LiDAR point cloud: the quality of the GPS and the Inertial Measurement Systems (IMU) in the aircraft, the GPS equipment on the ground, and the accuracy of the sensor. Aerial Services, Inc.’s (ASI) uses a Riegl VQ-480 LiDAR sensor. This sensor is designed to collect LiDAR data for corridors or small block areas and can achieve tremendous accuracies when used skillfully. Typical, projects well-suited to this type of sensor include Oil & Gas corridor, Electric Transmission Lines, Transportation Corridors, Land Development projects, Mining projects, Land Fill measurement assessments, Forestry assessments, and many others.
Despite how LiDAR is used, its fundamental purpose is to create a 3D representation of the area sensed by creating a Digital Elevation Model (DEM), a Digital Surface Model (DSM) or a Digital Terrain Model (DTM). All projects share a common need for information about where exactly features are on earth and relative to one another. Our clients want to know where “danger” trees are near power lines. Or they want to know where the pipeline is and calculate a depth of cover analysis. Transportation engineers frequently require “engineering grade” spatial information (point densities of 20-40 LiDAR points per square meter) to build roads and move dirt.
Riegl claims the accuracy of its unit is 2.5 cm on well-defined features. Achieving this demanding level of accuracy using LiDAR is generally not possible without using ground control. When designing a remote sensing project, Aerial Services’ geospatial experts will determine the optimal placement of ground control points within the project area so maximal positional accuracy is achieved. A 10 mile oil & gas pipeline corridor may require 2 points at each end and a few along the way. The number, placement, and type of ground control points will vary based on the intended use of the spatial information and the physical realities (location, accessibility, geography, geometry) of the project site.
Over the last year, ASI has provided aerial LiDAR solutions for numerous DOT and other projects. Transportation engineers typically require accuracies of 3.0 cm. We have been able to achieve positional accuracies of up to 1.5 – 2.1 cm accuracy on hard surfaces, which is better than the 2.5 cm the LiDAR manufacturer asserts. How is this possible? By using proper ground control! Positional accuracy reports are provided to our clients and it is recommended that project data is independently verified by the client or third parties to test and document the accuracy of each solution provided. A well-engineered project will include sufficient ground control to achieve, test, and objectively document the positional accuracy of remotely sensed data. It makes little economic sense to invest in LiDAR or aerial imagery and skimp on ground control. Your best insurance, your best return on investment, and your best assurance of a reputable geospatial services provider is to insist on a positional accuracy specification and ground control to verify this accuracy. Aerial Services’ geospatial experts are able to engineer all remote sensing solutions delivered with the needed positional accuracy for your next project.