Ground-based, or terrestrial, lidar systems are used for close-range, high-accuracy applications, such as bridge and dam monitoring, architectural restoration, facilities inventory, crime and accident scene analysis, landslide and erosion mapping, and manufacturing. They have even been used to create novel music videos and movie special effects. A combination of airborne and terrestrial lidar can provide realistic 3D models of built-infrastructure with accurate rooftops and realistic building facades.
Using an infrared or green wavelength laser, ground-based lidars pulse at rates up to 1000 Hz, and can map objects from about 1 meter up to 1000 meters away with accuracies on the order of millimeters to a few centimeters. The accuracy of individual points can be affected by atmospheric conditions, distance to the target, angle of incidence of the laser pulse upon the target, and the reflectivity of the target surface. Very shiny, polished surfaces and very black surfaces that absorb nearly all incident light are difficult to image.
Ground-based lidars are either static (on a stationary platform such as a tripod or mast) or dynamic (on a moving vehicle). It has been incorporated into surveying and metrology instruments and is often employed in mobile mapping systems. In a static implementation, a GPS/INS georeferencing system is not needed. The lidar is set up over a known point, and the scan angles for each point are recorded in the data set. Reference points on the target surface can also be surveyed to provide additional georeferencing control. In a dynamic implementation of ground-based lidar, GPS/IMU is utilized to provide georeferencing, just as it would be on an airborne platform. Mobile mapping systems are often used to develop 3D streetscapes in cities, where GPS signals can be blocked by tall buildings or affected by error-inducing "multipath." Mobile mapping systems employ additional motion-detecting sensors to provide corrections to the GPS/IMU in these applications.
Many ground-based lidars use the simple principle of laser pulse rangefinding introduced earlier in this lesson. These systems pulse at lower frequencies and can measure distances of several hundred meters with centimeter-level accuracy. Another technique, called "phase-differencing," is used in systems intended for very close range (less than 50 meters) work with millimeter-level accuracy. For those who are familiar with GPS, the difference between pulse rangefinding and phase differencing is analogous to the difference between GPS code-phase and carrier phase signal processing. For the interested student, these concepts are explained with more detail in Shan and Toth. We will only be dealing with pulse rangefinding lidars for the remainder of this course.
Three types of scanning systems are employed in ground-based lidar:
- Panoramic scanners rotate 360 degrees around the mounting axis, and scan 180 degrees vertically to provide seamless and total coverage of the surroundings.
- Single axis scanners also rotate 360 degrees but are limited to a 50-60 vertical swath.
- Camera scanners point in a fixed direction with limited angular range both horizontally and vertically.
Ground-based lidars can also be classified according to operational range:
- Short-range systems operate at ranges of 50 - 100 meters with panoramic scanning, and are often used to map building interiors or small objects.
- Medium range systems operate at distances of 150 - 250 meters, also achieving millimeter accuracies in high definition surveying and 3D modeling applications, such as bridge and dam monitoring.
- Long range systems can measure at distances of up to one kilometer and are frequently used in open-pit mining and topographic survey applications.