LiDAR is another tool in the surveyor’s toolbox. It is capable of providing a three-dimensional X,Y,Z position with a possible accuracy of 5 cm.. The key components of aerial LiDAR instruments include:
Positioning with Global Positioning System (GPS) control. This is accomplished either with airborne receivers within the aircraft, or with position tied to GPS base stations on the ground for survey control. There’s also post-processing kinematic (PPK) positioning for greater accuracy in the data processing phase.
The types of LiDAR instruments are fairly general, but the configuration and calibration of the instruments is specific to the application, with a fair degree of design flexibility according to the task. Topographic LiDAR can be used for either wide area mapping or corridor mapping. Bathymetric LiDAR determines water depth and can be useful in coastal areas.
Additional sensors can be flown in conjunction with LiDAR, including imagery, orthophotography and infrared. Each additional sensor input adds value unique to the design and output of the equipment.
Wide area LiDAR instruments are mounted on board of fixed-wing aircraft that fly at altitudes from 1,000 to 6,000 feet. They’re capable of wide swaths, but have a low point density, with one point per square meter on average. Wide area is lower resolution and lower accuracy than corridor mapping. The laser used is a non eye-safe high intensity laser that are safe on the ground, but not close to the instrument. Wide area LiDAR has an accuracy of 9 to 51 cm vertical and 15-65 cm horizontal. The square meter of laser point coverage translates into a half-meter horizontal accuracy.
Applications for wide area LiDAR include: base mapping, bare Earth digital elevation, flood plain mapping (contour, hyrologic, hydraulic), natural resource (biomass, tree height, timber volume), transportation/utility corridors, urban modeling (3D cityscape, urban planning, line of sight, viewshed analysis).
Corridor instruments are typically attached to booms from a helicopter that flies lower and slower than aerial (50 – 1,000 feet) with a narrow swath. The instrument shoots 12-250 laser points per square meter. The swath size is roughly equal to the flying height, with a higher point density the closer you travel to the ground. Corridor mapping is at a high density and high resolution, with 5-10 cm vertical accuracy and 8-20 cm horizontal accuracy. Typical flight speed is 45-60 mph. Typical corridor applications include: transmission lines, railways (positive train control), highways, levees and pipelines.
The LiDAR dataset is extremely large, with a typical project bringing in Terabytes of data. The data is processed for input within GIS or CAD to meet the project objectives.
There are some common misconceptions about LiDAR that should be put to rest.
Systems are all the same – This is not true. There are different configurations for each and every sensor and typically different calibration for each project. The sensors output is unique to the conditions as well as to the individual instrument.
The more points, the better – This isn’t necessarily the case. You pay more for level of detail, and the project requirements may not need the best possible resolution. Point density is purely a function of the laser pulse rate.
LiDAR only at “leaf-off” – While it’s true that there are fewer points on the ground with leaves on, it’s not a major factor for collecting accurate information. Each laser point has multiple returns (up to 16 on one pulse), so coverage around a tree can be nearly as dense as in bare Earth views.
LiDAR operates in all weather – While there are fewer limitations than aerial photography, LiDAR is not collected in all conditions. It does very poorly with snow cover, so is typically not collected on snowy surfaces. It can operate in rain, fog and light snow, but the imagery that’s typically collected at the same time isn’t good.
Advantages of LiDAR
The advantages of LiDAR include: