This technology involves a scanning and ranging laser system that produces pinpoint accurate, high-resolution, topographic maps. The original technology has been in existence for 20-30 years, but the commercial applications for LiDAR generated topographic maps have only developed within the last decade. Today the entire process of airborne laser mapping is highly automated from flight planning, to data acquisition, to the generation of digital terrain models.
The basic components of a LiDAR system are a laser scanner and cooling system, a Global Positioning System (GPS), and an Inertial Navigation System (INS). The laser scanner is mounted within a properly outfitted aircraft and emits infrared laser beams at a high frequency. The scanner records the difference in time between the emission of the laser pulses and the reception of the reflected signal. A mirror that is mounted in front of the laser rotates and causes the laser pulses to sweep at an angle, back and forth along a line. The position and orientation of the aircraft is determined using a phase differenced kinematic GPS. GPS systems are located in the aircraft and at several ground stations within the area to be mapped. The orientation of the aircraft is then controlled and determined by the INS.
The round trip travel time of the laser pulses from the aircraft to the ground are measured and recorded, along with the position and orientation of the aircraft at the time of the transmission of each pulse. After the flight, the vectors from the aircraft to the ground are combined with the aircraft position at the time of each measurement and the three dimensional XYZ coordinates of each ground point are computed.Video Produced by the Carl Vinson Institute of Government at University of Georgia
The system can be operated at various scan frequencies and at different altitudes depending on the measurement accuracy dictated by the project requirements, as well as by the regulated eye-safe range of the particular laser. By accurately timing the round trip travel time of the light pulses to the surface it is possible to determine the distance from the laser to the ground; typically with a precision of 10 to 25 centimeters. Typical operating specifications permit flying speeds of 50-200 knots, flying at heights of 100 to 5,000 meters, scanning angles up to + 20 degrees and pulse rates of 2,000 to more than 100,000 pulses per second. These parameters yield enough data points to create a highly accurate digital terrain model (DTM). Typical users of this technology have achieved accuracies of roughly 15 centimeters at up to 95 percent confidence interval vertically and 1 foot or 30 centimeters horizontally.
The post-flight processing combines precise aircraft trajectories developed from differential GPS solutions with the corrected laser ranging data and aircraft roll, pitch, and heading information. Integration of this data produces a precise horizontal position and vertical elevation for each laser pulse. Each data point can be identified by type, i.e. ground, vegetation, building, power line or other object. Once classified, it is simple to manipulate data, remove layers of data points and create digital terrain models (DTM).
Each pulse of light is accurately measured and later classified as ground, vegetation, structure, etc. After post-processing these points make up the Digital Terrain Model, depicting the ground with as dense point spacing as the customer desires.
By rapidly scanning the ground from left to right and back again along the aircraft's planned flight path, a ‘herringbone’ pattern of spot elevations are collected. The laser must scan quickly enough to prevent unwanted gaps along the outside edges of the flight path. Because the desired point spacing is typically between 2 and 10 feet, the system must be capable of scanning all the way across and back again before the aircraft has advanced beyond this distance along its flight line.
As you can imagine, the laser sometimes hits more than one object on its trek to the earth's surface. For example, it may pass through a vegetation canopy, touching leaves or branches before finding its way to the ground. The system Airborne 1 puts to work for its clients is capable of delivering only the ‘last return’ when only ground surface data is requested. However, we can simultaneously collect all ‘first returns’ when customers desire data containing tree and/or vegetation heights, quantities, and locations. Providing both sets of data allows users to view their project areas with or without existing vegetation, without having to fly a project twice.
Systems can be designed for a wide variety of altitudes and point-densities. One of the many tradeoffs is between laser power and altitudes allowable by federal safety regulations. The systems employed by Airborne 1 have always been built with safety as a first consideration. One important safety feature is an automatic disabling of the unit when altitudes that might expose people and animals on the ground to even the slightest risk of eye injury are reached.
When projects need a rapid turnaround, laser terrain mapping can deliver. By selecting the right service providers and/or system design firms, one can ensure that the proprietary software is capable of turning the volumes of collected data into usable and fully reliable ASCII XYZ points, with or without surface features. Unlike traditional photogrammetry methods LiDAR elevation data is only collected in a digital format. This eliminates the laborious process of converting analog data (e.g. paper photos or negatives) into digital. It also eliminates the interpretation errors possible from traditional methods of elevation data compilation.
Initial output is in generic ASCII XYZ data format, in WGS84. This data is typically converted according to the clients’ needs with respect to datum and coordinate systems.