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LiDAR in Mapping Dhananjay M. Satale1, Madhav N. Kulkarni Department of civil Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai- 400076. Tel: (91-22) 25767308, Fax: 25767302 Email: m1satale@civil.iitb.ac.in
1. Introduction
LiDAR is an acronym for Light Detection And Ranging, sometimes also referred to as Laser Altimetry or Airborne Laser Terrain Mapping (ALTM). The LiDAR system basically consists of integration of three technologies, namely, Inertial Navigation System (INS), LASER, and GPS. The Global Positioning System (GPS) has been fully operational for over a decade, and during this period, the technology has proved its potential in various application areas. Some of the important applications of GPS are crustal deformation studies, vehicle guidance systems, and more recently, in LiDAR. Geo Spatial Information is an important input for all planning and developmental activities especially in the present era of digital mapping and decision support systems. LiDAR is much faster than conventional photogrammetric technology and offers distinct advantage over photogrammetry in some application areas. Its development goes back to 1970s and 1980s, with the introduction of the early NASA-LiDAR systems, and other attempts in USA and Canada (Ackermann, 1999). The method has successfully established itself as an important data collection technique, within a few years, and quickly spread into practical applications. Early 1980's, second generation LiDAR systems were in use around the world but were expensive and had limited capability. With the enhanced computer power available today, and with the latest positioning and orientation systems, LiDAR systems have become a commercially viable alternative for development of Digital Elevation Models (DEM) of earth surface. 2. Technology in a glance A pulsed laser ranging system is mounted in an aircraft equipped with a precise kinematic GPS receiver and an Inertial Navigation System (INS). Solid-state lasers are now available that can produce thousands of pulses per second, each pulse having a duration of a few nanoseconds (10-9 seconds). The laser basically consists of an emitting diode that produces a light source at a very specific frequency. The signal is sent towards the earth where it is reflected off a feature back towards the aircraft. A receiver then captures the return pulse. Using accurate timing, the distance to the feature can be measured. By knowing a speed of the light and the time the signal takes to travel from the aircraft to the object and back to the aircraft, the distances can be computed. Using a rotating mirror inside the laser transmitter, the laser pulses can be made to sweep through an angle, tracing out a line on the ground. By reversing the direction of rotation at a selected angular interval, the laser pulses can be made to scan back and forth along a line. When such a laser ranging system is mounted in an aircraft with the scan line perpendicular to the direction of flight, it produces a saw tooth pattern along the flight path. The width of the strip or "swath" covered by the ranges, and the spacing between measurement points depends on the scan angle of the laser ranging system and the airplane height. Using a light twin or single engine aircraft, typical operating parameters are; flying speeds of 200 to 250 kilometers per hour (55 to 70 meters per second), flying heights of 300 to 1000 meters, scan angles generally ±30, to ±20 degrees, and pulse rates of 2000 to 50000 pulses per second. These parameters can be selected to yield a measurement point every few meters, with a footprint of 10 to 15 centimeters, providing enough information to create a Digital Terrain Model (DTM) adequate for most applications, including the mapping of storm damage to beaches, in a single pass. The primary factor in the final DTM accuracy is the airborne GPS data. Errors in the location and orientation of the aircraft, the beam director angle, atmospheric refraction model and several other sources degrade the co-ordinates of the surface point to 5 to 10 centimeters (Shrestha and Canter, 1998). An accuracy validation study showed that LiDAR has the vertical accuracy of 10-20 centimeters and the horizontal accuracy of approximately 1 meters (Murakami et al., 1999). 3. Comparison with photogrammetry LiDAR is useful for collection of elevation data in case of dense forests, where photogrammetry fails to reveal the accurate terrain information, due to dense canopy cover. Not limited by the environmental conditions restricting aerial photography, airborne LiDAR is emerging as an attractive alternative to the traditional technology for large-scale geospatial data capture. Because it is an active illumination sensor a laser system can collect data at night and can be operated in any weather and at low sun angles that prohibits aerial photography. Rural and remote areas can be surveyed easily and quickly because each XYZ point is individually geo-referenced, aerial triangulation or orthorectification of data is not required (Flood and Gutelius, 1997). Photogrammetric methods for DTM generation are very time consuming and labor intensive. In photogrammetric method of DTM generation using stereoplotters, firstly photogrammetric model needs to be formed into stereoplotter using interior, relative and absolute orientations. Stereo-compiler manually digitizes geomorphic feathers such as, drainage, road edges, sides and bottom of ditches in one layer. These lines are called as “hard break lines”. The undulations in the topography are mapped by so called, “soft breaklines” (shown in yellow colour). Then spot heights are added up at regular interval (cyan colour) manually by keeping floating mark on the ground (in model). Later on the DTM is generated from these breaklines and spot heights by using sophisticated softwares currently available in market. On the contrary, on an average, 90-100 sq. km. area can be measured in one hour using LiDAR system. Typical post-processing time for LiDAR are, two to three hours for every hour of recorded flight data with additional processing time required for more sophisticated analysis for target classification (Lohani, 2000). Studies showed that LiDAR requires only 25 to 33% of the budget needed for photogrammetric compilation (Petzold et al., 1999)  Figure 1 “hard/soft breaklines and spot heights” for DTM generation  Figure 2 Comparison of various steps involved in DTM generation, Photogrammetry vs. LiDAR
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