Abstract: Airborne Altimetric LiDAR, despite its recent emergence, has become an industry-standard-tool for collecting high-resolution topographic data. This paper briefly describes the principle and technical issues related with this technology and as well its advantages. Further, the varied application areas where this technology has been successfully employed are reviewed. An assessment is also made of the other application areas where there is a great potential of using this technology. The paper also outlines the commercial and scientific potential of this technology, particularly in Indian reference and raises issues regarding the use of this technology in India.
1. Introduction
Topographic data have been the core of any Geographical Information System (GIS) project. The accuracy and functionality of many GIS projects rely to a large extent on the accuracy of topographic data and the speed with which it can be collected. Furthermore, the data collection consumes a major slice of the project resources in terms of both time and finance. Topographic data collection, therefore, assumes considerable significance and forms an integral part of a GIS project.
The conventional methods of topographic data collection include land surveying and aerial photogrammetry. More recently, attempts have been made to use satellite stereogrammetry for this purpose. However, all the above techniques have limitations in terms of their accuracy, cost-effectiveness, time-consumption, feasibility, and applicability. The recently emerged technique of airborne altimetric LiDAR has gained considerable acceptance in both scientific and commercial communities as a tool for topographic measurement. This technique has the potential to remove several bottlenecks imposed by earlier methods.
2. Principle of LiDAR
The basic concepts of airborne LiDAR mapping are simple. The airborne LiDAR instrument transmits the laser pulses while scanning a swath of terrain, usually centred on and co-linear with, the flight path of the aircraft in which the instrument is mounted. The scan direction is orthogonal to the flight path. The round trip travel times of the laser pulses from the aircraft to the ground are measured with a precise interval timer. The time intervals are converted into range measurements, i.e. the distance of LiDAR instrument from the ground point struck by the laser pulse, employing the velocity of light. The position of aircraft at the instance of firing the pulse is determined by differential Global Positioning System (GPS). Rotational positions of the laser pulse direction are combined with aircraft roll, pitch, and heading values determined with an inertial navigation system (INS), and with the range measurements, to obtain range vectors from the aircraft to the ground points. When these vectors are combined with the aircraft locations they yield accurate coordinates of points on the surface of the terrain. A typical LiDAR campaign involves the following steps:
- Flight planning i.e. fixing LiDAR instrument and aerial platform parameters to control the density and coverage of topographic measurements.
- Fixing ground control points (GCPs) to place reference receivers for differential GPS positioning.
- Instrument calibration-- pre, during, and post-flight-- to ensure accuracy of data collected.
- Data collection i.e. obtaining INS, GPS, laser range and scan measurements.
- Data processing to determine aerial platform location using GPS and INS measurements and combining it with laser range and scan measurement to yield triplets i.e. x, y, and z for each ground point struck by the laser in WGS-84 system.
- Quality assurance/quality check to determine and quantify the errors present in data and, if needed, elimination or minimisation of the errors.
- Generation of data products i.e. DSM, DEM, contour plots, 3D visualisations, and fly-throughs.