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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:

1. Flight planning i.e. fixing LiDAR instrument and aerial platform parameters to control the density and coverage of topographic measurements.
2. Fixing ground control points (GCPs) to place reference receivers for differential GPS positioning.
3. Instrument calibration-- pre, during, and post-flight-- to ensure accuracy of data collected.
4. Data collection i.e. obtaining INS, GPS, laser range and scan measurements.
5. 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.
6. Quality assurance/quality check to determine and quantify the errors present in data and, if needed, elimination or minimisation of the errors.
7. Generation of data products i.e. DSM, DEM, contour plots, 3D visualisations, and fly-throughs.

3. Advantages of LiDAR

1. Accuracy: An accuracy of order of 10 - 15 cm in the vertical and 50 - 100 cm in the horizontal is claimed by manufactures and has been demonstrated by many field studies.
2. Time of data acquisition and processing: The data capture and processing time is significantly less for LiDAR compared to other techniques. LiDAR can allow surveying rates of up to 90 km2 per hour (Environment Agency, 1997) with post-processing times of two to three hours for every hour of recorded flight data (Martin and Gutelius, 1997).
3. Minimum user interference: User interference is minimum, as most of the data capture and processing steps are automatic except the maintenance of the ground GPS station.
4. Weather independence: LiDAR is an active sensor and can collect data at night and can be operated in slightly bad weather and low sun angle conditions, which prohibit the aerial photography.
5. Additional data: Besides relief information, laser reflectance may be used to generate intensity images to help in classifying the terrain features (Schreier et al., 1985). Further, the systems can have fluorosensors allowing pollutant identification and chlorophyll mapping (Environment Agency, 1997; Measures, 1984).
6. Canopy penetration: Unlike photogrammetry, LiDAR can see below canopy in forested areas and provide topographic measurements of the surface underneath. Additionally, LiDAR generates multiple returns from single pulse travel, thus providing information about understory.
7. Data density: LiDAR has the ability of measuring subtle changes in terrain as it generates a very high data density ( due to firing of 2000 - 80000 pulses per second).
8. Ground control point independence: Each LiDAR pulse is individually georeferenced using the onboard GPS, INS, and laser measurements. Only one or two GPS ground stations are required for improving the GPS accuracy by the differential method. Independence from GCPs makes it an ideal method for inaccessible or featureless areas like wastelands, ice sheets, deserts, forests, and tidal flats.
9. Digital compatibility: Data produced from LiDAR flights are in digital format with Easting, Northing, and Altitude values of each laser target. This makes importing of data to GIS and other image processing packages straightforward.
10. Cost: One of the major hindrances in the use of LiDAR had been the cost of the equipment. However, in recent years the purchase price of these instruments has been reduced so that cost is no longer a barrier to companies capable of investing in standard aerial photogrammetry equipment (Martin and Gutelius, 1997). Furthermore, with more and more users opting for LiDAR the cost of the system and operation is likely to go further down. Mason et al. (1999), on the basis of overall performance evaluation of available topographic techniques for coastal terrain, found that LiDAR could achieve good performance at a lower cost.

4. Review of LiDAR applications

4.1 Floods
High-resolution and accurate LiDAR data are suitable for improving the performance of flood models by providing a more reliable initial boundary condition (Bates, 1999). LiDAR data having multiple returns help in generating and understanding the 3D structure of obstructions (i.e. surface roughness, vegetation, buildings, and other structures). This information can yield the friction coefficient over the various parts of a floodplain (Cobby, 1999). LiDAR data are also being employed for flood hazard zoning (Hill et al., 2000). FEMA (Federal Emergency Management Agency, US) is using LiDAR on a mandatory basis to create Digital Flood Insurance Rating Maps.

4.2 Coastal applications
LiDAR has generated considerable interest among coastal researchers as a topographic tool. Highly accurate, dense, and rapidly obtained data sets are most suitable for coastal applications like sediment transport, coastal erosion, and coastal flood models (Brock et al., 1997; Gutierrez et al., 1998).

4.3 Bathymetry
LiDAR in its bathymetric form can map the bed topography up to a depth of 70 m (Wehr and Lohr, 1999). This information is useful for determining the siltation on navigation canals and ports and planning the construction details.

4.4 Hydrology
LiDAR data can be used to quantify gully and stream channel cross sections and roughness, gully and stream bank erosion and channel degradation, to estimate soil loss from gully or channel banks and to measure channel and flood plain roughness and cross sections for estimating flow rates (Ritchie, 1996). Further, coastal channels which are difficult to map otherwise can be automatically quantified using LiDAR data (Lohani, 1999).

4.5 Glacier and avalanche
LiDAR has been found ideally suited for mapping glacial topography (Krabill et al., 1995; Kennett and Eiken, 1997) and ice velocities (Abdalati and Krabill, 1999). The above studies have amply shown that LiDAR can be used to monitor snow glacier movement, snow accumulation, and predict the onset of avalanche. The data set can further be employed to estimate the risk from a particular avalanche (Wehr and Lohr, 1999).

4.6 Landslides
LiDAR has made it possible to monitor and predict slope failure by rapidly obtaining highly accurate and dense elevation data. In post-slide conditions rapid damage assessment and mapping can be realised using LiDAR.

4.7 Forest mapping
The unique feature of LiDAR of producing multiple returns from the canopy top, understory, and the ground has attracted many to use it for estimating forest biomass, timber volume, and other parameters (Nelson et al., 1984).

4.8 Volcano monitoring
Ridgway et al. (1997) have shown that subtle systematic changes (uplift of up to 4 cm per year) in volcano dome height can be monitored from time to time using LiDAR.

4.9 Transmission lines
As rightly noted by Ackermann (1996), LiDAR data due their typical characteristics are finding many new applications, which were not thought feasible hitherto with other data collection techniques. One such area is monitoring transmission lines. Long stretches of transmission lines can be mapped with speed (Medvedev, 1998) to determine the exact location of the transmission towers, accurate topography of the corridor, and the encroachment by vegetation for modification and repair purposes.

4.10 Route mapping
Highly dense data from LiDAR can be used to differentiate objects such as rails, mileposts, signals, switches, damage to road surface, accident sites, traffic density, and subtle changes in slope/grade on roadways and railways, without interrupting the services (Lembo et al., 1998). LiDAR can as well be employed for corridor mapping to plan oil and gas pipelines and their post-commission maintenance.

4.11 Cellular networks
Planning and managing cellular networks require terrain elevation, ground cover and building outlines (Hill et al., 2000). To ensure a clear line of sight and locate areas for development, accurate and detailed data sets containing information about natural and manmade obstructions, is highly important. LiDAR data have been found suitable for this purpose and increasing number of communication companies are relying on it.

Besides the above well-known and proven applications, the data, due to its typical characteristics has potential of being employed in a variety of prospective application areas. Simulation of the effects of hurricanes can become possible with the precise 3D information of natural and manmade objects (Hill et al., 2000). Using the ability of LiDAR data to generate 3D-city model (Mass and Vosselman, 1999), rapid and reliable post-disaster damage assessment (e.g. after an earthquake) can be carried out. Land subsidence can precisely be measured and monitored using LiDAR, which is significant for mining and landfill sites. The polluting gases travel through the atmosphere depending on various physical and climatic controls. LiDAR derived 3D city models, can be used within GIS framework to monitor these movements and develop emergency-response strategy to potential catastrophic chemical leaks and spills (Hill et al., 2000).

5. Indian perspective

5.1 Relevance of LiDAR technology in Indian scenario
As discussed above, LiDAR has great potential to provide solutions which were not deemed feasible hitherto. As well in many other cases, where conventional data collection tools were not meeting the required accuracy, speed, and resolution standards, LiDAR provides the desired solution.

India, still in the developmental phase, has to embark on several major infrastructure projects related to roadways, railways, oil and gas pipelines, electric transmission lines, communication network, and ports and harbours. Needless to state, speedy collection of accurate topographic data, which are the core of these projects, greatly reduces the costs. More importantly, the cost escalations resulting from the delays in project work due to the disadvantages of conventional data collection approaches may also be minimised. It is worth noting that surveying and mapping operations take most valuable time in a project's life, thus influencing the ultimate cost substantially. In comparison to the overall cost of projects the cost of a LiDAR equipment is generally minuscule but the resources saved will be substantially higher. Furthermore, LiDAR may prove to be the most suitable technology to measure Indian urban centres, which in absence of appropriate topographic data become difficult to manage.

India is prone to natural disasters of varied form resulting in heavy losses of life and wealth. LiDAR data have potential to be effective in many disaster management programmes (Lohani, 2000), including the most frequently occurring floods. High-resolution and accurate topography generated by LiDAR is most suitable to further the scientific understanding of natural phenomena, e.g. the floods and coastal environment.

5.2 Status of LiDAR technology in India
Despite the highly useful role that LiDAR can play in nation development it is contrary to note that no LiDAR survey has been conducted in India till date (However, it is expected that a few government departments may procure the instrument in near future). The reasons for the non-availability of this technology are many, but the first and foremost could be the excessively repulsive government policy, which regulates the airborne data collection in India. This could also be cited in general as the reason for near-complete absence of airborne remote sensing programmes in India except in a few government departments. Considering the typical needs of airborne remote sensing operations, this technology is most apt to grow within private sector in a highly competitive, liberal, and conducive environment. In addition to the above, the high initial cost of instrument (˜US $1M), apprehension of new technology, and the lack of awareness may also be cited as some of the other but secondary reasons.

6. Conclusion
It is amply clear that airborne laser altimetry technique has become a very prominent tool to collect accurate high-resolution topographic data. It offers many advantages over the conventional techniques of DEM generation and has been successfully used in a variety of applications. In addition, the typical characteristics of LiDAR data have opened up the possibility of using them for many other applications which were not thought of earlier. Notwithstanding the increasing use of this technology world over, it has not yet been available in India. However, this technology has the potential of saving the precious national resources and providing better understanding of several problems which are difficult to comprehend otherwise, due to the limitations imposed by conventional data collection techniques. In tune with the liberalisation of most of the government policies it is hoped that the skies will be opened for private operators without much time consuming regulations. This exactly is when LiDAR in particular and airborne remote sensing in general will see their true potential unfolding in India for nation development.

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