Dr. Bharat Lohani, Professor of Civil Engineering, IIT Kanpur
Sudharsan Ramamurthy; Rajasubramani Periasamy; Sasidaran; Sharvi Lohani, Geokno India Pvt. Ltd.
Source:THE Environment Management
A Quarterly E- Magazine on Environment and Sustainable Development
October – December, 2018
The ever-growing needs of our population, the recent KisanLongMarch and various food security schemes initiated by the Government of India, constantly remind us of the need to conserve our resources in a sustainable manner. Being the second most populous nation with agriculture being the major contributor to the GDP, India must be prudent about its water resources. India heavily depends on Monsoons for its water. The water resources of India are unevenly distributed in space and time. While some parts face severe droughts, the other regions are being inundated with floods. This surplus-deficit equation causes widespread socio-economic and political disparities among people. Construction of Dams and Inter-Linking of rivers have the potential to minimize these disparities. However, such efforts may have a direct or indirect impact on people from several states thus making political leaders wary of adopting these. In addition, water being an environmentally sensitive and scarce resource, any such project must go through stringent scientific scrutiny. A project of this nature includes handling interdisciplinary layers of hydrological, agricultural, and environmental data along with socio-economic and political aspects. The topographic data, also broadly referred to as Geospatial data, are integral and most important part of design and analysis for such projects. Considering the significance of topographic data, it is important that these data be collected accurately, covering entire project site, and at a finer
resolution, so that no important details are missed. Further, the data collection should be fast as delay in data collection results in delay in delivering the benefits of projects to people and loss to government exchequer.
Aerial LiDAR Technology:
Light Detection and Ranging (LiDAR) technology collects high-accuracy elevation data for large areas very quickly and at a lower cost than traditional methods. In LiDAR a Laser transmitter is used to throw a laser pulse, and a receiver is employed to detect the return of the pulse. The ‘Time of flight’ is then calculated and since the speed of light is known, the distance of the point can be calculated. Normally, based on its requirement, LiDAR can be used on three platforms namely Terrestrial, Mobile, and Airborne.
(FIG 1 top left, FIG 2 top right, FIG 3 bottom two
The aerial LiDAR technology can acquire highly accurate and dense elevation data of terrain surface quickly and accurately with support of high-precision GPS and Inertial Navigation System as shown in figure below. Therefore, a high-precision Digital Terrain Model can be built from it. LiDAR is emerging as a powerful tool for detailed hazard and resource mapping. The Digital Elevation Model (DEM) derived from LiDAR data has the highest spatial resolution amongst other available DEMs.
Figure 1: Principle of Light Detection and Ranging Figure: 2 Sensors used for LiDAR Technology
Figure 3: Left : Aerial photograph in which underlying ground details are not visible because of canopy cover; Right : DEM generated from LiDAR data wherein terrain is clearly visible and mapped including the area under forest cover
Aerial LiDAR system uses a combination of LiDAR equipment, GNSS unit for the position, Inertial Measurement Unit (IMU) for orientation and cameras for recording images (for coloring the output) The LiDAR unit is mounted on an aircraft/Helicopter as shown in Figure 1. This is useful for large area surveys such as irrigation, disaster mapping, flood mapping and corridor survey, alignment survey for irrigation canal and high-altitude roads. These data are highly useful in planning and designing greenfield infrastructure projects for mapping large tracts of land, which are otherwise not accessible with conventional technologies.
CASE STUDY: The Tale of two States Sharing of Godavari Water
The government of Telangana wanted to resolve the decade-old problem of river water sharing with Maharashtra, so that water getting discharged into the sea could be utilized for the benefit of both the states. The Government of Telangana had undertaken a complete reevaluation of the earlier designed irrigation projects.
Godavari is the second largest river in India after Ganges. The drainage area of Godavari River is 312,812 sq. km of which 48.6% lies in Maharashtra, 19.04% in Telangana, 4.76% in Andhra Pradesh, 18.7% in Chhattisgarh, 5.5% in Orissa, 2% in Madhya Pradesh and 1.4 % in Karnataka. The goal of the project is to identify drought-prone, upland and backward areas of Telangana Region to provide irrigation facilities to several districts in Telangana.
One of the critical inputs needed for resolving the dispute was the Topographic data which would correctly identify the actual submergence in both the states once the dams are constructed. The other two being the generation of Digital Elevation Model (DEM) and Contour Map which give an idea of the basic physical characteristics like drainage networks and drainage flow and a better understanding of the elevation profile of the catchment.
Figure 4: Contour Map generated from LiDAR data which enables better understanding of the catchment area for the administrators
Traditional surveying technology is severely inadequate as the large area of survey would take much longer time besides accuracy issues. Most of the disputes in the past could not be resolved because the levels cross-verified by inter-state departments were never found accurate and mutually acceptable and hence decision making was hampered.
This was caused due to the short-comings of the traditional DGPS/Total Station technologies which are incapable of generating true contours as only a few, and far-spaced points would be available for this purpose. Furthermore, due to extensive manual work involved in DGPS/TS survey, the survey data are prone to human-induced errors. Alternatively, imagery-based technologies like Satellite and Drone-based photogrammetry would also fail as, besides poorer accuracy, it is not possible to measure points under vegetation hence making the resulting data not useful for irrigation projects. Considering the above, the Government of Telangana and WAPCOS decided that the Aerial LiDAR technology is appropriate for this prestigious project. Using LiDAR technology 100 sq km to 200 sq km area can be mapped per day to high degree of accuracy. Moreover, as LiDAR technology can map below vegetation through small gaps in tree canopies, data below vegetation also becomes available. This results in an accurate 3D mapping of the surface features and bare earth which are required for irrigation design. As the technology is free from human intervention, the captured data are free from manual errors.
WAPCOS identified Geokno India Pvt Ltd which has now 10 years of extensive experience in conducting accurate LiDAR surveys of engineering grade in India for this prestigious project. Geokno was incubated in IIT Kanpur to spread the use of the revolutionary LiDAR technology in India. Heli-Mapper aerial LiDAR system was deployed for surveying the project area. Since the survey started towards the end of the rainy season, there was an additional benefit of using the Heli-mapper as the helicopter could be flown below the clouds and still capture data. For this multiple flight plans were made so that the survey could be completed at the earliest.
Figure 5: Helicopter ready for surveying with sensors fitted
The survey of over 3700 sq km was completed in a short span of 4 months which would have otherwise taken 3 years using traditional method. The surveyed region is Naxal affected zones making it difficult to map by field based methods. Moreover, the larger portion of the project area is comprised of cultivation land and forest where the LiDAR technology performs best. The Aerial LiDAR survey has also helped to improve the design of multiple irrigation schemes. Telangana Government was extremely satisfied with the project outputs, and the original 700 sq km of the area was extended to more than 3,700 sq km. In Phase 2 further a survey of more than 4,000 sq km of area was carried out. All the issues relating to Aerial LiDAR survey vetting, utilization of water, submergence and project designs were analyzed by the officials of the CDO (Central Design Organizations) of the two states in detail and approved. The biggest achievement of this survey was that it resulted in an MoU being signed between the Governments of Maharashtra and Telangana within 5 months of the start of the Aerial LiDAR survey. This MoU benefits lakhs of farmers of the two states. The project is expected to irrigate 16.4 lakh acres in Telangana and over 50,000 acres in tribal areas of Maharashtra.
The economic progress of India is not only tied with agricultural development, but also with industrial development for which water resources play major role. As seen in the Kaleshwaram project discussed in this article LiDAR technology has potential to play a key role in the development of nationwide digital elevation dataset. These data provide better understanding of various issues related to water. On one hand water needs can be met by better design of projects while on the other hand a better design also ensures environmental sustainability of the water resources. Better terrain data through LiDAR technology is proving a blessing towards these goals.
Comentarios