Every year in a country like India, natural disasters like floods and cyclones are fairly frequent. Earthquakes also occur time and again. The occurrences of such disasters, their intensity, the area/region of their occurrences and their impact has to be assessed, so as to have information/data about the damages caused by them to the area/population specific or probable damages or impact likely to be caused. Proper mapping will be helpful not only for pre-disaster preparedness but also in rescue and relief operations with greater accuracy and speed.
With the data/information collection, storage, retrieval becoming highly technological and scientific, new specialized techniques like Geographical Information System (GIS) are increasingly used for disaster mapping and these are proving to be very useful.
DEFINING A MAP
Map is defined as a representation of a portion of earth's surface in a two-dimensional depiction. Accordingly, mapping denotes the making of a map i.e. setting out various physical geographical features on a map. A map may show fixed features such as cities or mountains or variables such as temperatures or populations. Maps can be on computers (GIS). Pre-disaster and post-disaster maps will show the impact of disaster. In order to be precise and accurate, most maps are geo-referenced, i.e., they are portrayed with reference to universally standardized geographic references, as in latitude, longitude and altitude.
Techniques to Read and Interpret Maps
Traditionally, maps have been prepared based on physical surveying. These maps are then presented as hard copies, i.e., they are printed on paper, using colours, symbols, labels and other drawing elements, to represent geographic facts. Such a map is on 'scale', which means that its dimensions are proportionate to the actual area it represents. A typical town may be conveniently represented on a scale of 1 :20,000, meaning that one unit length on the map represents 20,000 unit lengths in reality. In other words, a distance of one c.m. shown on the map represents 20,000 c.m., or 200 m, of actual distance on ground. Various land uses, activities, zones etc., are shown with different colours, the colour code being explained in a legend, or key, on the map itself. Different structures or specific locations are shown using various symbols, which again are explained in the legend. A compilation or collection of maps is called atlas.
The usual problem with such maps is that they are tedious and time consuming to prepare, difficult to update and inconvenient to maintain. Therefore, now-a-days remote sensing is becoming popular as a means of collecting data for map preparation, and Geographical Information Systems (GIS) is being used for storage, analysis and retrieval. Under remote sensing techniques, maps are prepared using satellite data or aerial photographs, and are then digitised and stored on computers using GIS software. Once this is done, they can be retrieved and viewed on the computer any time. They can easily be enlarged or reduced, and even printed in multicolour format.
Disaster maps usually show risk zones or disaster impact zones. These could include flood zone maps, seismic zone maps, industrial risk zone maps etc. A number of such maps may be overlaid on a base map of the area to give a composite disaster map that covers the risk of a number of types of disasters because many geographical areas are prone to more than one type of disasters. The vulnerability atlas of India Contours are useful maps of this type covering various disasters.
TECHNIQUES FOR AREA MAPPING
Area mapping involves four basic steps. First is the task of data collection. This would include spatial data on physiography, hydrology, geology, population distribution, land use and activity pattern, structural conditions and socio-economic data. Some of the data, mainly the physical data, can be collected through remote sensing. The data would then have to be verified on ground. Thereafter the data would need to be plotted on a spatially referenced medium, either on paper, or using computers. After proper coding, the relevant information would be added to the map,
ZONING OF SEISMIC ACTIVITY
Zoning of seismic activity is a highly technical field and requires collection and detailed study of a time-series data on earthquakes stretching over decades, or even centuries. The seismic history of an entire region has to be analysed in detail, and depending on the frequency and intensity of seismic activity in different parts of the region, seismic zoning is carried out for that region. The presence of active faults "and ridges in the subterranean region are also studied and act as determinants in the process of seismic zoning. Seismic zoning has to be updated after every major seismic activity in low seismic zones, as happened in the case of Latur Earthquake in Maharashtra.
INDUSTRIAL ZONING AT MICRO SCALE
As compared to seismic zoning, which has to be carried out at regional scale, industrial zoning can be carried out at a micro scale due to the comparatively localized impact of industrial disasters. This is known as micro-zonation. Industrial risks may arise due to the hazardous process involved in the industry, or during storage, or transportation of hazardous materials. Depending on the nature of risk, as well as the activity pattern and intensity in the vicinity, risk zones are identified. The risk due to air pollution is generally most widespread. That of water may extend over longer distances in case of surface water, but would be more channeled. The risks of land degradation, and subsurface water and also noise pollution are more localized.
MAPPING PROMINENT DISASTER PRONE AREAS
Early warning of drought conditions -will help to undertake contingency agricultural strategies and to organize relief where and when it is most needed. Drought relief management efforts have been greatly helped by remote sensing and disaster mapping techniques. The condition of the crops as also the amount of water in the water bodies or even the extent of moisture in the ground can be mapped with the help of remote sensing techniques. Targeting of potential ground water sites for taking up emergency well digging programmes has been made possible by the use of satellite data. The success rate of such wells had been significantly higher than when using ground methods alone. The first phase of the Drinking Water Mission of Government of India consisted of district-wise groundwater potential maps using high-resolution Landsat/IRS data.
Long-term drought proofing programmes calling for base-line information on the natural resources of the district or river basin have been greatly helped by the use of satellite data. A drought monitoring project recently taken up by the Department of Space in collaboration with the State concerned covers management of water resources, agricultural and land resources and fodder resources, and integrates natural resource physical data base and socio-economic and demographic data base. Data from IRS satellite in 1:50,000 scale was used to generate resource data.
Resource maps such as soil, land-use and hydro-geomorphoiogy have been prepared using IRS data on 1:50,000 scale. Other maps such as slopes, drainage and watershed, transport network and settlement location and rainfall data have been prepared from the secondary data available on Survey of India topographical sheets and with the Census Department. The socio-economic data has been collected from the respective district administrations. The conventional resources of information, socio-economic and demographic data are integrated with the resource data obtained from the IRS satellite imagery in order to prescribe appropriate land use, fodder and water management practices.
Floods and Cyclones
Each year cyclones bring copious rains, which submerge lands. Rivers overflow during flood times and inundate land. The extent of such submersion varies from time to time depending upon the intensity of flood flows. The traditional method of mapping areas that gets flooded (for different levels of probabilities are to use a map of suitable scale (1:15,000) on which inundation information is added. Over a long period it would be possible to indicate different probabilities of flooding in these maps. The Central Water Commission in cooperation with the State has done pioneering work in India in this regard and has published the Flood Atlas of India long ago (CWC-1987).
In recent decade or two, satellite data has been mostly used for mapping and monitoring the flood-inundated areas, flood damage assessment, flood hazard zoning, and post-flood survey of river configuration and protection works. Near real time flood "mapping was performed in the year 1986 in respect of the unprecedented historic Godavari floods, as well as floods that occurred in parts of Ganga basin lying in Bihar and Uttar Pradesh. Near real time was continued during the flood period of 1987 in respect of various rivers like Ganga, Ghagra, Kosi, Gandhak, Mahananda, Brahmaputra, Teesta, Jhelum etc., in different parts of the country.
Floods in Jhelum and Brahmaputra rivers and in Orissa State during 1992 were mapped. Recent availability of satellite data has helped flood mapping inspite of cloud cover. This data has been used recently (1993) for flood mapping of the Brahmaputra, North Bengal and Punjab floods. Floods in Jhelum and Kosi have also been mapped.
Efforts are also being made to compute the flood damage in various parts of the country by combining remotely sensed data and conventional ground data. A pilot project is under progress in a part of Brahmaputra Basin. An analysis of database on flood inundation during successKe years can help delineate areas susceptible to floods of differing magnitudes. Preliminiary flood risk zone maps along Kosi and Brahmaputra rivers have been made using multi-year satellite imagery corresponding to differing flood magnitudes and return periods. This provides valuable inputs to regulating flood plain land use.
High-resolution satellite data has been used to map post-flood river configuration in order to identify vulnerable reaches of embankments to enable corrective action. Erosion prone areas along the Brahmaputra river have been identified through multi-year satellite data to delineate river reaches for flood protection works.
Volcanic Eruption and Fires
Though volcanic disasters are not common to India, Andaman Islands have two volcanoes that have been silent. These are the Narcondum and Barren Islands. The Barren Island volcano remained dormant for nearly 200 years and erupted in March 1991, which continued till November 1991. The volcanic eruption was monitored using Multidate satellite data of both day and night passes. The extent of reach of lava flows was monitored.
Satellite imagery in the infrared regions and actual ground/aerial photographs has been employed to map areas damaged by forest fires. The data has been used to study and map a number of forest fires and to monitor the vegetation regeneration over burnt areas. The 11-day repeat cycle provided by the Indian Remote Sensing Satellites (IRS) has been found to be extremely valuable in monitoring the forest fires.
Earthquake phase data collected by the National Seismic Telemetry Network for the past one hundred years were analysed using a computer, and epicentral parameters were determined. These locations were shown in maps.
The epicentral maps are used for preparing seismic hazard map. Seismic zoning map is the basis for the code for designing earthquake resistant structures. Apart from the earthquake data, geological factors, soil data etc., are used for preparing the building codes. Upgradation of this code is a continuous process for which purpose the building code is reviewed from time to time. However, in order to assess the exact nature of risk, several other important factors such as gravity, magnetic, geodetic, and climatic data are necessary. These data are then used to prepare microzonation maps, which are used, for urban and rural planning.
Landslide zonation map comprises of a map demarcating the stretches or areas of varying degrees of anticipated slope stability or instability. The map thus shows the factors conducive to landslides and hence it has an inbuilt element of forecasting and is therefore of probabilistic nature. Depending upon the methodology adopted and the comprehensiveness of the input data used, a landslide hazard zonation map be able to provide help concerning one or more of relevant aspects such as location, type of occurrence of landslide, and affected people area and mass movement of slope mass.
One of the early projects on zonation was carried out by the Central Road Research Institute, New Delhi in 1984, in whiclVhazard zonation techniques were used to choose a most suitable alignment from the possible alternative alignments on landslide affected stretches in Sikkim area. Subsequent monitoring has shown that the choices made have proved successful. During 1989, a landslide hazard .zonation map was prepared for a part of Kathgodam-Nainital highway. This map was prepared with the objective to evolve a suitable maintenance strategy to keep the hillslopes along the road free of landslide problem.
Preparation of a comprehensive landslide hazard zonation map requires intensive and sustained efforts. The problem is highly interdisciplinary in nature. A large amount of data concerning many variables, covering large slope areas has to be collected, stored, sorted and evaluated. Finally, the degree of risk of sliding has to be evaluated and zonation maps prepared. The use of aerial photographs and adoption of remote sensing techniques helps in the collection of data. For storage, retrieval and analysis, adoption of computerised techniques is useful.
Hazard zonation maps have multifarious uses, some of which are listed below:
In the preparation of development plans for townships, dams, roads, and other development.
General purpose Master Plans and Land Use Plans.
Discouraging new development in hazard prone areas.
Choice of optimum activity pattern based on risk zones.
Quick decision making in search, rescue and relief operations during disaster and post-disaster situations.
Clearly such maps have a large number of users, including several Government Departments, and private agencies as well as NGOs involved in any Type of development, construction of disaster management work.