Use of IRS Data in the Production of a Hydrology Map of Sri Lanka
Bandu Adhikari and Sarath Jayatilaka
Survey Department - Sri Lanka
1.0 Introduction
Sri Lank is situated between 79 E and 82 E longitude and 6 and 10 latitudes. It has a land area of about 65,000 sq.kms with a population of about 17 million. It is hilly in the central region sloping down to the northern and coastal regions.
The annual rainfall is between 1000 mm to 5000 mm dividing the country into three zones - the wet zone in the south-west, the intermediate in the south-west including the central hills and the dry zone in the rest of the country.
Land forms a basic resource and a fundamental component in the national development strategies. Reliable information about land and natural resources is a prerequisite for effective planning and land administration.
The collection of data about the spatial distribution of significant properties of the earth's surface had long been an important part in planning activities. These information were very often displayed on maps and charts.
The Hydrology of a country plays an very important part in the eco system of the country. Hence an attempt was made to map the hydrology using the Indian Satellite data from IRS in Sri Lanka at the Centre for Remote Sensing at the Survey Department. At the time of this study the available data was from the IRS 1A of 1992.
2.0 Use of Satellite Date for feature extraction
The potential of using satellite images for feature extraction has been provided by Remote Sensing scientist for a long time. As a result of segmentation of original grey scale image is transformed to an image in which the ground features could be independently labeled. Basically for monochrome images there are two types of general processes in image segmentation. i.e. Edge detection or line following and region growing method. In this study region growing method was employed. The principal approaches in this method were based on threshold, region growing, region splitting and merging.
2.1 Spectral Characteristics
IRS 1A satellite carries three state of the art cameras using Charge Couple Devices (CCD) as detectors. The design of the cameras is based on the concept
of "push broom" scanning, using Linear Imaging Self-Scanning Sensors (LISS). Each of the three cameras will provide data in four spectral bands three of which are in the visible region and the fourth is in the near infrared region.
| Band |
Spectral range |
Applications |
| 1 |
0.45 - 0.52 |
Coastal environmental studies
ChlorophyII absorption region |
| 2 |
0.52 - 0.59 |
Green vegetation useful for discrimination ofRokks & soil for their iron content |
| 3 |
0.62 - 0.68 |
Strong correlation with chlorophyII absorption in vegetation, discrimination of soil & geological boundaries |
| 4 |
0.77 - 0.86 |
Sensitive to green biomass opaque to water resulting in this contrast with vegetation |
3.0 Data used in the study
The data that was used in this study was from the IRS 1A data that was acquired in 1992 in BSQ format. The band 4 was selected as it was sensitive to green biomass and water, resulting in high contrast with vegetation.
