Interactions of EMR with the Earth’s Surface

1. Interactions of EMR with the Earth’s Surface • Radiation that is not absorbed or scattered in the atmosphere can reach and interact with the Earth's surface. • When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions are possible: • 1. Absorption (A) • 2. Transmission (T) • 3. Reflection (R) • The proportions of each will depend on the • - wavelength of the energy, • angle of incidence with the surface, and • roughness of the material and condition of the feature.

2. Reflectance is defined as the ratio of incident flux on a sample surface to reflected flux from the surface. Albedo is defined as the reflectance using the incident light source from the sun. A basic assumption in remote sensing is that spectral reflectance is unique and different from one object to an unlike object.

3. Two types of reflection that represent the two extreme ends of the way in which energy is reflected from a target are: 1. Specular reflectionand 2. Diffuse reflection. Specular or mirror-like reflection typically occurs when surface is smooth and all (or almost all) of the energy is directed away from the surface in a single direction. Diffuse or ‘Lambertian’ reflection occurs when the surface is rough and the energy is reflected almost uniformly in all directions.

4. Spectral Reflectance Curve The reflectance characteristics of the earth’s surface features may be quantified by measuring the portion of incident energy that is reflected. This energy is measured as a function of wavelength and is called spectral reflectance. It is defined as: Reflectance ranges from 0 to 1. The instrument used to measure reflectance is called spectrometer. A graph of spectral reflectance as a function of wavelength is called spectral reflectance curve.

5. Spectral Reflectance of Land Covers (Spectral Signature) Spectral reflectance is assumed to be different with respect to the type of land cover. Vegetation: A chemical compound in leaves called chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green wavelengths. The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths.

6. Water: Longer wavelength visible and near-infrared radiation is absorbed more by water than shorter visible wavelengths. Water typically looks blue or blue-green due to stronger reflectance at these shorter wavelengths, and darker if viewed at red or near infrared wavelengths.

7. Spectral Reflectance Curve for Vegetation and Water By measuring the energy that is reflected (or emitted) by targets on the Earth's surface over a variety of different wavelengths, it is possible to build up a spectral response for that object. By comparing the response patterns of different features we may be able to distinguish them.

8. Spectral Reflectance of Water, Vegetation, Soil and Rock TM = Thematic Mapper (Bands 1,2,3,4,5,7)

9. Spectral Reflectance of Water, Vegetation and Soil

10. Spectral Reflectance of Vegetation, Soil and Rock

13. Digital Image • A digital image is a regular grid array of squares (or rectangles). • The square is referred to as a ‘pixel’, which is a word formed from the term ‘picture element’. • Each square is assigned a digital number (DN) which is related to some parameter (such as reflectance or emittance measured by a remote sensing system sensor).

14. 480 rows, 492 columns, pixel size 10 m

15. Analog to Digital Conversion process

16. The DNs are simply positive integers that result from quantizing the original electronic signal from the sensor into positive integer values using a process called analog-to-digital (A to D) signal conversion.

18. RS and GIS Applications DEM, Stream network

19. RS and GIS Applications Land type, Land use, Soils Classification Agrometeorological Parameters Derivable from Satellites • Soil Surface and Canopy Temperature • Spectral Albedo / Integrated Albedo • Cloud cover / Cloud Albedo /Cloud top Temperature / Rainfall • Soil moisture : Diurnal Variation of Temperature, higher amplitude means low moisture and low amplitude means high moisture; besides, the lower Albedo of soil acts also as function of soil-moisture. • Crop biomass and crop phenology monitoring • Crop area and yields

20. RS and GIS Applications Crop yield Aman 1998 Crop Damage mapping and Assessment

21. RS and GIS Applications Crop yield Winter Rice 1999

22. RS and GIS Applications Crop yield : NDVI • ‘Normalized Difference Vegetation Index’ as an Indicator of Crop Yield • Satellite remote sensors can quantify what fraction of the photosynthetically active radiation is absorbed by vegetation. • In the late 1970s, scientists found that net photosynthesis is directly related to the amount of photosynthetically active radiation that plants absorb. • In short, the more a plant is absorbing visible sunlight (during the growing season), the more it is photosynthesizing and the more it is being productive. • Conversely, the less sunlight the plant absorbs, the less it is photosynthesizing, and the less it is being productive. Either scenario results in an NDVI value that, over time, can be averaged to establish the "normal" growing conditions for the vegetation in a given region for a given time of the year. • NDVI value lower than the normal over cropped areas during critical periods of the growth cycle can result in destroyed or underdeveloped crops with greatly depleted yields.

23. Exploration of SIDR affected area using remote sensing images (MODIS NDVI data)

24. MODIS NDVI of the Sundarbans

25. RS and GIS Applications Change in Waterbodies, Wetlands Water bodies in 1968 Fluvial Waterbody -11.65 km2 Inland Waterbody - 5.06 km2 Open Waterbody - 68.66 km2

26. RS and GIS Applications Change in Waterbodies, Wetlands Water bodies in 2001 Fluvial Waterbody - 10.90 km2 Inland Waterbody - 2.30 km2 Open Waterbody - 61.67km2

27. RS and GIS Applications Change in Waterbodies, Wetlands Change in Open Water bodies between 1968 and 2001