ENERGY INTERACTIONS WITH EARTH SURFACE FEATURES
- 1. ENERGY INTERACTIONS WITH EARTH SURFACE FEATURES Name: DIPON DEB NATH Department: Geography And Environmental Studies Session: 2016-17
- 2. ENERGY INTERACTIONS WITH EARTH SURFACE FEATURES OVERVIEW: There are three possible interaction with earth surface 1. Reflection depends on .wavelength of energy .geometry of surface .components of objects materials and its condition 2. Absorption 3. Transmission spectral reflectance : .calculating method . Reflectance curve and its characteristics Spectral Reflectance of Earth Surface Features
- 3. ENERGY INTERACTIONS WITH EARTH SURFACE FEATURES When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions are possible. These are: 1. Reflection (R) 2. Absorption (A) 3. Transmission (T) 1.Reflection(R): Reflection occurs when a ray of light is redirected as it strikes a non-transparent surface. According to the law of reflection, the angle of incidence is equal to the angle of reflection.
- 4. (continuous) Reflection depends on .energy of wavelength .geometry of surface .components of objects materials and its condition Energy of wavelength: Reflection is proportion to energy of wavelength. Lower wavelength, higher energy and higher wavelength lower energy. Geometry of surface: The Surface of features is two types. . Smooth surface (specular reflection occurs) . specular reflection is obtained where complete or nearly complete incident energy is reflected in one direction . Rough surface (diffuse or isotropic reflection occurs) Diffuse or isotropic reflection is the energy reflected uniformly in all directions
- 5. (continuous) components of objects materials and its condition: Different objects provide different reflection owing to the objects materials and its conditions
- 6. (continuous) 2. Absorption(A): Absorption occurs when radiation is absorbed by the target. The portion of the EM energy which is absorbed by the Earth’s surface is available for emission and as thermal radiation at longer wavelengths. 3.Transmission(T): Transmission occurs when energy pass through the object. It’s depends on the characteristics of medium, velocity of wavelength. According to the principle of energy conservation, energy can neither be created nor destroyed. It can be transferred. So we can measure the different form of energy by this equation:-
- 7. (spectral reflectance) Spectral reflectance is the percentage of incoming incident energy that is reflected. It is also known as albedo of the surface. It may vary from 0-100%. Equipment to measure reflectance is called spectrometer The energy that is reflected by features on the earth's surface over a variety of different wavelengths will give their spectral responses in the remote sensing systems. Each type of feature/object has a unique spectral response/ reflectance characteristics, also known as spectral signature, which can be used to identify the respective surface features & to study their properties
- 8. (spectral reflectance curve or signature) The graphical representation of the spectral response of an object over different wavelengths of the electromagnetic spectrum is termed as spectral reflectance curve. The graph is drawn between various wavelengths (μm) of EM spectrum on x- axis & the amount of reflectance (%) recorded by the R.S. system on the y- axis. These curves give an insight into the spectral characteristics of different objects. The curve for each of these object types is plotted as a “ribbon” (or “envelope”) of values, not as a single line. The peaks indicate strong reflection of incident energy and the valleys indicate predominant absorption of the energy in the corresponding wavelength bands. In visible portion, the spectral reflectance curves for each tree is same or approximately overlapped. Consequently, we can not distinguish the both trees. But in near-IR(from 0.7 to 1.3 μm ) we can separate them on the basis of spectral characteristics. On near-IR images, deciduous trees (having higher IR reflectance than conifers) generally appear much lighter than conifers.
- 9. 1. Spectral Reflectance Curve for Vegetation: Spectral Reflectance Curve for various objects Spectral reflectance curve for healthy green vegetation exhibits the "peak-and-valley" configuration. In general, healthy vegetation are very good absorbers of electromagnetic energy in the visible region (0.4- 0.7μm). A chemical compound in leaves called chlorophyll absorbs radiation in the blue(0.45) and red(0.67) μm wavelengths but strongly reflects green wavelengths. But reflection increases in the near infrared boundary(0.7 μm) owing to internal structure of leaves and nearly constant from (0.7-1.3 μm) then decreases.
- 10. Spectral Reflectance Curve for various objects The internal structure of healthy leaves act as excellent diffuse reflectors of near-infrared wavelengths. As internal structure varies among different plant species, the near infrared ray used to discriminate different plant species. In the region beyond 1.3 μm, leaf reflectance is approximately inversely related to the total water present in a leaf as water absorbs the energy. This total water is a function of both the moisture content and the thickness of the leaf. Dips in reflectance occur at 1.4, 1.9, and 2.7 μm (Fig.1.) as water in the leaf strongly absorbs the energy at these wavelengths. So, wavelengths in these spectral regions are referred to as water absorption bands. Reflectance peaks occur at 1.6 and 2.2 μm (Fig.1.), between the absorption bands.
- 11. If a plant is subject to some form of stress that interrupts its normal growth and productivity, it may decrease or cease chlorophyll production. The result is less chlorophyll absorption in the blue and red bands.That we see the plant turn yellow (combination of green and red). This can be seen in the spectral curve for dried grass. Spectral Reflectance Curve for various objects
- 12. Spectral Reflectance Curve for various objects Some of the factors effecting soil reflectance are moisture content, soil texture (proportion of sand, silt, and clay), surface roughness, presence of iron oxide and organic matter content. These factors are complex, variable, and interrelated. The presence of moisture in soil decreases its reflectance. This effect is greatest in the water absorption bands at 1.4, 1.9, and 2.1 μm. On the other hand, similar absorption characteristics are displayed by the clay soils. Clay soils have hydroxyl ion absorption bands at 1.4 and 2.2 μm. The soil curve is considerably less peak-and-valley variation in reflectance. Some factors that influence soil reflectance. 2. Spectral Reflectance curve for Soil:
- 13. Spectral Reflectance Curve for various objects 3. Spectral Reflectance curve for Water: Spectral reflectance of water varies with its physical condition. In the solid phase (ice or snow) water give good reflection at all visible wavelengths. On the other hand, reflection in the visible region is poor in case of water in liquid stage. This difference in reflectance is due to the difference in the atomic bond in the liquid and solid states. In the visible region between 0.4μm and 0.7μm, around 0.6μm water in the liquid form shows high reflectance. Wavelengths beyond 0.7μm are completely absorbed (i.e. no curve formed beyond 0.7μm). Thus clear water appears in darker tone in the NIR image. Locating and delineating water bodies with remote sensing data is done more easily in reflected infrared wavelengths because of this absorption property.
- 14. The reflectance from a water body can stem from an interaction with the water's surface (specular reflection), with material suspended in the water, or with the bottom surface of the water body. Even in deep water, where bottom effects are negligible, the reflectance properties of a water body are not only a function of the water, but also of the material in the water. Spectral Reflectance Curve for various objects The reflectance depends on .material suspended in the water (river highly reflect than lake. Because river highly contain sediment) .presence of chlorophyll (increase green wavelength reflection) Reflectance data in the visible region can also be used to differentiate shallow and deep waters, clear and turbid waters, as well as rough and smooth water bodies.
- 15. Spectral Response patterns We have already known that the spectral reflectance characteristics of vegetation, soil and water by the reflectance curves/ signatures/patterns. It is possible to ‘spectrally separate’ features of this type. The amount of separation will depend on ‘where we look’ in the electromagnetic spectrum. Water and vegetation, reflect nearly the same in visible wavelengths but in near-IR wavelengths they are distinctly separable. spectral responses of various features often permit to assess the type and/or condition of the features. spectral response patterns or curves may be quantitative, but they are not absolute. They may be distinctive, but they are not necessarily unique. The spectral responses curve can be form wavelengths of any part of the spectrum relevant for the application.
- 16. Spectral Response patterns We have already looked at some characteristics of objects that influence their spectral response patterns. Temporal effects and spatial effects can also enter into any given analysis. Temporal effects: Temporal effects are any factors that change the spectral characteristics of a feature over time. For example, the spectral characteristics of vegetation generally changed throughout a growing season. These changes can influence the sensor data we collect for a given application. An example of this process is detecting the change in suburban development near a metropolitan area by using data obtained on two different dates. Spatial effects: Spatial effects refer to factors that cause the same type of features at a given point in time, to have different characteristics at different geographic locations. For example, water body in tropical and polar region have some different characteristics which influence the reflectance curve.
- 17. Spectral response from surface features can be influenced by shadows. Within a shadow, the total reflected energy is reduced, and the spectral response is shifted toward shorter wavelengths. An example of a useful spatial effect is the change in the leaf morphology of trees when they are subjected to some form of stress.
- 18. Questions i. Describe the ways of energy interaction with earth surface? ii. What is spectral reflectance? iii. Discuss spectral reflectance curve and it’s function? iv. Illustrate the spectral reflectance characteristics of earth surface features?