Remote sensing principles-spectral signature-spectural range
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This document discusses the application of remote sensing and geographical information systems in civil engineering. It begins by defining remote sensing as the acquisition of information about an object without physical contact, typically by measuring electromagnetic radiation. It then defines geographical information systems as a system for capturing, storing, analyzing and presenting spatially referenced data. The document provides examples of how remote sensing data from sources like Google Earth can be spatially analyzed using a GIS. It proceeds to discuss key concepts in remote sensing including the electromagnetic spectrum, atmospheric interactions with radiation, and radiation measurement principles.
Remote sensing principles-spectral signature-spectural range
- 1. APPLICATION OF REMOTE SENSING AND GEOGRAPHICAL INFORMATION SYSTEM IN CIVIL ENGINEERING Date: INSTRUCTOR DR. MOHSIN SIDDIQUE ASSIST. PROFESSOR DEPARTMENT OF CIVIL ENGINEERING
- 2. Remote Sensing (RS) Remotely sensing the useful information of object (earth) Geographic Information System (GIS) A system that deals with all types of geographically referenced data Application of Remote Sensing and Geographical Information System in Civil Engineering 2
- 3. Remote Sensing (RS) Remotely sensing the useful information of object (earth) Process of recording, measuring and interpreting imagery and digital representations of energy patterns derived from noncontact sensor systems Geographic Information System (GIS) A system designed to capture, store, manipulate, analyze, manage, and present all types of geographically referenced data Application of Remote Sensing and Geographical Information System in Civil Engineering 3
- 4. Can you recall Google Earth ? While the representation and management of remotely sensed data on geographical locations is made possible through GIS The information in the Google earth is obtained through Remote Sensing 4
- 5. Can you recall Google Earth ? Lets look at small movies about Google earth to learn more about the remotely sensed information and its geographical referencing of the information 5
- 6. Remote sensing has been variously defined but basically it is the art or science of telling something about an object without touching it. (Fischer et al., 1976, p. 34) Remote sensing is the acquisition of physical data of an object without touch or contact. (Lintz and Simonett, 1976, p. 1) Remote sensing is the observation of a target by a device separated from it by some distance. (Barrett and Curtis, 1976, p. 3) The term “remote sensing” in its broadest sense merely means “reconnaissance at a distance.” (Colwell, 1966, p. 71) Remote sensing is the art, science and technology of obtaining reliable information about physical objects and the environment, through the process of recording, measuring and interpreting imagery and digital representations of energy patterns derived from noncontact sensor systems (Lecture Note by Wataru Takauchi, 2009) Remote Sensing 6
- 7. Remote sensing is the science of deriving information about an object from measurements made at a distance from the object, i.e., without actually coming in contact with it. The quantity most frequently measured in present- day remote sensing systems is the electromagnetic energy emanating from objects of interest, and although there are other possibilities (e.g., seismic waves, sonic waves, and gravitational force), our attention . . . is focused upon systems which measure electromagnetic energy. (D. A. Landgrebe, quoted in Swain and Davis, 1978, p. 1) Remote sensing is the practice of deriving information about the Earth’s land and water surfaces using images acquired from an overhead perspective, using electromagnetic radiation in one or more regions of the electromagnetic spectrum, reflected or emitted from the Earth’s surface. (James B. Campbell, Randolph H. Wynne (2011): Introduction to Remote Sensing) Remote Sensing 7 Is remote sensing limited to use of electromagnetic radiation ?
- 8. Various Steps in RS 8 (A) Energy Source (B) Radiation and the Atmosphere (C) Interaction with the Target (D) Recording of Energy by the Sensor (E) Transmission, Reception, and Processing (F) Interpretation and Analysis (G) Application
- 9. Source of Electromagnetic Radiation 9 Nuclear reactions within the Sun produce a full spectrum of electromagnetic radiation, which is transmitted through space without experiencing major changes. As this radiation approaches the Earth, it passes through the atmosphere before reaching the Earth’s surface. Some is reflected upward from the Earth’s surface; it is this radiation that forms the basis for photographs and similar images. Other solar radiation is absorbed at the surface of the Earth and is then reradiated as thermal energy.
- 10. Source of Electromagnetic Radiation By recording emitted or reflected radiation and applying knowledge of its behaviour as it passes through the Earth’s atmosphere and interacts with objects, remote sensing analysts develop knowledge of the character of features such as vegetation, structures, soils, rock, or water bodies on the Earth’s surface. 10 Remote sensing of reflected radiation Remote sensing of emitted radiation
- 11. The electric and magnetic components are oriented at right angles to one another and vary along an axis perpendicular to the axis of propagation Magnetic field (H) oriented at right angles to the electrical field is propagated in phase with the electrical field (E) Electromagnetic Radiations (EMR) λfc = Electric (E) and magnetic (H) components of EMR. smc /103 8 ×= 11
- 12. Two characteristics of electromagnetic radiation are particularly important for understanding remote sensing. These are the wavelength and frequency. Electromagnetic Radiations (EMR) f c =λ Wave length (λ) is the length of one wave cycle, which can be measured as the distance between successive wave crests. Wavelength is measured in metres (m) Frequency (f) refers to the number of cycles of a wave passing a fixed point per unit of time. Frequency is normally measured in hertz (Hz), equivalent to one cycle per second, and various multiples of hertz Remember ! Two are inversely related to each other. The shorter the wavelength, the higher the frequency. The longer the wavelength, the lower the frequency 12
- 13. Units used in RS 13
- 14. Electromagnetic (EM) Spectrum 14 The most familiar form of EMR is visible light, which forms only a small (but very important) portion of the full EM spectrum. The large segments of this spectrum that lie outside the visible range require our special attention because they may behave in ways that are quite foreign to our everyday experience with visible radiation.
- 16. Two important categories are not shown in above Table. The optical spectrum, from 0.30 to 15 µm, defines those wavelengths that can be reflected and refracted with lenses and mirrors. The reflective spectrum extends from about 0.38 to 3.0 µm; it defines that portion of the solar spectrum used directly for remote sensing. Electromagnetic (EM) Spectrum reflective spectrum Optical spectrum 16
- 17. The Visible Spectrum The color of an object is defined by the color of the light that it reflects . Thus a “blue” object is “blue” because it reflects blue light. Intermediate colors are formed when an object reflects two or more of the additive primaries. 17
- 18. Wavelengths longer than the red portion of the visible spectrum are designated as the infrared region The Infrared Spectrum Infrared (0.7-15µm) Near infrared (0.72-1.3µm) Mid infrared (1.3-3.0µm) Far infrared (3-15µm) A photo of the Orion constellation in visible (left) and infrared (right). Although the infrared provides little indication to the exact location of the stars, it detects gas clouds throughout the constellation and other features totally invisible in the optical spectrum 18
- 19. The portion of the spectrum of more recent interest to remote sensing is the microwave region from about 1 mm to 1m. This covers the longest wavelengths used for remote sensing. The shorter wavelengths have properties similar to the thermal infrared region while the longer wavelengths approach the wavelengths used for radio broadcasts. Microwave Spectrum The remote sensing using microwave spectrum is termed as microwave sensing 19
- 20. Basic Definitions The rate at which photons (quanta) strike a surface is the radiant flux, measured in watts (W); this measure specifies energy delivered to a surface in a unit of time. Irradiance is defined as radiant flux per unit area (usually measured as watts per square meter). Irradiance measures radiation that strikes a surface Radiant exitance defines the rate at which radiation is emitted from a unit area (also measured in watts per square meter). Radiation Laws 20
- 21. Black Body Radiation Radiation Laws A blackbody is a hypothetical source of energy that behaves in an idealized manner. It absorbs all incident radiation; none is reflected. A blackbody emits energy with perfect efficiency; its effectiveness as a radiator of energy varies only as temperature varies. where B is the spectral radiance, T is the absolute temperature of the black body, k is the Boltzmann constant, h is the Planck constant, c is the speed of light and λ is the wavelength. 21
- 22. Wein’s Displacement Law Radiation Laws 22
- 23. Stephan Boltzmann Law Radiation Laws 23
- 25. Emissivity Radiation Laws 25 All these radiation laws are important for understanding electromagnetic radiation. They have special significance of detection of radiation in the far infrared spectrum
- 26. All radiation used for remote sensing must pass through the Earth’s atmosphere Atmospheric effects may have substantial impact on the quality of images and data that the sensors generate Therefore, the practice of remote sensing requires knowledge of interactions of electromagnetic energy with the atmosphere Radiation Interaction with the Atmosphere As solar energy passes through the Earth’s atmosphere, it is subject to modification by several physical processes, including (1) scattering, (2) absorption, and (3) refraction 26
- 27. Scattering is the redirection of electromagnetic energy by particles suspended in the atmosphere or by large molecules of atmospheric gases The amount of scattering depends on the sizes of these particles, their abundance, the wavelength of the radiation, and the depth of the atmosphere through which the energy is travelling The size of a scattering particle is parameterized by the ratio α of its characteristic dimension D and wavelength λ: α = πD/λ Rayleigh scattering (α < 0.4) Mie scattering (0.4 < α < 3) Nonselective scattering /Discrete dipole approximation (α > 3) Scattering 27
- 28. Rayleigh scattering occurs when atmospheric particles have diameters that are very small relative to the wavelength of the radiation. Typically, such particles could be very small specks of dust or some of the larger molecules of atmospheric gases, such as nitrogen (N2) and oxygen (O2) Mie scattering is caused by large atmospheric particles, including dust, pollen, smoke, and water droplets. Such particles may seem to be very small by the standards of everyday experience, but they are many times larger than those responsible for Rayleigh scattering Non-selective scattering is caused by particles that are much larger than the wavelength of the scattered radiation. For radiation in and near the visible spectrum, such particles might be larger water droplets or large particles of airborne dust Scattering Rayleigh scattering of sunlight in clear atmosphere is the main reason why the sky is blue 28
- 29. Scattering behaviors of three classes of atmospheric particles. (a) Atmospheric dust and smoke form rather large irregular particles that create a strong forward-scattering peak, with a smaller degree of backscattering. (b) Atmospheric molecules are more nearly symmetric in shape, creating a pattern characterized by preferential forward- and backscattering, but without the pronounced peaks observed in the first example. (c) Large water droplets create a pronounced forward-scattering peak, with smaller backscattering peaks. Scattering Lynch and Livingston (1995) Mie Rayleigh Non-selective 29
- 30. Principal components of observed brightness I = Is + Io + Id Is: reflected from the ground Io: scattered by the atmosphere directly to the sensor Id: diffuse light directed to the ground then to the atmosphere scattered reflected Diffused/refracted 30
- 31. Changes in reflected, diffuse, scattered, and observed radiation over wavelength for dark (left) and bright (right) surfaces. Atmospheric effects constitute a larger proportion of brightness for dark objects than for bright objects especially at short wavelengths Magnitude of brightness components 31
- 32. Absorption is the other main mechanism when electromagnetic radiation interacts with the atmosphere. In contrast to scattering, this phenomenon causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapour are the three main atmospheric constituents which absorb radiation. Absorption 32
- 33. Absorption After passing from atmosphere Original EM spectrum 33
- 34. Refraction is the bending of light rays at the contact area between two media that transmit light. Refraction occurs at the lenses of cameras, magnifying glasses and in atmospheric layers of varied clarity, humidity and temperature Refraction 34
- 35. As electromagnetic energy reaches the earth’s surface, it must be reflected, absorbed or transmitted The proportions of these processes depend on three components; nature of the surface wavelength of the energy angle of illumination Reflection occurs when a ray of light is redirected as it strikes a non-transparent surface. It depends on sizes of surface irregularities (roughness or smoothness) in relation to the wavelength of the radiation If surface is smooth relative to wavelength, specular reflection occurs. If surface is rough, diffuse or isotropic reflection occurs Interaction with the surface 35
- 36. Reflection Specular (a) and diffuse (b) reflection. Specular reflection occurs when a smooth surface tends to direct incident radiation in a single direction. 36
- 37. Lambert’s cosine law, which states that the observed brightness (I) of such a surface is proportional to the cosine of the incidence angle (θ), where I is the brightness of the incident radiation as observed at zero incidence: I′ = I/cos θ This relationship is often combined with the equally important inverse square law, which states that observed brightness decreases according to the square of the distance (D) from the observer to the source: I′ = (I/D2) (cos θ) Illumination 37
- 38. Transmission of radiation occurs when radiation passes through a substance without significant attenuation From a given thickness, or depth, of a substance, the ability of a medium to transmit energy is measured as the transmittance (t): t = Transmitted radiation/Incident radiation Transmission 38 Incident radiation passes through an object without significant attenuation (left), or may be selectively transmitted (right). The object on the right would act as a yellow (“minus blue”) filter, as it would transmit all visible radiation except for blue light.
- 39. Fluorescence occurs when an object illuminated with radiation of one wavelength emits radiation at a different wavelength. The most familiar examples are some sulfide minerals, which emit visible radiation when illuminated with ultraviolet radiation Reflectance For many applications of remote sensing, the brightness of a surface is best represented as reflectance. Reflectance is expressed as the relative brightness of a surface as measured for a specific wavelength interval: Reflectance = Observed brightness/Irradiance Fluorescence Note: Irradiance measures radiation that strikes a surface 39
- 40. The polarization of electromagnetic radiation denotes the orientation of the oscillations within the electric field of electromagnetic energy Polarization Schematic representation of horizontally and vertically polarized radiation. The smaller arrows signify orientations of the electric fields. 40
- 41. Spectral characteristics of Energy sources, Atmospheric Effects and Sensing Systems Spectral sensitivity range of eye coincides with an atmospheric window and peak level of energy from the sun Photography Thermal Scanner Multispectral Scanner Radar and Passive Remote Sensing 41
- 42. Earth’s atmosphere is by no means completely transparent to electromagnetic radiation because the gases (O3, O2, CO2 & H2O ) together form important barriers to transmission of electromagnetic radiation through the atmosphere. Atmosphere selectively transmits energy of certain wavelengths; those wavelengths that are relatively easily transmitted through the atmosphere are referred to as atmospheric windows. Atmospheric Windows Atmospheric windows are vitally important to the development of sensors for remote sensing. 42
- 43. Everything in nature has its own unique distribution of reflected, emitted and absorbed radiation. These spectral characteristics can – if ingeniously exploited - be used to distinguish one thing from another or to obtain information about shape, size, and other physical and chemical properties. A set of observations or measurements that constitutes a spectral response pattern is called the spectral signature of an object. Concept of Spectral Signature 43
- 44. Which bands are most useful for distinguishing between these classes ? Spectral response patterns (spectral patterns) 44
- 46. Both types of trees will appear as similar shades of green to the naked eye Imagery (or photography) using the visible portion of the spectrum may not be useful In near-infrared, they are clearly separable Spectral response pattern (Reflectance curves) 46
- 47. Reflectance curves Oblique normal color aerial photograph showing portion of Univ. of Wisconsin- Madison Oblique Infrared aerial photograph showing portion of Univ. of Wisconsin- Madison 47
- 48. Assignment: Discuss the spectral reflectance of each category in accordance with division of EM spectrum Typical Spectral Reflectance curves for Vegetation, Soil and Water48
- 49. Spectral sensitivity of the sensors available Presence or absence of atmospheric windows in the spectral range(s) in which one wishes to sense Source, magnitude, and spectral composition of the energy available in these ranges Manner in which the energy interacts with the features under investigation (Spectral signature and spectral patterns) Sensor Selection 49
- 50. Type of Remote Sensing (RS) 50 Passive RS Natural (EMR from Sun) Example Optical and thermal remote sensing (passive) (A technology to measure reflected and emitted energy in visible and thermal wavelength) RS using reflected solar radiation RS using emitted terrestrial radiation
- 51. Type of Remote Sensing (RS) 51 Active RS Technological Assisted Radiation Example Microwave remote sensing (active) (A technology to measure a time (distance) between sensor and an object) RS using senor’s transmitted radiation
- 52. Uniform Energy Source Source would provide energy over all wavelengths, at a constant, known, high level of output, irrespective of time and place Non interfering atmosphere Atmosphere would not modify the energy from the source in any manner Unique Energy/ Matter Interactions at the Earth's Surface Reflectance is invariant and unique to each and every earth surface feature Ideal Remote Sensing System 52 Super Sensor Highly sensitive to all wavelengths Simple, reliable, require virtually no power or space, be accurate, and economical to operate Real-Time Data Handling System Derived data would provide insight into the physical-chemical- biological state of each feature of interest Multiple Data Users Knowledge in subject domain & RS image interpretation Same set of data would become various forms of information
- 53. Energy Source Solar energy Microwave for Active remote sensing RS at specific local time Atmosphere Atmospheric windows Energy/Matter Interactions Spectral signature and Spectral similarity Sensor All sensors have fixed range of spectral sensitivity Limitation on spatial resolution Real Remote Sensing System 53 Real-Time Data Handling System Capability of current remote sensors to generate data far exceeds the current capacity to handle these data Multiple Data Users No single combination of data acquisition and analysis procedures will satisfy the needs of all data users
- 54. Comments…. Questions…. Suggestions…. 54 I am greatly thankful to all the information sources (regarding remote sensing and GIS) on internet that I accessed and utilized for the preparation of present lecture. Thank you ! Feel free to contact smohsin@uet.edu.pk
- 55. Assignment: Discuss the spectral reflectance of each category in accordance with division of EM spectrum Typical Spectral Reflectance curves for Vegetation, Soil and Water 55
- 56. 'Peak and Valley' configuration VISIBLE RANGE Valleys in the visible portion are dictated by the pigments in plant leaves Chlorophyll strongly absorbs energy in 0.45-0.65 μm (Chlorophyll Absorption band) If Vegetation is subjected to stress, chlorophyll content reduces and red reflectance increases NIR RANGE (0.7 to 1.3 µm) Very high reflectance (50%) Remaining energy transmitted (very little absorption) Depends on Plant leaf structure Useful for identification of different species Useful for vegetation condition monitoring VEGETATION (Healthy Green Vegetation) 56
- 57. Spectral reflectance of oak leaves 57
- 58. Reflectance from Forest canopy and Layered vegetation 58
- 59. Leaf Structure and Reflectance 59
- 60. BEYOND 1.3 μm Essentially reflects or absorbs with little transmittance At 1.4, 1.9, and 2.7 µm water in leaf absorbs strongly (Water Absorption Bands) Leaf reflectance is approximately inversely related to the total water present in a leaf VEGETATION (Contd..) Reflectance of a leaf to decreased relative water content 60
- 61. Factors affecting soil reflectance Moisture content Soil texture (proportion of sand, silt, and clay) Surface roughness (reduces reflectance) Iron oxide (reduces reflectance) Organic matter content (reduces reflectance) Inter-related Coarse textured dry soils will have more reflectance than fine textured soils (reverses if water is present) Rocks Aggregates of minerals Reflectance depends on mineral composition Weathered surface SOIL (Dry Bare Soil – Grey-brown Loam) 61
- 62. Most of the energy is either absorbed or transmitted VISIBLE RANGE Little energy is reflected only in this range Water quality studies Shallow Vs Deep water Clear Vs Turbid water Rough Vs Smooth NIR RANGE (0.7 to 1.3 µm) Completely absorbs Useful for delineating water bodies Algal bloom and/ or Phytoplankton results in reflection WATER (clear deep water body) 62
- 63. Water surface, subsurface, volumetric and bottom radiance63
- 64. Attenuation in pure water by absorption and scattering
- 66. Ability to view large parts of the globe at different scales Capability to monitor regions which may be very remote or where access is denied Ability to analyse different surfaces at wavelengths not detectable to the human visual system Ability to obtain imagery of an area at regular intervals over many years in order that changes in the landscape can be evaluated Capability to see human-induced effects on our planet Disadvantages Certain skill level is required to interpret the imagery Interpretation based solely on remotely sensed data should be treated with caution unless supported by ground verification data. Advantages of Remote Sensing 66
- 67. Exploring both geographical and thematic components of data in a holistic way Stresses geographical aspects of a research question Allows handling and exploration of large volumes of data Allows integration of data from widely disparate sources Allows analysis of data to explicitly incorporate location Allows a wide variety of forms of visualisation Disadvantages of GIS Data are expensive Learning curve on GIS software can be long Shows spatial relationships but does not provide absolute solutions Origins in the Earth sciences and computer science. Solutions may not be appropriate for humanities research Advantages of GIS 67
- 68. Thank you