REMOTE SENSING COURSES. Syllabus - Theory 1 - Basic Principles of Remote Sensing

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1 REMOTE SENSING COURSES Syllabus - Theory 1 - Basic Principles of Remote Sensing Prof. dr. Frank Veroustraete and Prof. dr. ir. Roeland Samson (Department of Bioscience engineering) Contents Remote Sensing Courses... 1 Syllabus - Theory 1 - Basic Principles of Remote Sensing... 1 Introduction... 2 Basic Principles of Electromagnetic Wave Propagation... 3 Basic Concepts, Equations and Terms Spherical spreading Absorption Scattering Resolution References... 9 The Electromagnetic Spectrum Measuring the electromagnetic spectrum Wavelength, Energy, and Frequency A Radio Wave is not a Gamma-Ray, a Microwave is not an X-ray... or is it? Why do we have to be in space to enable the observation of the complete EM Spectrum? Light as a Wave Exercises

2 INTRODUCTION Remote sensing has been defined in many ways. It can be thought of as including traditional aerial photography, geophysical measurements such as surveys of the Earth's gravity and magnetic fields, and even seismic and sonar surveys. However, in the context of the ICT era, the term remote sensing usually implies digital measurements of electromagnetic energy often for wavelengths that are not visible to the human eye. A big advantage in remote sensing is that the final product is usually an image ("picture") of the Earth's surface which we can visualize and interpret as if it were an analogue photograph. Hence, many of the terms and concepts (e.g., brightness, contrast, colour, intensity) are familiar, and we have a physical intuition of their meaning. Very often the ultimate goal of a remote sensing study is simply to be able to "see" some feature or changes in it sufficiently well. Thus, the definition of a good result can be as subjective as deciding which of a series of photographs is the best. However, one must be careful to remember that remote sensing images are more than just digital pictures, and we need to have a good understanding of their physical meaning. Remote sensing techniques measure the interaction of the Earth's surface (or better, its uppermost meters) with electromagnetic energy from the sun and are therefore inherently a form of geographic information. Hence, the use of geographic information systems (GIS) to store and display remote sensing information is so common that the terms remote sensing and GIS are almost synonymous. The use and generation of digital elevation models is an example of how these two fields have a merged zone. When properly geographically referenced (e.g., the location of each measurement is carefully determined), images ("pictures") Aerial photograph of district of El Paso, Texas. the downtown Close-up view of downtown El Paso, Texas taken from an airplane. Note building shadows and clear resolution of image. Rectified USGS Digital Orthophoto Quarter Quadrangle El Paso SW January 28, Image by John Seeley, PACES. -2-

3 BASIC PRINCIPLES OF ELECTROMAGNETIC WAVE PROPAGATION Most of the key physical principals we formulate to obtain a basic understanding of electromagnetic energy are familiar to us. In remote sensing, we classify electromagnetic energy by its wavelength. Visible light is the type of electromagnetic energy with which we are most familiar, but there is much to be learned from waves whose wavelengths are longer or shorter that those of visible light. The basic theory needed to understand electromagnetic energy sufficiently well and to use remote sensing techniques intelligently, is surprisingly simple. The mathematics needed is not complex, and the physical principles are straightforward. The trick is to link a training in mathematics and physics with practical knowledge of physical phenomena (like light, X-rays, radar, and radio waves) to develop an intuitive understanding of the propagation of electromagnetic waves through the atmosphere and their interaction with the Earth's surface. One advantage is that many terms used in remote sensing are familiar and have the same meaning as in everyday life (i.e., bright, dark, high and low contrast, and intensity). In physics, one learns that light (and electromagnetic energy of similar wavelengths) can be thought of as either a propagating wave or a stream of particles. In remote sensing, we usually think in terms of waves, and sensors are designed to detect waves whose wavelengths are located in specific bands (ranges of wavelengths). Landsat Thematic Mapper (TM) Image West-central El Paso, Texas. (Image by John Seeley) -3-

4 BASIC CONCEPTS, EQUATIONS AND TERMS There are a number of basic equations (and the terms involved in them) which are basic to the understanding of electromagnetic wave properties and how to use them in practical applications. Electromagnetic (EM) energy is classified by wavelength or wave number to represent the electromagnetic spectrum. Illustration of the electromagnetic spectrum (EM spectrum) The EM spectrum shows that visible light occupies a very narrow band of wavelengths only in the total EM spectrum. Gamma rays, X-rays, and most shortwave ultra-violet energy do not penetrate the atmosphere. Hence, they cannot be applied in remote sensing of land surfaces. However, infrared and microwave (radar) energy is measured regularly and is very useful in addition to energy at visible wavelengths. Remember that most imagery made with remote sensing digital data is so-called false colour imagery and therefore, they are not comparable with photographs in the sense that humans will never see the Earth in this particular mode of viewing or in other wavelinghts than the visible ones (by definition). The sensors in fact detect reflected EM radiation from objects which cannot be "seen" by the naked eye. Primarily as the result of the presence of ozone (O3), CO2, and water vapour in the atmosphere solar radiation is absorbs in several discrete bands. Earth observation sensors are designed to avoid these bands since little energy survives transmission through the atmosphere. Most remote sensing devices are passive in the sense that they measure the intensity of the Sun's incoming electromagnetic energy after reflection at the surface of the Earth or when it is emitted (radiated) as heat. However, some systems (radar in particular) are active in the sense that the measured microwave energy is generated by the radar measuring platform and not the Sun. As electromagnetic energy interacts with the atmosphere and the surface of the Earth, the most important concept to remember is the conservation of energy (i.e., the total EM energy remains constant). -4-

5 Our main concern is the fate of the energy that makes it through the atmosphere to hit the surface of the Earth. The three main interactions at this interface are reflection (scattering is a form of reflection), absorption (radiation energy is converted into heat and some is emitted with a change in wavelength and a time delay, which is called fluorescence), and transmission. Each of these radiation interactions are a function of wavelength ( ). The conservation of energy leads to the following equation: E i ( ) = E ( ) + E ( ) + E ( ) where, E i ( ) = incident energy; E ( ) = reflected energy; E ( ) = absorbed energy; E ( ) = transmitted energy. In the case of a lens, almost all of the incident energy is transmitted. However, in the case of the Earth's surface, most of the radiation energy is reflected or absorbed. At a particular location, the ratio of the reflected energy to the incident energy as a function of wavelength (expressed as a fraction) is called a reflectance spectrum, and is expressed as reflected over incident radiation energy: ( ) = E ( ) / E i ( ) The interaction of the incident energy with the atomic and molecular structures in soils, rocks, plants, bodies of water, man-made objects, etc. governs how much energy is absorbed and thus how much is reflected and transmitted. Materials such as minerals and leaves have a (at least somewhat) distinctive reflectance spectrum which can be measured with a laboratory or field-portable spectrometer. These values can then be compared with remote sensing data to identify which materials are present in the field of view of an image. When electromagnetic waves travel through a medium, they encounter objects (discontinuities in velocity) that reflect some energy like a mirror and transmit some energy after changing the travel path (refract light like a lens). -5-

6 A basic principle of optics that applies here is Snell's law: sin(i)/c 1 = sin(r)/c 2 The ratio of the velocities of radiation is called the index of refraction (n) n = V 1 /V 2 This type of reflection (technically called specular reflection) is what we usually think of when we consider physical processes in optics, seismology, or billiards. However, it only occurs when the surface is smooth (typically when irregularities in the reflecting matrix are very small relative to the wavelength of the incident electromagnetic radiation). Examples are the reflection of objects in a mirror or a body of quiet water. However, in the case of the Sun's energy reflecting from the Earth s surface, this reflection type only partly describes the processes occurring. Specular reflection is a function of wavelength, the size of the reflecting particle, and the index of refraction of the particle. In addition, some energy is usually transmitted by a few particles very near the surface and thus, is affected by their absorptive properties (which are function of wavelength) as well as adjacent particles that are encountered (see figure at right). The end result is that reflected energy leaves the surface at all angles (diffuse reflection) after interacting with particles near the surface. The resulting variation of reflectance with wavelength is the process which we sense as colour when we see it in the visible band of the EM spectrum. It also is the energy we study in remote sensing. Hence, reflectance is basically a simple concept, but we must remember that it is a function of wavelength of incident radiation energy (and to some extent the angle of This is what happens as light interacts with a surface. -6-

7 incidence), the index of refraction and absorption of the materials which make up the reflecting surface, and the size of the particles which make up the surface. The distance (d) an electromagnetic wave travels in a certain time (t) depends on the velocity through the material (v) in which the wave is travelling; d = v t. Since all electromagnetic energy travels at the speed of light in vacuum (usually denoted by c) this is equation is very simple to apply. For example, we can generate radar waves and time their return after reflection from the Earth's surface. The distance to the reflecting object can be obtained from this simple equation. The velocity (c), frequency (v), and wavelength ( ) of an electromagnetic wave are related by the equation: c = v Strictly speaking, this equation applies to simple harmonic motion in which we are dealing with a wave that consists of motion with a single frequency (i.e. a sine wave), but in remote sensing, one usually treats measurements that are made for a narrow band of frequencies as representing a single frequency at the middle of the band. In the figure to the right, the EM wave is depicted by taking a particular point as the wave passes. The horizontal axis in this diagram is in units of time and the distance between two peaks is the period. We can also think of depicting the wave as if we take a snapshot of it at one point in time. In +this case, we are looking at a spatial picture in which the horizontal axis is in units of distance and the distance between two peaks on the wave form is the wavelength. We usually think of electromagnetic waves as a constant stream of energy from the Sun at particular frequencies. Thus, the scheme in the figure above for an EM wave applies. In radar applications, the platform generates its own energy and in this case, it is appropriate to think of these waves as a pulse (wavelet) of energy that is as short as possible in time. Strictly speaking, it is no longer simple to think of the wavelength or frequency of this wave although such terms are still commonly used in radar studies. One reason for this usage is the utility of wavelength as a measure of the spatial dimension of an electromagnetic wave relative to some object whose interaction with the wave is of interest. We can draw on the analogy of a rock dropped into a pond to define a wave front. The circular ripples for which wave motion is the same (a particular peak or trough) are wave fronts. In the case of most remote sensing applications, these fronts are spherical and spread out in three dimensions from the source (the Sun). Because of our great distance from the Sun, we think of the incoming wave front as being planar, and in radar, we go to great lengths to generate a planar wave front (we call this synthetic aperture radar and almost all modern data is of this type). In this way, one can think of the propagating -7-

8 electromagnetic energy as being very organized. When a wave front encounters discontinuities that are large relative to the wavelength, the energy is reflected (and refracted for example through a lens) also in an organized fashion (Snell's law above). Thus, light whose wavelength is such that it is red looks red to us when it is reflected. It is quite appropriate to look at the amplitude of a electromagnetic wave and think of it as a measure of the energy in that wave. Obviously, the larger the EM wave amplitude, the more energetic the wave. However, technically we must remember that introductory physical treatments of simple harmonic motion show that energy is proportional to the square of the amplitude. Electromagnetic waves lose energy (amplitude) as they travel because of several phenomena. 1.1 Spherical spreading This phenomenon is straightforward related with the die-off of amplitude with distance travelled (e.g., the further from the source, the weaker the signal). This physical effect is not different from the observation that: As one gets further and further from a source of sound, the weaker and weaker the sound is heard. Technically, as the wave travels, the wave front is a sphere whose radius becomes larger and larger. Since the area of a sphere is 4 R 2, at some radius (distance travelled), the energy per unit area on the wave front is equal to the original energy divided by 4 R 2. Since the energy is proportional to the square of the amplitude, the amplitude is proportional to 1/R. Strictly speaking, the energy is not diminished by this effect, but it is simply spread along the surface areas of larger and larger spheres. 1.2 Absorption It is appropriate to think of absorption as the tendency of materials to simply soak electromagnetic energy and to convert it into heat (or fluorescence). We can measure some of this energy as radiated (emitted) heat at a longer wavelength than the original incoming energy. The same is true fro fluorescence. 1.3 Scattering The interaction of electromagnetic waves with small objects relative to the wavelength (i.e., molecules in the atmosphere) leads to energy being reflected (scattered) in a random type fashion. This term is very intuitive, and we can just think of the energy being randomly dispersed in three dimensions. The primary scattering process is the so-called Rayleigh scattering which is proportional to 1/ 4. Hence in the atmosphere, short wavelengths are subject to selective scattering which is basically due to the fact that many particles and molecules are too small to affect the longer wavelengths significantly. Short wavelength radiation like UV and blue light are scattered about twice as strongly as red light. For wavelengths in the visible light range, selective scattering causes us to see a blue sky, which is the effect of selective scattering of blue light. It is the classic example of this effect. This effect (along with absorption) is a fundamental restriction in remote sensing and limits the range of wavelengths that can be applied. When large particles like water droplets are considered, the scattering affects a broad range of wavelengths (non-selective). We then simply observe diffuse white light. Classic examples of this effect are clouds and fog. -8-

9 1.4 Resolution A question which constantly arises in electromagnetic studies is whether the target of a remote sensing study can actually be "seen" (resolved) in the data. When one looks at a printed copy of a particular image or picture, resolution can be quantified to some extent but is really not more complex than to determine whether an interpreter can see the object, pattern, signature, texture, etc. of interest. In the case of digital imagery, resolution is a technical issue with two aspects. Firstly by its very nature, a digital image is composed of pixels (ground resolution cells) of finite size. The quantity for each cell is the intensity of reflected energy for a particular band of wavelengths while the size of the pixel is a measure of spatial resolution. The simple fact is that the smaller the pixels, the higher the resolution becomes. A good resolution is of course desirable, and as new satellites and airborne systems become available, the tendency is to decrease pixel size (30x30 m² for Landsat Thematic Mapper, 10x10 m² for SPOT panchromatic, 5x5 m² for IRS panchromatic). There is also the question of spectral resolution. In this case, the question is how well we can determine the variation of reflectance with wavelength (the reflectance spectral profile). Practically, this is a question of how many bands a particular device is capable of measuring over a specific spectral range and what is their bandwidth. A large number of narrow bands is desirable and again technology is providing data with better resolution. For example, Landsat Thematic Mapper produces measurements with 7 spectral bands while the AVIRIS airborne system makes measurements using 224 bands. Hence, in digital terms, the denser the measurement, the better the resolution. However, the data volume also increases importantly, as sampling rate (spatially or in wavelength) increases, and this will affect the speed and time needed for image data processing. SPOT Panchromatic: Southern Tip of Franklin Mountains, El Paso, Texas 1.5 References Lilles and Thomas M., and R. W. Kiefer, 1987, Remote Sensing and Image Interpretation: John Wiley and Sons, New York, 721pp. Sabins, Floyd F., 1997, Remote Sensing: Principals and Interpretation: W. H. Freeman and Company, New York, 494pp. Vincent, Robert K., 1997, Fundamentals of Geological and Environmental Remote Sensing: Prentice Hall, Upper Saddle River, New Jersey, 366pp. -9-

10 THE ELECTROMAGNETIC SPECTRUM 1.6 Measuring the electromagnetic spectrum You actually know more about it than you may think! The electromagnetic (EM) spectrum is just a name that scientists give to a bunch of radiation types when they want to talk about them as a group. Radiation is energy that travels and disperses as it goes-- visible light emitted by a lamp or radio waves from a radio station antenna are two types of electromagnetic radiation. Other examples of EM radiation are microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more energetic objects and events emit higher thermal energy radiation than cooler objects. Only extremely hot objects or particles moving at very high velocities can create high-energy radiation like X-rays and gammarays. Here are the different types of radiation in the EM spectrum, in order from the lowest energy level to the highest: Radio EM. This is the same kind of energy that radio stations emit into the air for your boom box to capture to turn into your favorite Mozart, Madonna, or Coolio tunes. But radio waves are also emitted by other things... such as stars and gases in space. You may not be able to dance to what these objects emit, but you can use it to learn what they are made of. Microwaves. They will cook your popcorn in just a few minutes! In space, microwaves are used by astronomers to learn about the structure of nearby galaxies, including our own Milky Way! Infrared. We often think of this as being the same thing as 'heat', because it makes our skin feel warm. In space, IR light is used to map the dust between stars. Visible. Yes, this is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs to stars... also by fastmoving particles hitting other particles. Ultraviolet. We know that the Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin to burn! Stars and other "hot" objects in space emit UV radiation. -10-

11 X-Rays. Your doctor uses them to look at your bones and your dentist to look at your teeth. Hot gases in the Universe also emit X-rays. Gamma-rays. Radioactive materials (some natural and others made by man in things like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use to help them understand what matter is made of can sometimes generate gamma-rays. But the biggest gamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways. 1.7 Wavelength, Energy, and Frequency The speed of light in a vacuum is commonly given the symbol c. It is a universal constant that has the value: c = 3x10 10 cm/s The speed of light in a medium is generally less than this. Normally the term "speed of light", without further qualification, refers to the speed of light under vacuum conditions. An EM wave can be characterized by its wavelength, but we can also characterize it by the frequency (how many wavelengths pass a fixed point in a given time; think of sitting on the dock---of the bay---counting the number of water waves passing in one minute) and the energy that it carries (think of a water wave knocking you over in heavy surf). For light waves the relationship among the wavelength (usually denoted by Greek "lambda"), the frequency (usually denoted by Greek "nu"), and the energy E are where c is the speed of light and h is another universal constant called Planck's Constant that has the values: h = 4.135x10-15 ev-sec = 6.625x10-27 erg-sec in two different useful sets of units (ev stands for "electron volts". Electron volts and ergs are two common units of energy). Thus, these equations allow us to freely interconvert among frequency, wavelength, and energy for electromagnetic waves. Specifying one also specifies the others. -11-

12 1.8 A Radio Wave is not a Gamma-Ray, a Microwave is not an X-ray... or is it? Radio waves, visible light, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing, electromagnetic radiation. We may think that radio waves are completely different physical objects or events than gamma-rays. They are produced in very different ways, and we detect them in different ways. But are they really different things? The answer is 'no'. Radio waves, visible light, X- rays, and all the other parts of the EM spectrum are fundamentally of the same physical nature. They are all electromagnetic radiation. Electromagnetic radiation can be described in terms of a stream of photons, which is a particle stream without mass travelling in a wave-like pattern and moving at the speed of light under vacuum. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and... the most energetic of all... gammarays. The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each way of thinking about the EM spectrum is related to the others in a precise -12-

13 mathematical way. So why do we have three ways of describing things, each with a different set of physical units? After all, frequency is measured in cycles per second (which is also named Hertz), wavelength is measured in meters, and energy is measured in electron-volts. The answer is that scientists don't like to use big numbers when they don't have to. It is much easier to say or write "two kilometers or 2 km" than "two thousand meters or 2,000 m". So generally, scientists use whatever units are easiest for whatever they are working with. In radio astronomy, astronomers tend to use wavelengths or frequencies. This is because most of the radio part of the EM spectrum falls in the range from a about 1 cm to 1 km (30 gigahertz (GHz) to 100 kilohertz (khz)). The radio is a very broad part of the EM spectrum. Infrared astronomers also use wavelength to describe their part of the EM spectrum. They tend to use microns (or millionths of meters) for wavelengths, so that they can say their part of the EM spectrum falls in the range 1 to 100 microns. Optical astronomers use wavelengths as well. In the older "CGS" version of the metric system, the units used are angstroms. An angstrom is equal to meters (10-10 m in scientific notation)! In the newer "SI" version of the metric system, we think of visible light in units of nanometers or meters (10-9 m). In this system, the violet, blue, green, yellow, orange, and red light we know so well has wavelengths between 400 and 700 nanometers. This range is only a small part of the entire EM spectrum, so you can tell that the light we see is just a little fraction of all the EM radiation around us! By the time you get to the ultraviolet, X-ray, and gamma-ray regions of the EM spectrum, lengths have become too tiny to think about any more. Scientists usually refer to these photons by their energies, which are measured in electron volts. Ultraviolet radiation falls in the range from a few electron volts (ev) to about 100 ev. X-ray photons have energies in the range 100 ev to 100,000 ev (or 100 kev). Gamma-rays then are all the photons with energies greater than 100 kev. 1.9 Why do we have to be in space to enable the observation of the complete EM Spectrum? White ranges are regions where radiation is blocked in the atmosphere. -13-

14 EM from space is unable to reach the surface of the Earth except at a limited set of wavelengths, such as the visible spectrum, radio frequencies, and some ultraviolet wavelengths (see figure above). Astronomers can measure above enough of the Earth's atmosphere to observe at some infrared wavelengths from mountain tops or by flying their telescopes in an aircraft. Experiments can also be taken up to altitudes as high as 35 km by balloons which can operate for months. Rocket flights can take instruments all the way above the Earth's atmosphere for just a few minutes before they fall back to Earth, but a great many important first results in astronomy and astrophysics came from just those few minutes of observations. For long-term observations, however, it is best to have your detector on an orbiting satellite platform...and get above it all! In modern physics, light or electromagnetic radiation may be viewed in one of two complementary ways: as a wave in an abstract electromagnetic field, or as a stream of zeromass particles called photons. Although either is an acceptable description of light, for our purposes in introductory astronomy the wave description will be more useful Light as a Wave The quantity that is "waving" is the electromagnetic field, an esoteric but quite measurable entity: your lights shine and your microwave runs and your radio plays because the electromagnetic field exists. As illustrated in the adjacent image, a wave has a wavelength associated with it. It is normal to give the wavelength of light the symbol Greek "lambda". Some common units of length employed for wavelengths of electromagnetic radiation are summarized in the following table. (The micrometer is often still called by its old name, the micron.) Units of Wavelength Unit Symbol Length centimeter cm 10-2 meters Angstrom 10-8 centimeters nanometer nm 10-9 meters micrometer nm 10-6 meters -14-

15 EXERCISES -15-