Fundamentals of Remote Sensing.pdf

Fundamentals of Remote Sensing Outline Photogrammetry Aerial Photogrammetry Remote Sensing Remote Sensing Process Electromagnetic Spectrum Spectral Indices Atmospheric Window Energy Interactions with Earth Surface Spectral Reflectance Curve Photogrammetry What d...

Fundamentals of Remote Sensing Outline Photogrammetry Aerial Photogrammetry Remote Sensing Remote Sensing Process Electromagnetic Spectrum Spectral Indices Atmospheric Window Energy Interactions with Earth Surface Spectral Reflectance Curve Photogrammetry What does Photogrammetry mean? “Photogrammetry” word derived from three Greek words: 1. phos or phot : light 2. gramma: something drawn or written 3. metrein: to measure Hence, photogrammetry is measuring something by the means of light. Photogrammetry According to the American Society for Photogrammetry (1956), Photogrammetry is the science or art of obtaining reliable measurements by the means of photography. According to the American Society for Photogrammetry (1987), it is the art, science and technology of obtaining reliable information about physical objects and the environment by recording, measuring and interpreting photographic images. Introduction to Aerial Photogrammetry Aerial photogrammetry is a technique that uses photographs taken from aircraft or drones to create maps, 3D models, and other spatial data. Applications of Aerial Photogrammetry: 1. Topographic mapping 2. Urban planning 3. Agriculture 4. Disaster management 5. Environmental monitoring Advantages of Aerial Photogrammetry: 1. Large area can be covered in less time 2. No details are missed 3. Can also be used in inaccessible areas 4. Time and cost efficient 5. 3D visualization Application of Aerial Photogrammetry in Agriculture 1. Crop monitoring and health assessment 2. Yield Estimation and analysis 3. Precision Agriculture 4. Irrigation Management 5. Land Use and Field Planning 6. Environmental Monitoring 7. Flood and Drought Assessment Types of Aerial Photographs: Vertical Photographs: Camera axis is nearly vertical as possible In these photos, the shape of objects remains mostly in their actual state as visible from above. Due to various weather conditions and air turbulence, it is difficult to obtain a true vertical photograph. Tilt Photographs In this type of photograph, the vertical line is unintentionally tilted with plumb line. This is most widely used as a true vertical photograph is not easy to obtain. The tilt is normally 1°-3° Tilt is due to the motion of aircraft and consequently the disturbance in camera position. Oblique Photographs In this type of photograph, the vertical line is intentionally tilted with plumb line. Tilt is according to the purpose of photographs. Types of Oblique Photographs Low Oblique High Oblique There is intentional 15°-30° deviation in camera There is intentional more than 30° deviation in axis from the vertical axis. camera axis from the vertical axis. Horizon is not visible. Horizon is visible. Remote Sensing Remote Sensing is an art, science and technology of observing an object, scene or phenomenon by instrument-based techniques without physical contact. The process of use of electromagnetic radiation by sensors to record images of the environment ,which can be interpreted to produce useful information. This is done by sensing and recording reflected or emitted energy and processing, analyzing, and applying that information. “Knowing Without going” Remote Sensing Process 1. Energy Source or Illumination (A) 2. Radiation and the Atmosphere (B) 3. Interaction with the Target (C) 4. Recording of Energy by the Sensor (D) 5. Transmission, Reception, and Processing (E) 6. Interpretation and Analysis (F) 7. Application (G) 1. Energy Source or Illumination (A) The first requirement for remote sensing is to have an energy source which illuminates or provides electromagnetic energy to the target of interest. 2. Radiation and the Atmosphere (B) As the energy travels from its source to the target, it will come in contact with and interact with the atmosphere it passes through. This interaction may take place a second time as the energy travels from the target to the sensor. 3. Interaction with the Target (C) Once the energy makes its way to the target through the atmosphere, it interacts with the target depending on the properties of both the target and the radiation. 4. Recording of Energy by the Sensor (D) After the energy has been scattered by, or emitted from the target, we require a sensor (remote - not in contact with the target) to collect and record the electromagnetic radiation. 5. Transmission, Reception, and Processing (E) The energy recorded by the sensor has to be transmitted, often in electronic form, to a receiving and processing station where the data are processed into an image (hardcopy and/or digital). 6. Interpretation and Analysis (F) The processed image is interpreted, visually and/or digitally or electronically, to extract information about the target which was illuminated. 7. Application (G) The final element of the remote sensing process is achieved when we apply the information we have been able to extract from the imagery about the target in order to better understand it, reveal some new information, or assist in solving a particular problem. Electromagnetic Radiation The basis for most of remote sensing methods and systems is simply that of measuring the varying response to electromagnetic radiation (photon) of an object. Variation in photon energies are tied to the parameter wavelength or its inverse, frequency. When any target material is excited by internal processes or by interaction with incoming EM radiation, it will emit or reflect photons of varying wavelengths whose radiometric quantities differ at different wavelengths in a way diagnostic of the material Photon energy received at detectors is commonly stated in power units such as Watts per square meter per wavelength unit.  Electromagnetic radiation consists of an electric field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is travelling and a magnetic field (M) oriented at right angles to the electrical field. Both these fields travel at the speed of light (C) Electromagnetic Spectrum The electromagnetic spectrum is the range of all types of electromagnetic radiation. Radiation is energy that travels and spreads out as it goes, and this energy comes in different wavelengths and frequencies. The electromagnetic spectrum ranges from the shorter wavelengths (including gamma and x-rays) to the longer wavelengths (including microwaves and broadcast radio waves). The EM spectrum has been arbitrarily divided into regions or intervals to which descriptive names have been applied. The Electromagnetic spectrum ranges At the very energetic (high frequency; short wavelength) end are gamma rays and x-rays. Radiation in the ultraviolet region extends from about 1 nanometer to about 0.36 micrometers. It is convenient to measure the mid regions of the spectrum in these micrometers and nanometers. The visible region occupies the range between 0.38 to 0.7 micrometer or equivalent to 380 to 700 nanometers. The infrared region, spans between 0.7 to 100 micrometers. At shorter wavelength (near 0.7micrometer) infrared radiation can be detected by special film, while at longer wavelengths it is felt as heat. Useful regions of Electromagnetic Spectrum 1. Visible Light (380-700 nm) The visible light spectrum is the segment of the electromagnetic spectrum that the human eye can view. It helps in analyzing land use, vegetation, and water bodies. 2. Near - Infrared (NIR) (380-1300 nm) Crucial for vegetation analysis. Healthy plants reflect more NIR light. Used to monitor crop health and water content 3. Thermal Infrared (TIR) (3000-15000 nm) Measures heat emitted from surfaces. Useful in monitoring temperature changes, irrigation needs, and surface heat islands. Spectral Indices Spectral indices are mathematical combinations of reflectance measurements at different wavelengths, designed to highlight specific features or conditions on the Earth's surface. Spectral indices help enhance certain characteristics, such as vegetation health, soil moisture, or water bodies, making it easier to analyze satellite or aerial imagery for specific purposes. There are different types of spectral indices like NDVI, NDWI, NDMI, SAVI etc. Atmospheric Window The atmosphere selectively transmits energy of certain wavelengths. Atmospheric windows are specific ranges of wavelengths in the electromagnetic spectrum where the Earth's atmosphere allows radiation to pass through with little absorption or scattering. Atmospheric windows are present in the visible part (0.38 m -.70 m) and the infrared regions of the EM spectrum. In the visible part transmission is mainly affected by ozone absorption and by molecular scattering. The atmosphere is transparent again beyond about λ = 1mm, the region used for microwave remote sensing. Energy Interaction with Earth Surface When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature is possible. Various fraction of energy incident on the elements are reflected, absorbed and or transmitted.  Applying the principle of conservation of energy, we can state the interrelationship between these three energy interaction as 𝐸𝐼 (⅄) = 𝐸𝑅 (⅄) + 𝐸𝐴 (⅄) + 𝐸𝑇 (⅄) Where 𝐸𝐼 denotes incident energy 𝐸𝑅 denotes reflected energy 𝐸𝐴 denotes absorbed energy and 𝐸𝑇 denotes transmitted energy, will all energy being function of wavelength.  Two points concerning this relationship should be noted. First, the proportion of reflected, absorbed and transmitted will vary for different earth features, depending on their material type and condition. Second, the wavelength dependency means that, even within a given feature type, the proportion of reflected, absorbed and transmitted energy will vary at different wavelength. The geometric manner in which an object reflects energy is also an important consideration. This factor is primarily a function of the surface roughness of the object.  We refer to two types of reflection, which represent the two extreme ends of the way in which energy is reflected from a target: 1. Specular reflection 2. Diffuse reflection 1. Specular Reflection When a surface is smooth we get specular or mirror-like reflection where all (or almost all) of the energy is directed away from the surface in a single direction 2. Diffuse Reflection Diffuse reflection occurs when the surface is rough and the energy is reflected almost uniformly in all directions. Spectral Reflectance Curve A spectral reflectance curve shows how much light (or electromagnetic energy) a surface reflects at different wavelengths across the electromagnetic spectrum. When light hits a surface (like soil, vegetation, or water), some of it is absorbed, some passes through, and some is reflected. The reflectance at each wavelength is measured and plotted on a graph to create the curve. Different materials reflect light differently at various wavelengths, so their spectral reflectance curves help identify and differentiate between them. Spectral Reflectance of Different Surfaces 1. Soil Soil generally has a moderate reflectance across visible and near-infrared (NIR) wavelengths. The reflectance increases with increasing wavelength in the visible region (from blue to red). In the NIR region, the reflectance is relatively high, but it varies depending on the soil's moisture content, texture, and organic matter. 2. Vegetation In the visible spectrum, especially in the blue and red regions, vegetation reflects very little light because chlorophyll absorbs it for photosynthesis. In the green region, vegetation reflects more light, which is why healthy plants appear green. In the NIR region, healthy vegetation reflects a large amount of light due to the structure of plant leaves, which is why NIR is crucial for vegetation analysis. In healthy vegetation, there is high reflectance in NIR, low in red (used to calculate indices like NDVI for plant health). In unhealthy vegetation, reflectance in NIR decreases, indicating issues like disease or drought. 3. Water Water absorbs most light in the visible and NIR regions, so it typically has low reflectance, especially in the NIR. The reflectance is highest in the blue region, which is why clear water bodies often appear blue. Assignments 1. Describe the process of remote sensing in your own words. 2. Describe spectral reflectance curves and write how spectral reflectance curves help identify different materials and their condition. 3. Explore various spectral indices used in agricultural field and explain how they might be useful in agriculture. 4. Differentiate between Specular reflection and Diffuse reflection.