746A27 Remote Sensing and GIS

Transcription

1 746A27 Remote Sensing and GIS Lecture 1 Concepts of remote sensing and Basic principle of Photogrammetry Chandan Roy Guest Lecturer Department of Computer and Information Science Linköping University

2 What is Remote Sensing? Remote Sensing is defined as the science (and to some extent art) of acquiring information about the objects of interest without actually being in contact with it. Basic components of Remote Sensing (RS): Energy source or illumination (A) Radiation and the Atmosphere (B) Interaction with the target (C) Recording of the energy by the sensor (D) Transmission, reception and processing (E) Interpretation and analysis (F) Application (G)

3 (A)Energy Source or Illumination Electromagnetic Radiation (EMR) The first requirement is to have a source of energy which will illuminate the earth s surface objects. Usually the energy is in the form of EMR EMR consists of an electrical field and a magnetic field Two characteristics of EMR is very important in the field of RS. Those are Wavelength, and Frequency

4 Electromagnetic spectrum Electromagnetic radiation occurs as a continuum of wavelengths and frequencies from short wavelength, high frequency cosmic waves to long wavelength, low frequency radio waves. And this systematic arrangement of these different electromagnetic waves is called electromagnetic spectrum

5 Visible portion of light A narrow range of EMR extending from 0.4 to 0.7 µm, the interval detected by the human eye, is known as the visible region The visible light is only a short part of the EMR used in RS. Wavelengths of visible portion of light Violet: Blue: Green: Yellow: Orange: Red: μm μm μm μm μm μm

6 Electromagnetic Spectrum used in Remote Sensing Near UV (ultra-violet): Visible light: Infrared (IR): Microwave: μm Blue: μm Green: μm Red: μm Near IR: μm Shortwave IR: μm Thermal IR: 8-14 μm 1 mm - 1 m

7 Spectral signature For any given material, the amount of solar radiation that it reflects, absorbs, transmits, or emits varies with wavelength. When that amount (usually intensity, as a percent of maximum) coming from the material is plotted over a range of wavelengths, the connected points produce a curve called the material's spectral signature (spectral response curve). For example, at some wavelengths, sand reflects more energy than green vegetation but at other wavelengths it absorbs more (reflects less) than does the vegetation. In principle, we can recognize various kinds of surface materials and distinguish them from each other by these differences in reflectance. NASA remote sensing tutorial

8 Reflectance pattern of different ground objects Spectral signature Using reflectance differences, we can distinguish the four common surface materials in the above signatures (GL = grasslands; PW = pinewoods; RS = red sand; SW = silty water) simply by plotting the reflectances of each material at two wavelengths.

9 (B) Radiation and the atmosphere 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. Interaction of light with the atmosphere Light interacts with the atmosphere two times. Once coming to the earth s surface and for second time when going back after reflection. During coming in and going back as light passes through atmosphere, it is affected through three processes 1. Scattering 2. Absorption, and 3. Refraction Scattering Absorption

10 Atmospheric Window The sun light before falling upon the earth s surface and after being reflected from the earth s surface has to travel through the atmosphere. And light while traveling through the atmosphere the suspended particles of varying size present in the atmosphere causes scattering effect. Except this effect when the light moves through the atmosphere certain portion of it is absorbed by ozone, carbon dioxide, and water molecules etc. which are present in the atmosphere. This effect is called absorption. Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows

11 Atmospheric Window (C) Interaction with the target 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.

12 EMR interactions with earth surface features When electromagnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible. Various fractions of the energy incident on the element are reflected, absorbed, and/or transmitted. Applying the principle of conservation of energy, the interrelationship between these three energy interactions can be stated as E1 (λ) = ER (λ) + EA (λ) + ET (λ) where, E1= Incident energy ER= Reflected energy EA= Absorbed energy ET= Transmitted energy E 1 (λ)= Incident energy E 1 (λ) = E R (λ) + E A (λ) + E T (λ) E R (λ)= Reflected energy

13 (D) Recording of Energy by the Sensor After the energy has been scattered by, or emitted from the target, a sensor (remote - not in contact with the target) is required to collect and record the electromagnetic radiation. Remote Sensing methods Based on the source of energy remote sensing methods can be of two types: passive and active Passive Remote Sensing In passive remote sensing energy that is reflected or emitted from the observed scene is recorded. A variety of passive remote sensors are used to record the energy.

14 Radiometer: An instrument that quantitatively measures the intensity of electromagnetic radiation in some band of wavelengths in the spectrum. Imaging Radiometer: A radiometer that includes a scanning capability to provide a two-dimensional array of pixels from which an image may be produced is called an imaging radiometer. Spectrometer: A device designed to detect, measure, and analyze the spectral content of the incident electromagnetic radiation Spectroradiometer: A radiometer that can measure the intensity of radiation in multiple wavelength bands

15 Active Remote Sensing In active remote sensing energy (electromagnetic radiation) is provided to illuminate the object or scene the sensor observe. A pulse of energy is sent from the sensor to the object and then receive the radiation that is reflected or backscattered from that object. Radar (Radio Detection and Ranging): A radar uses a transmitter operating at either radio or microwave requencies to emit electromagnetic radiation and a directional antenna or receiver to measure the time of arrival of reflected or backscattred pulses of radiation from distant objects.

16 RADAR Radio Detection And Ranging

17 Scatterometer: A scatterometer is a high frequency microwave radar designed specifically to measure backscattered radiation. Lidar (Light Detection and Ranging): A lidar uses a laser (light amplification by stimulated emission of radiation) to transmit a light pulse and a receiver with sensitive detectors to measure the backscattered or reflected light. Laser Altimeter: A laser altimeter uses a lidar to measure the height of the instrument platform above the surface. Platforms Based on altitude platforms can be divided into three types 1. Ground based 2. Aircraft borne, and 3. Satellite borne

18 (E) Transmission, Reception, and Processing The energy recorded by the sensor is transmitted to a receiving and processing station where the data are processed into an image (hardcopy and/or digital) (E) Interpretation and Analysis The processed image is interpreted, visually and/or digitally or electronically, to extract information about the target which was illuminated. (E) Application The final element of the remote sensing process is using the image to solve different problems.

19 Basic Principles of Photogrammetry Ptotogrammetry is the science and technology of obtaining spatial measurements and other geometrically reliable derived products from photographs. Lilleasand and Kiefer Geometric characteristics Types of aerial photographs Gemetrically aerial photographs are divided into two types vertical photographs, and oblique photographs

20 Taking vertical aerial photographs Vertical aerial photograph of Linköping University area

21 An example of oblique aerial photograph (Gherkin Tower, London)

22 Scale of aerial photographs Scale is one of the most fundamental and frequently used geometric characteristics of aerial photographs. It is expressed as unit equivalents, representative fraction or ratios (1mm = 25m, or 1/25000, or 1:25000). The scale of the aerial photographs depend on the flight height basically. Ground coverage The ground coverage of a photograph is a function of the camera format size. For example, an image having taken with a camera having a 230 X 230 format (on 240 mm film) has about 17.5 times the ground coverage of an image of equal scale taken with a camera having a 55 X 55 mm format (70 mm film). Ground coverage is also affected by flight height. If the aircraft is flying at a higher altitude then ground coverage will be greater than it flies at a low altitude.

23 Area measurement Measuring the area using aerial photograph can be done in many ways but the accuracy depends on the relief. The area of a rectangular field can be calculated only through multiplying the height and width of the photograph. Application of aerial photographs Preparation of topographic maps Landuse survey Construction and engineering purpose Military purpose

24 Topographic map of Linköping university area A photo of the first big raid by Boeing B- 17 aircraft 'Flying Fortress' of the 8th Air Force dropped bombs on the Focke Wulf plant at Marienburg, Germany 1943