Introduction to Remote Sensing page 1 Remote Sensing of Environment (RSE) with TNTmips ® TNTview ® Introduction to I N T R O T O R S E Introduction to Remote Sensing page 2 Before Getting Started You can print or read this booklet in color from MicroImages’ Web site. The Web site is also your source for the newest tutorial booklets on other topics. You can download an installation guide, sample data, and the latest version of TNTmips. http://www.microimages.com Imagery acquired by airborne or satellite sensors provides an important source of information for mapping and monitoring the natural and manmade features on the land surface. Interpretation and analysis of remotely sensed imagery requires an understanding of the processes that determine the relationships between the prop- erty the sensor actually measures and the surface properties we are interested in identifying and studying. Knowledge of these relationships is a prerequisite for appropriate processing and interpretation. This booklet presents a brief overview of the major fundamental concepts related to remote sensing of environmental features on the land surface. Sample Data The illustrations in this booklet show many examples of remote sensing imagery. You can find many additional examples of imagery in the sample data that is distributed with the TNT products. If you do not have access to a TNT products CD, you can download the data from MicroImages’ Web site. In particu- lar, the CB_DATA, SF_DATA, BEREA, and COMBRAST data collections include sample files with remote sensing imagery that you can view and study. More Documentation This booklet is intended only as an introduction to basic concepts governing the acquisition, processing, and interpretation of remote sensing imagery. You can view all types of imagery in TNTmips using the standard Dis- play process, which is introduced in the tutorial booklet entitled Displaying Geospatial Data. Many other processes in TNTmips can be used to process, enhance, or analyze imagery. Some of the most important ones are mentioned on the appropriate pages in this booklet, along with a reference to an accompanying tutorial booklet. TNTmips ® Pro and TNTmips Free TNTmips (the Map and Image Processing System) comes in three versions: the professional version of TNTmips (TNTmips Pro), the low-cost TNTmips Basic version, and the TNTmips Free version. All versions run exactly the same code from the TNT products DVD and have nearly the same features. If you did not purchase the professional version (which re- quires a software license key) or TNTmips Basic, then TNTmips operates in TNTmips Free mode. Randall B. Smith, Ph.D., 4 January 2012 ©MicroImages, Inc., 2001–2012 Introduction to Remote Sensing page 3 Introduction to Remote Sensing Remote sensing is the sci- ence of obtaining and interpreting information from a distance, using sen- sors that are not in physical contact with the object be- ing observed. Though you may not realize it, you are familiar with many examples. Biological evolution has exploited many natural phenomena and forms of energy to enable animals (including people) to sense their environment. Your eyes detect electro- magnetic energy in the form of visible light. Your ears detect acoustic (sound) energy, while your nose contains sensitive chemical receptors that respond to minute amounts of airborne chemicals given off by the materials in our surroundings. Some research suggests that migrating birds can sense variations in Earth’s magnetic field, which helps explain their re- markable navigational ability. The science of remote sensing in its broadest sense includes aerial, satellite, and spacecraft observations of the surfaces and atmospheres of the planets in our solar system, though the Earth is obviously the most frequent target of study. The term is customar- ily restricted to methods that detect and measure electromagnetic energy, including visible light, that has interacted with surface materials and the atmo- sphere. Remote sensing of the Earth has many purposes, including making and updating planimet- ric maps, weather forecasting, and gathering military intelligence. Our focus in this booklet will be on remote sensing of the environment and resources of Earth’s surface. We will explore the physical con- cepts that underlie the acquisition and interpretation of remotely sensed images, the important character- istics of images from different types of sensors, and some common methods of processing images to en- hance their information content. Fundamental concepts of electromagnetic radiation and its interactions with surface materials and the atmosphere are introduced on pages 4-9. Image acquisition and various concepts of image resolution are discussed on pages 10-16. Pages 17-23 focus on images acquired in the spectral range from visible to middle infrared radiation, including visual image interpretation and common processes used to correct or enhance the information content of multispectral images. Pages 23-24 discuss images acquired on multiple dates and their spatial registration and normalization. You can learn some basic concepts of thermal infrared imagery on pages 26-27, and radar imagery on pages 28-29. Page 30 presents an example of combine images from different sensors. Sources of additional information on remote sensing are listed on page 31. Artist’s depiction of the Landsat 7 satellite in orbit, courtesy of NASA. Launched in late 1999, this satellite acquires multispectral images using reflected visible and infrared ra- diation. Introduction to Remote Sensing page 4 The Electromagnetic Spectrum The field of remote sensing began with aerial pho- tography, using visible light from the sun as the energy source. But visible light makes up only a small part of the electromagnetic spectrum, a con- tinuum that ranges from high energy, short wavelength gamma rays, to lower energy, long wave- length radio waves. Illustrated below is the portion of the electromagnetic spectrum that is useful in re- mote sensing of the Earth’s surface. The Earth is naturally illuminated by electromagnetic radiation from the Sun. The peak solar energy is in the wavelength range of visible light (between 0.4 and 0.7 µm). It’s no wonder that the visual systems of most animals are sensitive to these wavelengths! Although visible light includes the entire range of colors seen in a rainbow, a cruder subdivision into blue, green, and red wavelength regions is sufficient in many remote sensing studies. Other substantial fractions of incoming solar energy are in the form of invisible ultraviolet and infrared radiation. Only tiny amounts of solar radiation extend into the microwave region of the spectrum. Imaging radar systems used in remote sensing generate and broadcast micro- waves, then measure the portion of the signal that has returned to the sensor from the Earth’s surface. Electromagnetic radiation behaves in part as wavelike energy fluctuations traveling at the speed of light. The wave is actually composite, involving electric and mag- netic fields fluctuating at right angles to each other and to the direction of travel. A fundamental descriptive feature of a waveform is its wavelength, or distance be- tween succeeding peaks or troughs. In remote sensing, wavelength is most often measured in micrometers, each of which equals one millionth of a meter. The variation in wavelength of electromagnetic radiation is so vast that it is usually shown on a logarithmic scale. UNITS 1 micrometer (µm) = 1 x 10 -6 meters 1 millimeter (mm) = 1 x 10 -3 meters 1 centimeter (cm) = 1 x 10 -2 meters Wavelength Wavelength (logarithmic scale) Incoming from Sun Emitted by Earth 0.4 0.5 0.6 0.7 Blue Green Red MICROWAVE (RADAR) INFRARED 1 m10 cm1 cm 100 µm10 µm1 µm 1 mm 0.1 µm VISIBLE ULTRAVIOLET Energy Introduction to Remote Sensing page 5 Interaction Processes Remote sensors measure electromagnetic (EM) ra- diation that has interacted with the Earth’s surface. Interactions with matter can change the direction, intensity, wavelength content, and polarization of EM radiation. The nature of these changes is dependent on the chemical make-up and physical structure of the material exposed to the EM radiation. Changes in EM radiation resulting from its interactions with the Earth’s surface therefore provide major clues to the characteristics of the surface materials. The fundamental interactions between EM radiation and matter are diagrammed to the right. Electro- magnetic radiation that is transmitted passes through a material (or through the boundary between two materials) with little change in intensity. Materials can also absorb EM radiation. Usually absorption is wavelength-specific: that is, more energy is ab- sorbed at some wavelengths than at others. EM radiation that is absorbed is transformed into heat energy, which raises the material’s temperature. Some of that heat energy may then be emitted as EM radiation at a wavelength dependent on the material’s temperature. The lower the temperature, the longer the wavelength of the emitted radiation. As a result of solar heating, the Earth’s surface emits energy in the form of longer-wavelength infrared radiation (see illustration on the preceding page). For this reason the portion of the infrared spectrum with wavelengths greater than 3 µm is commonly called the thermal infrared region. Electromagnetic radiation encountering a boundary such as the Earth’s surface can also be reflected. If the surface is smooth at a scale comparable to the wavelength of the incident energy, specular reflec- tion occurs: most of the energy is reflected in a single direction, at an angle equal to the angle of incidence. Rougher surfaces cause scattering, or diffuse reflec- tion in all directions. Matter - EM Energy Interaction Processes The horizontal line represents a boundary between two materials. Specular Reflection Scattering (Diffuse Reflection) Absorption Emission Transmission Introduction to Remote Sensing page 6 Interaction Processes in Remote Sensing Typical EMR interactions in the atmosphere and at the Earth’s surface. To understand how different interaction processes impact the acquisition of aerial and satellite images, let’s analyze the reflected solar radiation that is measured at a satellite sensor. As sunlight initially enters the atmosphere, it encounters gas molecules, suspended dust particles, and aerosols. These materials tend to scatter a portion of the incoming radiation in all directions, with shorter wavelengths experiencing the strongest effect. (The preferential scattering of blue light in comparison to green and red light accounts for the blue color of the daytime sky. Clouds appear opaque because of intense scattering of visible light by tiny water droplets.) Although most of the remaining light is transmitted to the surface, some atmospheric gases are very effective at absorbing particular wavelengths. (The absorption of dangerous ultraviolet radiation by ozone is a well-known ex- ample). As a result of these effects, the illumination reaching the surface is a combination of highly filtered solar radiation transmitted directly to the ground and more diffuse light scattered from all parts of the sky, which helps illuminate shadowed areas. As this modified solar radiation reaches the ground, it may encounter soil, rock surfaces, vegetation, or other materials that absorb a portion of the radiation. The amount of energy absorbed varies in wavelength for each material in a character- istic way, creating a sort of spectral signature. (The selective absorption of different wavelengths of visible light determines what we perceive as a material’s color). Most of the radiation not absorbed is diffusely reflected (scattered) back up into the atmosphere, some of it in the direction of the satellite. This upwelling radia- tion undergoes a further round of scattering and absorption as it passes through the atmosphere before finally being detected and measured by the sensor. If the sensor is capable of detecting thermal infrared radiation, it will also pick up radia- tion emitted by surface objects as a result of solar heating. EMR Source Sensor Absorption Absorption Absorption Scattering Scattering Scattering Scattering Emission Transmission Introduction to Remote Sensing page 7 Atmospheric Effects Scattering and absorption of EM radiation by the at- mosphere have significant effects that impact sensor design as well as the processing and interpretation of images. When the concentration of scattering agents is high, scattering produces the visual effect we call haze. Haze increases the overall brightness of a scene and reduces the contrast between different ground materials. A hazy atmosphere scatters some light upward, so a portion of the radiation recorded by a remote sensor, called path radiance, is the re- sult of this scattering process. Since the amount of scattering varies with wavelength, so does the con- tribution of path radiance to remotely sensed images. As shown by the figure to the right, the path radi- ance effect is greatest for the shortest wavelengths, falling off rapidly with increasing wavelength. When images are captured over several wavelength ranges, the differential path radiance effect complicates com- parison of brightness values at the different wavelengths. Simple methods for correcting for path radiance are discussed later in this booklet. The atmospheric components that are effective ab- sorbers of solar radiation are water vapor, carbon dioxide, and ozone. Each of these gases tends to absorb energy in specific wavelength ranges. Some wavelengths are almost completely absorbed. Con- sequently, most broad-band remote sensors have been designed to detect radiation in the “atmospheric win- dows”, those wavelength ranges for which absorption is minimal, and, conversely, transmission is high. Relative Scattering 0.4 0.6 0.8 1.0 Wavelength, µµ µµ µm Range of scattering for typical atmospheric conditions (colored area) versus wavelength. Scattering increases with increasing humidity and particulate load but decreases with increasing wavelength. In most cases the path radiance produced by scattering is negligible at wavelengths longer than the near infrared. 100 1 m 0.3 µm1 µm 10 µm 100 µm 1 mm Transmission (%) 0 Visible Ultraviolet Thermal Infrared Near IR Middle IR Microwave Variation in atmospheric transmission with wavelength of EM radiation, due to wavelength-selective absorption by atmospheric gases. Only wavelength ranges with moderate to high transmission values are suitable for use in remote sensing. Introduction to Remote Sensing page 8 All remote sensing systems designed to monitor the Earth’s surface rely on energy that is either diffusely reflected by or emitted from surface features. Current re- mote sensing systems fall into three categories on the basis of the source of the electromagnetic radiation and the relevant interactions of that energy with the surface. Reflected solar radiation sensors These sensor systems detect solar radiation that has been diffusely reflected (scattered) upward from surface features. The wavelength ranges that provide useful information include the ultraviolet, visible, near infrared and middle infrared ranges. Reflected solar sensing systems dis- criminate materials that have differing patterns of wavelength-specific absorption, which relate to the chemical make-up and physical struc- ture of the material. Because they depend on sunlight as a source, these systems can only provide useful images during daylight hours, and changing atmospheric condi- tions and changes in illumination with time of day and season can pose interpretive problems. Reflected solar remote sensing systems are the most common type used to monitor Earth resources, and are the primary focus of this booklet. Thermal infrared sensors Sensors that can detect the thermal infrared radiation emitted by surface features can reveal information about the thermal properties of these materials. Like reflected solar sensors, these are passive systems that rely on solar radiation as the ulti- mate energy source. Because the temperature of surface features changes during the day, thermal infrared sens- ing systems are sensitive to time of day at which the images are acquired. Imaging radar sensors Rather than relying on a natural source, these “active” systems “illuminate” the surface with broadcast micro- wave radiation, then measure the energy that is diffusely reflected back to the sensor. The returning energy pro- vides information about the surface roughness and water content of surface materials and the shape of the land surface. Long-wavelength microwaves suffer little scat- tering in the atmosphere, even penetrating thick cloud cover. Imaging radar is therefore particularly useful in cloud-prone tropical regions. EMR Sources, Interactions, and Sensors Reflected red image Thermal Infrared image Radar image Introduction to Remote Sensing page 9 Spectral Signatures The spectral signatures produced by wavelength-dependent absorption provide the key to discriminating different materials in images of reflected solar energy. The property used to quantify these spectral signatures is called spectral reflec- tance: the ratio of reflected energy to incident energy as a function of wavelength. The spectral reflectance of different materials can be measured in the laboratory or in the field, providing reference data that can be used to interpret images. As an example, the illustration below shows contrasting spectral reflectance curves for three very common natural materials: dry soil, green vegetation, and water. The reflectance of dry soil rises uniformly through the visible and near infrared wavelength ranges, peaking in the middle infrared range. It shows only minor dips in the middle infrared range due to absorption by clay minerals. Green veg- etation has a very different spectrum. Reflectance is relatively low in the visible range, but is higher for green light than for red or blue, producing the green color we see. The reflectance pattern of green vegetation in the visible wavelengths is due to selective absorption by chlorophyll, the primary photosynthetic pigment in green plants. The most noticeable feature of the vegetation spectrum is the dra- matic rise in reflectance across the visible-near infrared boundary, and the high near infrared reflectance. Infrared radiation penetrates plant leaves, and is in- tensely scattered by the leaves’ complex internal structure, resulting in high reflectance. The dips in the middle infrared portion of the plant spectrum are due to absorption by water. Deep clear water bodies effectively absorb all wavelengths longer than the visible range, which results in very low reflectivity for infrared radiation. Reflectance 0 0.2 0.4 0.6 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 Wavelength ( µµ µµ µm) Clear Water Body Green Vegetation Dry Bare Soil Near Infrared Middle Infrared Red Grn Blue Reflected Infrared Introduction to Remote Sensing page 10 Image Acquisition 52 71 74 102 113 144 1196570 6489 125 90 6687 87 80 89111 77 95 111115 67 74 We have seen that the radiant energy that is measured by an aerial or satellite sensor is influenced by the radiation source, interaction of the energy with surface materials, and the passage of the energy through the atmosphere. In addition, the illumination geometry (source position, surface slope, slope direction, and shad- owing) can also affect the brightness of the upwelling energy. Together these effects produce a composite “signal” that varies spatially and with the time of day or season. In order to produce an image which we can interpret, the remote sens- ing system must first detect and measure this energy. The electromagnetic energy returned from the Earth’s surface can be detected by a light-sensitive film, as in aerial photography, or by an array of electronic sen- sors. Light striking photographic film causes a chemical reaction, with the rate of the reaction varying with the amount of energy received by each point on the film. Developing the film converts the pattern of energy varia- tions into a pattern of lighter and darker areas that can be interpreted visually. Electronic sensors generate an electrical signal with a strength proportional to the amount of energy received. The signal from each detector in an array can be recorded and transmitted elec- tronically in digital form (as a series of numbers). Today’s digital still and video cam- eras are examples of imaging systems that use electronic sensors. All modern satellite imag- ing systems also use some form of electronic detectors. An image from an electronic sensor array (or a digitally scanned photograph) consists of a two-dimensional rectangular grid of numeri- cal values that represent differing brightness levels. Each value represents the average brightness for a portion of the surface, represented by the square unit areas in the image. In computer terms the grid is commonly known as a raster, and the square units are cells or pixels. When displayed on your com- puter, the brightness values in the image raster are translated into display brightness on the screen. [...]... of remote sensing, including thermal images and radar, with many sample images Remote Sensing Tutorials created by the Canada Centre for Remote Sensing http://www.nrcan.gc.ca/earth-sciences/geography-boundary/remotesensing/1599#tutor On-line tuturials in remote sensing fundamentals, radar and stereoscopy, and digital image analysis page 31 Advanced Software for Geospatial Analysis Introduction to Remote. .. tutorial booklet for more information) Colors in the combined image differentiate fields by degree of plant cover (red hue) and plant structure (intensity) page 30 Introduction to Remote Sensing Other Sources of Information This booklet has provided a brief overview of the rich and complex field of remote sensing of environmental resources If you are interested in exploring further, you may wish to. .. of the Thematic Mapper scene enlarged to the same scale as the IKONOS image, revealing the larger cells in the Landsat image Introduction to Remote Sensing Spectral Resolution The spectral resolution of a remote sensing system can be described as its ability to distinguish different parts of the range of measured wavelengths In essence, this amounts to the number of wavelength intervals (“bands”) that... imagery from any of these sensors into the TNTmips Project File format using the Import / Export process Each image band is stored as a raster object page 15 * Single satellite, nadir † view at equator With off-nadir pointing Introduction to Remote Sensing Radiometric Resolution In order to digitally record the energy received by an individual detector in a sensor, the continuous range of incoming energy... shape of the land surface and the direction of illumination One of the most important characteristics of an image band is its distribution of brightness levels, which is most commonly represented as a histogram (You can view an image histogram using the Histogram tool in the TNTmips Spatial Data Display process.) A sample image and its histogram are shown below The horizontal axis of the histogram... You can perform both types of classification in TNTmips using the Automatic Classification process, which is described in the tutorial booklet entitled Image Classification page 23 Introduction to Remote Sensing Temporal Resolution The surface environment of the Earth is dynamic, with change occurring on time scales ranging from seconds to decades or longer The seasonal cycle of plant growth that affects... interpretability of grayscale (and color) images by using the Contrast Enhancement procedure in the TNTmips Spatial Data Display process to spread the brightness values over more of the display brightness range (See the tutorial booklet entitled Getting Good Color for more information.) page 18 Introduction to Remote Sensing Color Combinations of Visible-MIR Bands Four image areas are shown below to illustrate.. .Introduction to Remote Sensing Spatial Resolution The spatial, spectral, and temporal components of an image or set of images all provide information that we can use to form interpretations about surface materals and conditions For each of these properties we can define the resolution of the images produced by the sensor system These image resolution factors place limits on what... imagery of the same area through the growing season adds to our ability to recognize and distinguish plant or crop types A time-series of images can also be used to monitor changes in surface features due to other natural processes or human activity The time-interval separating successive images in such a series can be considered to define the temporal resolution of the image sequence This sequence of Landsat... booklet page 25 Introduction to Remote Sensing Thermal Infrared Images Thermal infrared images add another dimension to passive remote sensing techniques They provide information about surface temperatures and the thermal properties of surface materials Many applications of thermal infrared images are possible, including mapping rock types, soils, and soil moisture variations, and monitoring vegetation . Introduction to Remote Sensing page 1 Remote Sensing of Environment (RSE) with TNTmips ® TNTview ® Introduction to I N T R O T O R S E Introduction to Remote Sensing page 2 Before. 4 January 2012 ©MicroImages, Inc., 2001–2012 Introduction to Remote Sensing page 3 Introduction to Remote Sensing Remote sensing is the sci- ence of obtaining and interpreting information from. Landsat im- age. Introduction to Remote Sensing page 12 The spectral resolution of a remote sensing system can be described as its ability to distinguish different parts of the range of measured wavelengths.