Remote sensing is the collection of data without directly measuring the object it relies on the

Transcription

1 Chapter 8 Remote Sensing Chapter Overview Remote sensing is the collection of data without directly measuring the object it relies on the reflectance of natural or emitted electromagnetic radiation (EMR). EMR can be emitted by the sun and sensed by photographic film, or it can be sent by a transmitter, e.g., radar and the returned energy sensed. Remote sensing has become a key means of data collection for a number of reasons. Mainly though, it allows for systematic and accurate collection of geographic information. Remote sensing is defined very broadly in this chapter: A measurement of an object s characteristics from a distance using reflected or emitted electro-magnetic energy. This definition means remote sensing includes all kinds of photography, aerial imagery, satellite sensors, and any kind of laser. Remote sensing involves different types of sensor technologies ranging from photographic emulsions to digital chips. It also involves a vast array of storage media including everything from photographic film to computer files. As you can imagine, the broad definition means remote sensing overlaps with a number of other fields. This is indeed true and important. For example, the discipline of surveying has changed enormously with the introduction of laserbased distance finding technology. 1

2 The reason for defining remote sensing so broadly is that it is a very important geographic information technology. Remote sensing, in general, offers three advantages over other forms of data collection and geographic information. First, it makes it much easier to systematically recognize things and events over a large area. Second, it makes it easier and less costly to revise most maps. Third, digital remote sensing images can be used directly by other applications. There are some caveats to these advantages that you will find out about in this chapter. This chapter is purely introductory in nature and will skim over many of the crucial details and physics, but you should end up with a solid understanding of what remote sensing involves and what some of key issues and applications for remote sensing are. 2

3 Figure 1 Different sensor types. Passive sensors use only reflected EMR. Active sensor use emitted EMR Principles Electromagnetic radiation Following the definition for this chapter, any understanding of remote sensing, regardless of the sensor technology, storage media, or application starts with understanding electromagnetic radiation. First off, remote sensing s detection of EMR has three characteristics. 3

4 1. It only detects EMR from the surface of an object, although some sensors allow for penetration 2. There is no contact between the sensor and the object. 3. All remote sensing measurements use reflected energy (usually from the sun) or emitted energy (from a radar station or plants) for example Figure 2 Emitted and reflected electormagnetic energy The EMR detected by remote sensing technologies varies. It depends on the desired application 4

5 as well as on the cost of different remote sensing data collections. Figure 3 The electromagnet spectrum showing common examples (Based on: Spectral Signature The EMR emitted or reflected by a thing or event varies. These differences are the basis for distinguishing things and events. The reflections and emissions of a particular thing or event can be associated with a particular spectral signature that is used to identify where these things and events are located in a remote sensing image. 5

6 Figure 4 Examples of spectral signatures. Note that a micrometer is 10-6 meter (based on image from It also varies by time of day, season, weather conditions, moisture levels in the soil, wind, and a number of other factors. The physics involved in addressing these differences are critical to the success of remote sensing. They are also very complex, but you need to be aware of the differences and a common-place solution. This solution is called ground-truthing and involves having some people in the field before, during, or after data collection who may take similar sensor measurements or observations. These measurements and observations can be used later to verify the remote sensing image or data and possibly define correction parameters for adjusting the remotely sensed data to correspond to ground observations. Needless to say, this is highly complex and requires very well trained specialists to assess these factors and detect patterns in the remote sensing data. 6

7 Bands The detection of patterns is helped by the use of different ranges or bands of EMR in sensing technology. Each band, as they are called commonly, refers to a particular range of wavelength for that sensor. The bands available for a particular sensor depend greatly on the purpose of the sensor and technical characteristics of the sensor. Some sensors have only a few bands in a narrow range of the total EMR, others are much broader. For example, Landsat 7 has seven bands: Band µm Blue-Green Band µm Green Band µm Red Band µm Near IR Band µm Mid-IR Band µm Thermal IR Band µm Mid-IR The following figure (xx) shows the different bands and how they can be combined for an application. 7

8 8 Figure 5 Illustration of different bandwidths used by Landsat 7 (Source:

9 Another widely used satellite, SPOT 5, offers a different set of bandwidths. [insert table ch8-table1-spot bands.doc] [insert table ch8-table2-landsat bands.doc] Resolution Resolution of remote sensing distinguishes between spatial and temporal. Spatial is the size of the unit recognized by the sensor, temporal is the frequency that a satellite visits the same place. Spatial resolution is usually given in a distance measurement. For example, most SPOT sensors have a resolution of 10 meters; some have a higher resolution of 2.5 meters. The resolution does not mean that an object of that size can be consistently detected and identified. Various atmospheric and situational characteristics play into this, and you might rather want to think of this as simply the measure of side of one of raster cells detected by the remote sensing technology. A raster cell is often also referred to as a pixel. 9

10 Figure 6 Comparison of spatial resolutions (Source: Temporal resolution depends greatly on the spatial resolution of the sensing technology. High spatial resolutions will record a great amount of data for a small area, requiring much longer to return to a place than low spatial resolution sensors. For example, Landsat with 30 m spatial resolution revisits a place only once every 16 days. The Advanced Very High-Resolution Radiometer (AVHRR) has a spatial resolution of 1.1 km and revisits a place once every day. 10

11 Types of Sensors The discussion of principles focused on satellite based remote sensing technology. This is only part of available remote sensing technologies. The same technologies used for satellites, or adaptions thereof are often used for remote sensing technologies used by airplanes, helicopters, and in some case hand-held formats. Photography Photography is the most common remote sensing technology because it is very commonplace. In fact, some of the first military remote sensing satellites used cameras with film in the 1960s. The film was dropped out of the satellite in a special heat-resistance re-entry container with a parachute and picked up out of the air by an airplane. Satellites still use cameras, but most of the images are now captured and stored digitally. Satellite sensor technologies detecting EMR in this range are often called panchromatic. The resolutions of photographic images are very high, and the main issue related to determining spatial and temporal resolution is the cost. Of course, many governments and companies use aerial photography as a means of data collection. Collected using ground reference points and calculations to remove subtle changes in the airplanes movements, two aerial photographs made simultaneously can be used to make a stereoscopic image. They are a very useful type of remote sensing because when viewed with some additional equipment, it is possible for most people to distinguish heights and elevation changes. A single photographic image which also has the effects of elevation change removed (called planimetric) is called an orthophoto and is georeferenced to a coordinate system. 11

12 Infrared Usually when we refer to photographic remote sensing we mean recording EMR in the visible wavelength spectrum, but this can be broadened to include infrared. This can be done with the chemical applied to photographic film (called an emulsion) or by using digital devices built and calibrated to detect this EMR spectrum. Multispectrum The data collected and images made with Landsat, SPOT, and similar sensing technologies are known as multispectrum because of the different bands. The variability of multispectrum remote sensors opens up a vast number of application possibilities. Hyperspectral This type of sensor technology collects more than 16 bands simultaneously. For example the Hyperion satellite collects 220 bands from blue to short wave infrared in equal steps (from 0.4 to 2.5 µm) with a 30 meter spatial resolution. Flying in formation with Landsat 7, images from Hyperion can be used easily with Landsat 7 images and data. Radar Radar is an important remote sensing sensor type. It s ability to penetrate through cloud cover and into the ground make it very useful for applications in areas with frequent cloud-cover and 12

13 for geological work. Laser (LiDAR) Not used on satellites, but on planes, helicopters, and from the ground, LIght Detection And Ranging (LiDAR) uses laser generate light pulses in the same way radar uses radio waves. LiDAR is a highly accurate and cost-effective means of collecting elevation data. Because of its speed, hand-held units are now being introduced to quickly scan an area, e.g., a crime or accident scene. Applications Images acquired by satellites have been used to produce local, regional, national, and global composite multispectral mosaics. They have been used in countless applications including monitoring timber losses in the U.S. Pacific Northwest, establishing urban growth, and measuring forest cover. Remote sensing images have also been used in military operations, locate mineral deposits, monitor strip mining, and assess natural changes due to fires and insect infestations. Data collection in general Thinking about remote sensing in a most general sense, we can easily distinguish types of data collection by the platform and by sensor technology. If the remote sensing is based on satellite images or data, in most cases we are likely to have multispectral, hyperspectral, or radar images or data. If it is airplane based, then we are more likely to have aerial photography, multispectral, 13

14 or LiDAR images or data. If it is ground based, then we are most likely to find photography, multispectral, or LiDAR images and data. These rules of thumb have exceptions of course, and will change as certain types of sensor technology and remote sensing systems become cheaper. They are simply helpful in seeing the relationship between costs, types of data, and application types. Applications in smaller areas tend to use airplane based or ground based sensor technologies; larger areas tend towards satellite based remote sensing. Coastal monitoring An important application area is coastal monitoring. Because of the key role of dynamic processes in coastal erosion coastal monitoring applications tend to use remote sensing sources that can repeat their observations often. Aerial and LiDAR photography and data may be suitable for smaller areas if the area is generally cloud-free; multispectral satellite images and data may be useful for larger areas, and radar may be used for large areas, or areas with frequent cloud cover. 14

15 Figure 7 Multispectral sensors produce data and imagery to help monitor and model complex coastal changes (Source: Global change With an increase in average temperatures world-wide, the study of changes to glaciers and artic and Antarctic ice fields has benefited greatly from the use of remote sensing images and data. The frequency of observations helps scientists keep track of changes to ice fields and even ice bergs in the water. Detailed observations, combined with measurements on the ground, help researchers monitor minute changes in ice fields. Made available online to other researchers, these measurements, images, and data have become a crucial part of a key area of global change research. 15

16 Figure 8 Atmospheric monitoring is a crucial part of global change research, which sensors on the TIMED spacecraft are especially designed to observe (Source: RST 16

17 Figure 9 A composite of different multispectral data to produce a 'picture-like' image of the world (Source: Urban Dynamics Because of the frequency of observation, satellite based remote sensing images and data have 17

18 proven to be very useful in documenting and assessing the growth of large cities around the world and distinguishing changes and processes. Urban dynamics are complex, but individual changes in a single area can be compared to assess the impacts of various policies and urban planning programs. This data and models developed to understand past growth can also be used to make predictions of future growth and assess alternative policy and planning proposals Figure 10 Aerial imagery (here from a digitized aerial photograph) can show a great amount of detail (Source: USGS) Precision Farming Detailed remote sensing images and data, from a variety of platforms, are used by farmers to reduce and become more efficient in the application of fertilizers and pesticides. Agricultural factors including plant health, plant cover and soil moisture can be monitored with remote sensing data. By combining the remote sensing images and data from different sources, 18

19 deficiencies of one remote sensing system can be made up. For instance, Landsat provides multispectral data on average only once every 16 days for any place in the continental US and is impaired by cloud coverage, even partially cloudy weather. By using radar data, scientists have been able to help farmers keep track of changing soil and plant conditions more frequently that is especially critical during particular phases of plant growth, e.g., pollination. 19

20 Figure 11 Center pivot irrigation systems create red circles of healthy vegetation in this image of croplands near Garden City, Kansas (Source: 20

21 Chapter 8 Web Resources One of the most consumer-friendly remote sensing-based web applications (registration required for full access) Information about LandSat 7 NASA provides many fascinating images at this web site Documentation of wetland destruction using animations This website offers in depth discussion of everything related to remote sensing with an emphasis on Landsat, but covering other sensor technologies in great detail For information about SPOT satellites 21

22 Another source for information about Landsat satellites An excellent interactive tutorial and various aspects of remote sensing A tutorial introduction to LiDAR 22

23 Review Questions 1. What does the term LiDAR stand for? 2. What does the term panchromatic stand for? 3. What often prevents the wider use of remote sensing? 4. What is the oldest commercial satellite system that is still in use? 5. What are some general characteristics of using remote sensing data? 6. How are surveying and remote sensing growing together? 7. What were some of the first applications for radar-based remote sensing? 8. What is the highest panchromatic remote sensing now available? 9. When was remote sensing first used? 10. How is remote sensing data usually stored? Chapter 8 Chapter Readings Gibson, Paul J. Introductory remote sensing: principles and concepts. London, New York, Routledge,

24 Sabins, Floyd F. Remote sensing: principles and interpretation. 3rd ed. New York, W. H. Freeman and Co., c1997. Lillesand, Thomas M., Ralph W. Kiefer, and Jonathan W. Chipman. Remote sensing and image interpretation. 5th ed. New York, Wiley, c2004. Conway, Eric D. An introduction to satellite image interpretation. Baltimore, Johns Hopkins University Press, c1997. Chapter Glossary Note: These entries are largely based on the glossary from NASA s Remote Sensing Tutorial (RST) Landsat -a series of US satellites that acquire multispectral images. 24

25 Oblique photograph - photograph acquired with a camera directed at an angle between horizontal and vertical orientations. Orthophotograph - a vertical aerial photograph from which the distortions due to varying elevation, tilt, and surface topography have been removed. Radar - acronym for radio detection and ranging. SPOT - Système Probatoire d'observation del la Terre, French remote sensing satellite system Stereoscope - binocular optical device for viewing overlapping images or diagrams. The left eye sees only the left image, and the right eye sees only the right image. When configured correctly, the viewer sees the images in three dimensions. Supervised classification - information analysis technique in which the operator provides information that the computer uses to assign pixels to categories. Unsupervised classification - information analysis technique in which the computer assigns pixels to categories with no instructions from the operator. 25