North Carolina Property Mappers Association

Transcription

1 North Carolina Property Mappers Association SECTION 1 AERIAL PHOTOGRAMMETRY

2 1.1 Introduction Photogrammetry is defined by the American Society of Photogrammetry as the art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring, and interpreting photographic images. As implied by its name, the science originally consisted of analyzing photographs. It has expanded to include the analysis of other phenomena, but photographs are still the principal source of information used by photogrammetrists. Crucial to the reliability of cadastral maps is an accurate base map. The principles of photogrammetry are important to the cadastral mapper because they quantify terrain features mentioned in deeds and plats. Because of the sophisticated nature of acquiring and producing an orthophoto base map, county s must contract with a company that specializes in photogrammetric techniques. This course will help the reader prepare for the project by illustrating the technology and its terms. The photogrammetric procedures outlined in this course cover base maps obtained by using sophisticated instrumentation and complex computational techniques. We will cover only the basic aspects of the broad subject of photogrammetry. Our first objective is to provide you with the fundamental understanding of how base maps are produced. 1.2 Aerial Film The film used in aerial photography is either panchromatic (black and white) or color. The vast majority of mapping projects use black and white film because color film is more expensive. However, more and more counties are electing to have color imagery because of its diverse use with many departments. As the demand grows and the technology improves you will begin to see a stronger shift to color. Aerial film is usually 10 inches wide and comes in a roll length ranging from 125 feet to 500 feet. Black and white rolls are usually 250 feet in length. Color film is usually 200 feet long. Aerial film is composed of three layers as shown in Figure 1-1. Black & White Film Emulsion Layer Film Base Antihalation Backing 1-2

3 1.2.1 Emulsion Layer The emulsion layer in black and white film is a single layer. It lies on top of the base layer and reacts to light to cause a chemical reaction that creates a gray scale image. This sensitive layer consists of silver salt (bromide, chloride, and halide) crystals suspended in a pure gelatin coating. A characteristic of these crystals is that when exposed to light the bond between the silver and halide is weakened. An emulsion that has been exposed to light contains an invisible image of the object called the latent image. When the latent image is developed, areas of the emulsion that were exposed to intense light turn to free silver and become black. The degree of darkness of developed images is a function of the total exposure, which originally sensitized the emulsion to form the latent image. In any photographic exposure there will be variations in illuminance received from different objects in the photographed scene, and therefore between black and white there will exist various tones of gray which result from these variations in illuminance. Actually the crystals turn black, not gray, when exposed to sufficient light. However, if the light received in a particular area is sufficient to sensitize only a portion of the crystals, a gray tone results from a mixture of the resulting black and white. The greater the exposure, the greater the percentage of black in the mixture and hence the darker the shade of gray. The color emulsion layer acts upon the same principle as black and white when it come to causing a chemical reaction of the silver salt crystals. A blue layer is followed by a filter because of the dominance of blue light waves in the atmosphere. The filter layer is followed by a green and red layer. This various color combination creates the imagery seen in color film. Color Film Filter Emulsion Emulsio on n Layer Layer Layer Antihalation Backing Film Base 1-3

4 1.2.2 Film Base The film base consists of a low shrinkage plastic material, which stabilizes the film and acts as a structure to support the emulsion Antihalation Layer This layer is a coating of light-absorbing material that traps the light that passes through the emulsion layer. It prevents light from rebounding back through the emulsion layer and thus creating a blurred image. 1.3 Aerial Cameras Aerial film is loaded into an aerial camera (see Figure 1-2 ). An aerial camera is a very specialized, costly, highly precision instrument. Aerial cameras are mounted in aircraft in a specially installed mount located over a hole in the belly of the aircraft. The mount has dampener devices, which prevent aircraft vibrations from being transmitted to the camera. The mount is also designed so that the camera can be rotated in azimuth to correct for crab. Figure 1-2 Aerial camera. (Courtesy of L. Robert Kimball and Associates, Inc., Ebensburg, PA) 1-4

5 Figure 1-3 represents the components of a typical aerial camera. Figure 1-3 An unexposed film canister is placed in the camera and film passes over the focal plane to the exposed film reel. A vacuum is applied to the film at the instant of exposure so that film is held flat against the focal plane. Otherwise air bubbles would collect beneath the film causing distortions in the photographic image. The lens assembly consists of several elements of polished glass. These lens elements are the most important (and most expensive) parts of the camera. The lens assembly gathers light rays from the object space and brings them to focus in the focal plane behind the lens. A filter may be placed on the camera lens to control lighting. A filter serves three purposes: (1) it reduces the effect of atmospheric haze, (2) it helps provide uniform light distribution over the entire film surface, and (3) it protects the expensive outer lens elements from damage and dust. The shutter and diaphragm together regulated the length of time and intensity at which light is allowed to pass through the lens to make the exposure. The shutter controls the length of time and the diaphragm regulates the intensity by varying the size of the aperture to control the amount of light passing through the lens. Aperture settings are referred to as F- stops. F-stops of aerial cameras typically range from about f-4 down to f-22. Thus, for a nominal 6 inch (152-mm) focal length lens, the diameter of the aperture would range from about 38mm at f-4 to about 7mm at f-22. The diaphragm is normally located in the air space 1-5

6 between the lens elements of an aerial camera and consists of a series of leaves, which can be rotated to vary the size of the opening. Figure 1-4 illustrates the function of a diaphragm. Figure 1-4 The important thing to remember is that this sequence represents the varying hole size in a lens (the aperture) through which light passes. Although they are not regularly written as such, f-stop numbers are actually fractions. Thinking of them as fractions will help you realize that the larger the number, the smaller the hole actually is, and therefore less light is passed through the lens. For example, f/2 represents a fairly large hole that admits a lot of light, while f/22 is quite a small opening, admitting only a small amount of light. Think of these as fractions: ½ is a lot larger that 1/ Camera Calibration Reports Every 2 or 3 years the aerial camera should be submitted to the U. S. Geological Survey for calibration and testing. USGS technicians measure a number of operating parameters to ensure that the camera is functioning correctly and meets published standards of precision and accuracy. An example of a USGS camera calibration report is included in Appendix A. Some of the items enumerated in the report are inputted into the control matrix of a stereoplotter to aid in stereomodel orientation. 1-6

7 1.5 Aircraft Fixed winged aircraft is used to produce aerial photography. Both single engine and multi engine aircraft are used. The crew inside the aircraft can consist of one, two, or three individuals. Figure 1-6 is a photograph of typical twin engine aircraft used in aerial photography. Figure 1-6 Aerial Photo Plane (Courtesy of L. Robert Kimball and Associates, Inc., Ebensburg, PA) 1.6 Season of Year The season of the year is a major factor in aerial photography because it affects ground cover conditions and the sun's altitude. If photography is being taken for cadastral mapping the photos should be taken when the deciduous trees are bare so that leaves do not obscure the ground. Normally aerial photography is not taken when the ground is snow-covered. Heavy snow not only obscures the ground but also causes difficulties in interpretation and in stereo viewing. A factor to be considered in planning aerial photography is the sun s altitude. Low sun angles produce long shadows, which can be objectionable, because they obscure detail. Generally about a 30 sun angle is the minimum acceptable for photography. During the winter months of November through February, the sun never reaches a 30 altitude in some northern parts of the United States due to the sun's southerly declination. Aerial photography should therefore be avoided in those areas during these winter months. For the other months, photography should be exposed during the middle portion of the day after the sun rises above 1-7

8 30 and before it falls below that altitude. Flights should be launched to commence photography at least three hours after sunrise. For certain purposes, shadows may be desirable, since they aid in identifying objects. Shadows of trees, for example, help to identify the species. Shadows may also be helpful in locating photo-identifiable features such as fence posts, power poles, etc., to serve as photo control points. In many places, the combination of sun angle, bare trees and lack of snow occurs twice a year for short periods in the late fall and in early spring. The best time to produce aerial photography is in the spring before the budding of foliage, but after the sun angle is high enough to minimize shadow. 1.7 Weather conditions The weather, which in most locations is uncertain for any given day, is a very important consideration in aerial photography. In most cases, an ideal day for aerial photography is one that is free from clouds; although if the sky is less than 10 percent cloud-covered, the day may be considered satisfactory. A particular day can be cloudless and still be unsuitable for aerial photography due to atmospheric haze, smog, dust, smoke, high winds, or air turbulence. Atmospheric haze scatters primarily the blue portion of the spectrum and it can therefore be effectively eliminated from the photographs by using a yellow filter in front of the camera lens. Smog, dust, and smoke scatter throughout the entire spectrum and cannot be filtered out satisfactorily. Best days for photographing over industrial areas, which are susceptible to smog, dust, and smoke, occur after heavy rains or during moving cold fronts which clear the air. Windy, turbulent days can create excessive image motion and cause difficulties in keeping the camera oriented for vertical photography, in staying on planned flight lines, and maintaining constant flying heights. The decision to fly or not to fly is one that must be made daily. The flight crew should be capable of interpreting weather conditions and of making sound decisions as to when satisfactory photography can be obtained. If possible, the flight crew should be based near the project so that they can observe the weather firsthand and quickly take advantage of satisfactory conditions. 1.8 Flight Inconsistencies Since the aircraft is not an absolutely stable platform, not all photographs are truly vertical. Wind currents and turbulence affect the path of the aircraft, which prevents the camera from continually remaining straight and level relative to the ground. Several inconsistencies can be encountered, and a single exposure may contain any one or a combination of all these inconsistencies. 1-8

9 1.8.1 Drift Since high pressure systems are required to clear out clouds, the wind associated with these systems will push the aircraft off the flight path to a position that is parallel to the intended path. Drift can result in diminished or excessive sidelap with adjacent flight lines or possible gaps in photo coverage. Figure 1-7 illustrates how drift can effect camera exposures. Figure 1-7 Wind Photo 3 Photo 2 Photo 1 Drift Crab Crab is the horizontal displacement, or twisting, of a photograph from the flight line axis. It is commonly caused by the pilot turning the aircraft into the wind in order to stay on the flight line. Crab should be no greater than 3 degrees. An illustration of crab is shown in Figure

10 Figure 1-8 (a) Camera exposing aerial photography with crab present. (b) Crabbed overlapping aerial photographs. (c) Overlapping aerial photographs with no crab. Camera (uncorrected for crab) Direction of Heading Crab Angle TravelDirection Camera (uncorrected for crab) Wind ( ) ℵ I R B Reduced width of coverage ℵ I R (C) Tilt Tilt is a condition in which the aircraft rotates around either the X or Y axis. (See figure 1-9). When tilts are introduced into a photograph, the ground area covered by the image actually becomes a parallelogram rather than a square (see Figure 1-10). Both tilts have the same effect in the imagery and should not exceed

11 Figure 1-9 Tilt X-Tilt Y-Tilt Figure 1-10 Effects of Tilt Negative Lens Axis Ground Shape of photograph Shape of area on ground covered by photograph 1-11

12 X - Tilt During flight the aircraft rolls on its X axis in the line of flight, causing the wings to tilt away from being parallel with the earth. This rotation introduces X-tilt into the photographic image. Y - Tilt During flight the aircraft pitches on its Y axis in the line of flight, causing the nose to incline up or down away from a level flight. This rotation introduces Y-tilt into the photographic image. 1.9 Flight Line Map A flight map depicts project boundaries and the direction the pilot must fly to obtain the desired coverage. The flight map is prepared on an existing map, which shows the project area. USGS Digital Line Graph (DLG) files are often used as the base upon which to create a flight map. With the advent of GPS navigation systems in aircraft, flight maps are commonly created using a computer and flight planning software. The computer creates a digital flight map referenced to the USGS DLG base map (or other suitable base map data). The software creates a files containing end point coordinates of each flight line. The coordinates are uploaded to a GPS navigation system in the aircraft, which is used to guide the aircraft along each flight line. Figure 1-11 Flight Map Strip 1 Strip 2 Strip

13 1.10 Overlap and Sidelap Aerial photos must overlap so that every object is on at least two, and sometimes as many as four, photographs. Forward overlap is the common image area on consecutive photographs along a flight line. This overlapping portion of two successive aerial photos, which creates the three dimensional effect necessary for mapping, is known as a stereomodel. Endlap as illustrated in Figure 1-12 ranges between 55 percent and 82 percent for most mapping projects. Figure 1-12 Endlap 1-13

14 Sidelap is the overlapping area of photographs between adjacent flight lines. Sidelap is planned so that there are no gaps in the three dimensional coverage. Figure 1-13 illustrates sidelap. Sidelap ranges between 25 and 35 percent of the width of the photo. Figure 1-13 Sidelap 1-14

15 Figure 1-14 illustrates all of these. Figure 1-14 Endlap, sidelap 1-15

16 1.11 Exposure Information Aerial cameras produce an image area which is 9 inches by 9 inches with a half-inch border along all four sides. (See Figure 1-15). Along this half inch strip is information that is imprinted onto the negative at the time of exposure. This information consist of a time of exposure, an altimeter vernier denoting height of the aircraft, a bubble target with its position indicating whether the camera is level, and a blank square on which the date of exposure or crew names can be noted. Figure 1-15 Aerial photograph. (Courtesy of L. Robert Kimball and Associates, Ebensburg, PA.) 1-16

17 1-17

18 1.13 Film Processing Once the film has been exposed it is developed in an automatic processing machine. Aerial film developing procedures require that the exposed negative roll be run through at least four chemical solutions. See Figure The developer is an alkaline solution, which chemically reacts with the silver salts to form a negative image. The stop bath is acetic acid, similar to vinegar, which neutralizes the alkaline developer to chemically terminate image development. The fixer (also known as hypo) reduces the unexposed silver salts. This metallic silver can be reclaimed from the used fluid for reuse. The wash is a water bath that rinses away all of the chemical residues from the previous three processes. This ensures that the image, stored in a controlled environment, will undergo no further changes. 1-18

19 1.14 Ground Control Before any aerial photos are taken, certain ground survey information is required to relate the photogrammetric spatial models to their true geographic locations.. These ground features are identified in two ways. One is by identifiable photo image detail, the other is ground targets. Most projects need a combination of both Photo Image Points Photo image points must be easily identified from pictorial image features, which are selected after the aerial flight is completed. Horizontal control points should be relatively small and sharply defined. Such features could be a fence corner or pole. Vertical control points do not have to be as small or sharply defined but should be fairly flat on unobstructed ground. These may be such features as road intersections, sidewalk intersections, manholes, or railroad grade crossings. In general, images of acceptable photo image points must satisfy two requirements: (1) they must be sharp, well defined, and positively identified on all photos, and (2) they must lie in favorable locations within the photograph. See figure Figure

20 Ground Targets A ground target is a panel point that is placed on unobstructed ground prior to aerial photography. Since the target must be visible on the aerial photo they cannot be placed in areas where overhead obstructions such as trees, buildings or overpasses will hide them. These targets create a discrete image point and can lead to better mapping accuracy. They are made of various material, most often plastic. The material is approximately one foot wide and comes in a roll. The targets, usually referred to as panel markers, are placed in a variety of configurations to resemble the letters T, Y, V, L, or X. White paint is also used to identify points. Painted targets such as sewer manhole covers are typically a circle or a square. Artificial targets or panel markers have the advantage of showing up very well on the photography. Their unique appearance makes misidentification unlikely. Disadvantages of artificial targets are that (1) extra work and expense are incurred in placing the targets, (2) the targets could be moved between the time of their placement and the time of photography, and (3) the targets may not appear in favorable locations on the photographs. To guard against the second disadvantage, the photography should be obtained as near as possible to the time of placing targets. Control Point Descriptions Control points, also known as benchmarks, are a 3 ½ diameter disks set in concrete or bedrock. Figure 1-20 illustrates two disks. These benchmarks are named and have a station description. These descriptions include the datum(s) used (Datum s are covered in another section) and the station's geodetic latitude and longitude. Also given are the state plane coordinates, UTM coordinates, and elevation. Some stations give geodetic and plane azimuths to a nearby station or stations. These stations are all part of the National Geodetic Reference System (NGRS). This systems consists of more than 270,000 horizontal control monuments and 600,000 benchmarks throughout the United States. 1-20

21 Figure 1-20 Bronze disks used by National Geodetic Survey to mark horizontal and vertical control stations Aerotriangulation Aerotriangulation is the term most frequently applied to the process of determining X, Y, and Z ground coordinates of individual points based on measurements from photographs. An aerotriangulation solution simultaneously determines the three-dimensional space coordinate values of each point on overlapping photos. Aerotriangulation is an analytical procedure, which allows a photogrammetrist to utilize a skeletal pattern of field survey control points to analytically generate sufficient photo control points to map a county. Thus, when the adjustment is complete, ground coordinates of unknown ground points are known. These supplemental control points are called pass points. The aerotriangulation geometry along a strip of photography is illustrated in Figure Photogrammetric control extension requires that a group of photographs be oriented with respect to one another in a continuous strip or block configuration. The exterior orientation of any photograph that does not contain ground control is determined entirely by the orientation of the adjacent photographs. In a given aerotriangulation configuration, each photograph contributes to the exterior orientation of the adjacent photographs through the pass points located in the triple overlap areas. The term triple overlap refers to the ground area shared by two adjacent stereomodels along the strip. The triple overlap area is imaged on three consecutive photographs. 1-21

22 Figure 1-21 Skeletal control pattern for analytical control Horizontal control point paneled prior to photography. (True position) Pass point image of, for example, tree or fence post shared by three consecutive photographs Triangulation measurements 3 Point number R 1-22

23 Therefore, a final solution to the analytical control illustration in figure 1-21 could be: Point Number Point Y Coordinate X Coordinate Z Etc. A pass point is an image point that is shared by three consecutive photographs along a strip. As the pass point is positioned by one stereomodel it can be used as control to orient the adjacent stereomodel. Pass points for aerotriangulation are normally selected in the general photographic locations shown in Figure The points may be images of natural well-defined objects that appear in the required photo areas, but if such points are unavailable, pass points may be artificially marked. Point-marking devices make small holes in the emulsion which become the pass points, as illustrated in Figure Even though satisfactory natural points may exist in the required general locations on the photographs, many photogrammetrists prefer to mark pass points artificially for two reasons. First, a more discrete point is obtained so that more accurate measurements of its position can be obtained. Second, the likelihood of misidentifying pass points is greatly reduced. 1-23

24 Figure 1-23 Figure Schematic of pugged point. The measured coordinates of the pass points, along with the field control point information, are imported into the computer to complete the aerotriangulation process. Fully analytical aerotriangulation is a simultaneous adjustment solution of all points in a strip or block of photography. The adjustment is often referred to as a bundle adjustment. 1-24

25 1.16 Orthophotography An aerial photograph without any control has little metric value. Metric vale in this context means the ability to use the image as if it were a scalable map. An aerial photograph cannot be used to perform measurements (such as scaling distances or finding angles between identified features) or to extract spatially accurate positions because it does not have a constant scale throughout the entire image. An aerial photograph s scale is defined as the ratio between the focal length of the camera (typically six inches) and the height of the camera above the surface. This scale is correct only for one point in the entire image (usually somewhere around the center of the photograph). All other points (or features) have different scales caused by the perspective nature of the image, by the tilt of the camera at the instance of exposure and by changes in elevation. These scale distortions are illustrated in the following picture: The image shows that the scale of the features on the top of the building are larger than those at the bottom. Also, the size of the top of the building appears to be larger than that of the bottom. These distortions are due to the perspective projection of the image. In general, the scale of a feature in the photograph decreases as a function of its distance from the center of the photograph. To use an aerial photograph as a map it must first be transformed into an 1-25

26 orthophoto. An orthophotograph is an image of an aerial photograph in which displacements caused by the camera and terrain have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map. Orhtophoto s are produced from stereo pairs of diapositives through a process of differential rectification. An orthophotograph is an image of an aerial photograph in which displacements caused by the camera and terrain have been removed. It combines the image characteristics of a photograph with the geometric qualities of a map. The conventional orthophoto process of removing displacement begins with an analytical stereoplotter. Stereoplotters are machines, incorporating complex stereoscopic viewing optics with precision mechanical instrumentation. These components are employed in extracting digital data from aerial photographs. All stereoplotters operate with a spatial stereomodel. A spatial model is a stereopair of photographs which resides within a stereoplotter referenced to its true geographic location. The operator inserts diapositives of two successive aerial photo exposures into receptacles residing within the machine. Even though the compiler is actually viewing two separate images, proper relative alignment of the photos allows the operator s mind to fuse the dual images into a reduced three-dimensional model of the terrain floating in spatial limbo. Once the photos are aligned for three-dimensional viewing, the operator, after importing the photo control information, orients the photos to terrain coordinates. There is an internal reference mark, which gives the visual impression of floating inside the machine. This reference mark is maneuvered horizontally by handwheels or a cursor, and is controlled vertically by a footwheel or a cursor. When the photos are properly oriented, this reference mark can read the true XYZ (east/north/elevation) geographic position of any point on the stereomodel. The operator at this point, is collecting breaklines and points to compile a digital elevation model (DEM). Therefore, beneath the surface of a digital ortho lies a DEM or surface model which accurately depicts the shape and form of the earth s surface. Then like a canvas over tent poles, the photographic surface details are draped over the surface model creating a highly accurate orthophoto base map. Figure 1-24 illustrates this process. Photogrammetric companies also use a process called softcopy to collect a DEM. Softcopy is a term covering a range of software/hardware environments that can be used to produce orthophotos. These softcopy systems produce mapping accuracy equivalent to that generated by conventional analytical stereoplotters. They create DEM s internally through computer-aided differential parallax measurements from a raster image viewed on a graphic screen. Rather than viewing the stereomodel created by passing a light stream through film diapositives, as in the use of analytical stereoplotters (figure 1-25) and digital stereoplotters which function by overlapping images from raster files scanned from the photographs. A very high-quality 1-26

27 image scanner is used to transform the image from film to a digital format. The next step is to compute the location of the camera and the orientation of each photograph at the time of exposure with respect to some coordinate system. After orienting the images to one another and georeferencing the fused image to ground control, the spatial model is observed stereoscopically on the graphic screen. The computer then generates the DEM. Figure 1-24 Once the DEM has been collected either by conventional means or by softcopy, a Photogrammetrist uses a computer to orient each pixel of the stereomodel to its proper XY and Z positions. Figure 1-25 Analytical Steroplotter 1-27

28 At first glance, an orthophoto looks the same as a perspective photo. But upon comparison of an orthophoto and enlargement of the same area, differences can be detected. Figure 1-26 illustrates the differences. The orthophoto on the left shows the terrain in its exact geographic position whereas the photo enlargement illustrates how relief displacement shows the location of those same ground features by the crooked lines. Figure 1-26 Differences between orthophoto and photo enlargement. Orthophoto Photo Enlargement Because of these characteristics, orthophotos make excellent base maps for compiling data to be input to GIS or overlaying and editing data already incorporated in a GIS. Orthophotos also enhance the communication of spatial data, since data users can often relate better to an orthophoto than a conventional planimetric map. 1-28

29 The capability of being able to correlate infinite number of ground objects makes the orthophotograph an asset to cadastral mappers. Mappers can see fence lines and other key items of evidence in order to determine boundary line placement. A digital orthophoto is simply a computerized version of a conventional orthophoto: a raster image of ground features in their true map positions. A raster image is a grid of computer pixels, or picture elements. Each pixel has a row and column "address" (a x, y value) and an intensity value ranging from 0 to 255. The zero grade is no reflectance (black) to 255 which is full reflectance (white). (See Figure 1-27.) Figure 1-27 Raster data format. Columns Rows Pixel Raster Data Format 1-29

30 A black and white digital orthophoto is a continuous tone raster image; all pixels are seen, but at varying intensities of black, white and gray. Color orthophotos have varying intensities of blue, red, and green. Digital orthophotos are created in a five-step process as follows: (1) Aerial photographs are taken in the same manner as previously described. (2) Sufficient horizontal and vertical ground control is acquired as in conventional hard copy orthophotographs. (3) The aerial photographs are scanned, producing continuous - tone, digital raster images of the photography. (Digital camera technology for aerial photography will eliminate the need for image scanning.) (4) The aerial photographs are used to compile a digital elevation model (DEM), a series of x, y, z, coordinates that accurately depicts ground elevations. The DEM must be both accurate and dense enough to adequately define the terrain. Two kinds of features are imputed to create a DEM: breaklines, which delineate abrupt change in elevation, and spot elevations in a grid type pattern (mass points). (5) The DEM is a valuable by-product of the imaging process. It may be used to generate secondary products like contours, profiles, cut and fill calculations, volumes, perspective views, (viewsheds), line-of-sight analyses, slope and aspect maps, etc., when loaded to a geographic information system. The following illustrates this concept. 1-30

31 (6) A technical benefit of digital orthophotographs is that the image map minimizes the abstraction of real world data. All the detail which may be interpreted from the aerial photograph is contained within the image map. The user may perform his or her own interpretation. The imagery also provides instant verification of completeness, correctness, and accuracy of vector datasets, as well as providing a very strong visualization tool in digitizing directly on the screen using the image as a template. The standard digital orthophoto produced by the USGS, is a black and white, color, or colorinfrared, 1-meter ground resolution quarter-quadrangle image covering 3.75 minutes of latitude by 3.75 minutes of longitude. This image is called a digital orthophoto quadrangle (DOQ). DOQ s are cast on the Universal Transverse Mercator projection based on the North American Datum of Some counties have chosen to purchase imagery from the US Geological Survey (USGS). The accuracy and quality of USGS digital orthophoto must meet National Map Accuracy Standards. Most county applications require an accuracy level greater than the national standard of 1-meter resolution. The following figure illustrates the resolution difference between three-foot USGS resolution and some State specified one-foot resolution. 1-31

32 1.17 Remote Sensing Remote sensing is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation. As you read these words, you are employing remote sensing. Your eyes are acting as sensors that respond to the light reflected from this page. The "data" your eyes acquire are impulses corresponding to the amount of light reflected from the dark and light areas on the page. These data are analyzed, or interpreted, in your mental computer to enable you to explain the dark areas on the page as a collection of letters forming words. Beyond this, you recognize that the words form sentences and you interpret the information that the sentences convey. In many respects, remote sensing can be thought of as a reading process. Using various sensors, we remotely collect data that may be analyzed to obtain information about the objects, areas, or phenomena being investigated. The remotely collected data can be of many forms, including variations in force distributions, acoustic wave distributions, or electromagnetic energy distributions. For example, a gravity meter acquires data on variations in the distribution of the force of gravity. Sonar, like a bat's navigation system, obtains data on variations in acoustic wave distributions. Our eyes acquire data on variations in electromagnetic energy distributions. When you analyze a photograph you are practicing remote sensing, we simply call in photo interpretation Remote Sensing Systems A remote sensor is an instrument, which gathers thematic information from a distance. An aerial camera is one remote sensing instrument. Satellites are another. They collect images in raster format. These satellite mounted "cameras" are called sensors because they record a surface characteristic by its spectral signature. The recorded spectral signatures are subsequently processed into photograph-like images. 1-32

33 Figure 1-28 LandSat VII 1-33

34 SPOT has three French satellites orbiting the earth. Unlike LANDSAT (nadir viewing only), SPOT can view both nadir (vertical), and off nadir (oblique). (See Figure 1-29.) There are many earth resource satellite systems revolving about the world in sunsynchronous orbits including: LANDSAT (American), SPOT (French), IRS (Indian), RADARSAT (Canada), and ERS (European Space Agency). The first Landsats were originally created ERTS for the Earth Resources Technology Satellite. 1-34

35 This allows the sensor system to capture the same scene on several different passes over the earth. Figure 1-30 illustrates the orbit of the LANDSAT satellites. Figure

36 1.18 Image Interpretation Three general methods have been used to interpret and extract information from Landsat and other remote-sensing images. 1. Photointerpretation: Visual interpretation of images based on feature tone (color), pattern, shape, texture, etc. 2. Spectral analysis: Identification of surface materials on the basis of spectral signatures. 3. Data integration: Merging of remote-sensing data with other types of data, such as digital elevation models (DEM) Visual interpretation The outstanding aspect of Landsat imagery is complete, small-scale coverage of the Earth at moderate resolution. Computers are used routinely to handle the large volume and quantitative nature of Landsat data. However, the images can still be usefully analyzed with classical techniques of photointerpretation. You should use these general guidelines for interpreting Landsat imagery. h h h h Look for differences in gray tones and/or color. Such differences are often paramount in recognizing the presence of specific surface features and defining boundaries between different classes. Rely on shapes of ground features and their spatial patterns as further clues to identify different features. Compare two or more scenes from different seasons or years to resolve uncertainties about some unknown features. If a feature remains similar in tone or color from season to season, it is probably not related to vegetation or to transient events. Features in short-ir that appear light during the growing season and noticeably darker in winter scenes are almost certainly vegetation. Seek out supporting information from maps, aerial photographs, reports, and other publications, as well as ground observations, as aids to feature identification. 1-36

37 h Draw upon your own experiences, observations, and knowledge of the region in question to help in recognizing features and interpreting the geographic and geologic aspects. Many features change with a seasonal regularity deciduous vegetation, soil moisture, and sun angle; hence the season and time of year must always be taken into account. Other features, such as forest fires, volcanic dust, drought, defoliation due to insects, and so on, are transient events. Interpretation of Landsat requires an appreciation of all aspects of the scene in order to recognize and analyze the features of interest Spectral Signatures The combination of energy reflected and emitted from an object is its spectral signature. Under clear, sunlit conditions, many objects have characteristic spectral signatures in the visible and short infrared portions of the spectrum Merging Landsat with other data Many innovative techniques can be used to create dramatic images of Landsat data combined with other kinds of remotely sensed or ground-based data. Digital elevation models (DEM) are especially useful in this regard. DEM s may be displayed as contour maps, block diagrams, or shaded-relief images. Possible combinations include: h h Landsat image draped over DEM block diagram; perspective view of the landscape with color coding from Landsat image. Landsat image added to shaded-relief image; enhancement of topography visible in Landsat image. Any of these approaches requires considerable computation, in which the remotely sensed data must be rectified for accurate fit onto the ground coordinate system of the DEM. Resampling of pixel, size, shape, and position may be necessary. 1-37

38 1.19 Applications Landsat images and data have been used for diverse applications for all manner of studies involving the Earth s surficial environments. h h h Cartography: basic cartography and new map projection. Hydrology: rivers and lakes, flooding, erosion, and deposition, coastal and wetland environments, irrigation, water quality, lake sediment and organic activity, snow cover, glaciers, ice sheets, and permafrost. Land use, environment, and conservation: land-use monitoring, urban growth, mining, and industrial activity, deforestation, wetland habitats, and storm damage. 1-38

39 Section 1 Review Questions 1. Three flight inconsistencies may occur while the aircraft is exposing film. They are,, and. 2. The two tilt conditions are, and. 3. In order to insure proper stereo coverage, aerial photos must overlap. The two types of overlapping are, and. 4. Proper film processing consists of running the film through four chemical solutions. They are,,, and. 5. is an analytical procedure, which allows a photogrammetrist to utilize a skeletal pattern of field survey points to control aerial photography. 6. Orthophoto s are produced from aerial photos from a process called. 7. A computerized version of a conventional orthophoto is known as a. 8. is the science and art of obtaining information about an object, area, or phenomenon through the analysis of data by a device not in contact with the area being investigated. 1-39