Aerial Photography and Photointerpretation Note#5

Transcription

1 Aerial Photography and Photointerpretation Note#5 Aerial photographs Aerial photographs are taken from an aircraft to capture a series of images using a large roll of special photographic film. The film is then processed and cut into negatives. The most common size for negatives is 9" x 9" (23cm x 23cm). The basis for aerial photography, like all kinds of photography, is light-sensitive chemicals in the film emulsion. These chemicals may react to ultraviolet, visible, and/or near infrared portions of the spectrum, from 0.3 µm to 0.9 µm wavelength. Airphotos are often used as data source for updating vector data that is stored and manipulated in GIS. Airphotos are especially important for applications that require the data of very high spatial resolution. Vertical Aerial Photography The camera s optical axis is within 3 of being vertical (perpendicular) to the Earth s level surface. Two overlapping vertical aerial photograph obtained from slightly different viewing position may be used in photogrammetry to derive: 1) Accurate planimetric (x,y location) base maps of natural and man-made features; 2) Topographic (z, elevation) base maps (contours); 3) Raster digital elevation model; and 4) Accurate orthophotographs-aerial photographs that are geometrically accurate in x, y. USGS 7.5-minute 1:24,000 map series and DEMs are derived based on vertical aerial photographs; The people are not used to viewing the tops of objects such as buildings, trees, etc. It takes considerable practice and training to efficiently and accurately interpret a vertical aerial photograph. Oblique Aerial Photography Obtained if the camera s optical axis is more than 3 off of vertical (perpendicular) to the earth s level surface; If the horizon is not visible, it is called low-oblique photograph; If the horizon is visible, it is called high-oblique photograph; People tend to be better able to interpret oblique photographs as we are accustomed to seeing. Basics of Aerial Cameras Camera body Lens-focal plane and focal length; 1

2 Shutter-control exposure speed Diaphragm-control apertures Focal plane and Focal Length The flat surface where the film is held is called focal plane. The focal length is the distance from the lens at which parallel rays of light are focused to a point Thin-lens equation: = + f o i Where f is the focal length of the camera, o is the distance between the object and the camera, and I is the distance between the lens and the image plane. In aerial photography, o can be considered infinite, so aerial cameras are manufactured with their film plane located precisely at the fixed distance f from the lens, namely, make i=f. Most metric aerial cameras have a fixed focal length, such as 152mm (6 in), 305mm (12 in). Military photo-reconnaisance operations commonly employ lenses 3 to 6 ft to obtain detailed photographs from extremely high altitudes. Exposure Exposure is controlled by several factors: 2 sd t E = 2 4 f where E is film exposure, s is scene brightness, d is the diameter of the lens opening, t is the exposure time, and f is the focal length. F/stop: in camera terms, the diameter of the opening is often referred to in terms of the ratio between the lens focal length and the diameter of the lens opening: f f / stop = d Shutter speed: the length of time the shutter is open is called exposure time. Typically, speeds are in the tenths or hundredths of seconds. To avoid the blurring, the shutter speed must be fast. Angular field of view (AFOV) narrow <60; normal (60-75); wide-angle ; super-wide-angle > 100. The wider the angular field of view, the greater the ground coverage. Annotations of Aerial photograph Image annotation around the perimeter of the photograph display numerous types of ancillary information: Grayscale step wedge Notepad Altimeter 2

3 Fiducial marks Clock Lens cone serial number Focal length Project frame number Mission name and date Navigation data Types of Aerial Cameras 1. Single-lens mapping (metric) cameras Provide the highest geometric and radiometric quality aerial photography to map the planimetric (x,y) location of features and to derive topographic (contour) map. Individual exposures are typically 9x9in. (23x23 cm). 2. Multiple-lens camera Each of the lens (camera) simultaneously records photographs of the same area, but using different film and/or filter combination. This creates multiple-band photographs. 3. Panoramic cameras Uses a rotating lens (or prism) to produce a narrow strip of imagery perpendicular to the flightline. Very common in military reconnaissance, but much less in civilian applications due to poor geometric integrity. 4. Digital camera Uses an area array of solid-state charge-coupled-device (CCD) detectors. The detectors are arranged in a matrix and located at the film plane. To replicate the spatial resolution of standard 9x9 in. metric aerial photograph, a digital camera would require approximately 20,000x20,000 detectors. Aerial Photograph Filtration Additive color theory Blue, green, and red are the primary colors for lights (EM radiation); The three primaries can be mixed to create all color shades Subtractive color theory Based on the use of pigments or dyes and not light. Use subtractive color theory when we paint or work with filters. The complementary color dyes: yellow, cyan, magenta Filters Filter out (subtract) certain types of unwanted wavelengths of light before they can reach the film plane and expose the film. A filter will appear the color of light that is allowed to pass through it. 3

4 Haze filter: When collecting natural color aerial photography, it is desirable to eliminate much of the scattering of ultraviolet radiation caused by atmospheric haze. Haze filter (HF) were developed to absorb light shorter than 400nm. Yellow filter (minus-blue filter): when collecting color-infrared aerial photography, a yellow filter is used, which subtract almost all of the blue light (wavelengths < 500nm). This reduces the effects of atmospheric Rayleigh scattering. It absorbs blue and allows green and red light to be transmitted. A mixture of red and green light is yellow. Band-pass filter: configure a film and filter combination so that the camera only records a very specific band of reflected EM energy. Polarizing filter: will allow the vibration of a light ray in just one plane to be passed. Aerial Photography Films The sensitivity of a photographic emulsion depends on the size, shape, and number of silver halide crystals in the emulsion per unit area and the wavelengths of light to which the grains are sensitive. Faster films can be used advantageously when photographing objects that are moving rapidly as in aerial photography. As sensitivity and grain size increase, the resulting image becomes more coarse and grainy, and resolution (sharpness/crispness) will be reduced. Types of films 1) Panchromatic Often called black and white, is sensitive to the same range of light wavelengths as perceived by the human eye (the "visible" wavelengths blue, green and red spanning 0.4 to 0.7 micrometers). Panchromatic photos are most commonly used for planimetric and/or topographic mapping. A yellow filter is normally used for exposure on panchromatic film to reduce the fogging effect that atmospheric haze causes. Unfiltered panchromatic film is used for penetration through clear water. 2) Natural color Often called true color, is also sensitive to the same wavelengths of light as perceived by the human eye. Normal color film is especially useful for identifying soil types, rock types and surficial deposits, water-surface patterns, and various forms of polluted water for clear water. Color film has good penetration qualities and is therefore valuable for recording underwater features. Its penetration through clear water can exceed 25m. Color photography is also useful fort detecting forest damage caused by various insects. Natural photos are commonly used for creating photo maps, or for mapping applications that require discrimination of the color of features. A haze filter is normally used to prevent ultraviolet light from exposing the film. 3) Black-and-white Infrared (IR) is sensitive to a range of wavelengths that includes the green, red, and near infrared portions of the spectrum. 4

5 One of the main values of infrared photographs is their ability to differentiate different types of vegetation. Healthy deciduous (broadleaf) vegetation is recorded in light tones, and coniferous (needle-leaf) vegetation tends to reflect less near infrared radiation and consequently registers in darker tones. Black-and-white infrared film is also useful for differentiating dry and moist soils. Moist soils, because of the presence of pore water, absorb near infrared radiation and appear in dark tones, whereas dry soil is more reflective and appears in lighter tones. The near infrared wavelengths ( ) cannot be perceived by the human eye, so they provide information that beyond the human perception system. Objects Panchromatic Infrared Snow white white Clouds white white Sky (high oblique) medium gray black Clear water dark gray black Silty water light gray medium gray Deciduous foliage dark gray white Coniferous foliage dark gray medium gray Autumn foliage (yellow) light gray light gray White sand (dry) light gray light gray White sand (moist) medium gray dark gray Red sandstone (dry) medium gray light gray Red sandstone (moist) medium gray dark gray Swamp dark gray black Asphalt dark gray black Concrete light gray medium gray 4) Color infrared (CIR) Often called false color, was developed during World War II for detecting camouflaged military targets. Like natural color, they are usually displayed using the RGB color system to re-create the same colors as on the photo print. Vegetation usually appears red on these images, thus the term false color. CIR photos are commonly used for agricultural, forestry and wetland studies because the IR band provides valuable information on vegetation health, species and biomass. Color-infrared (CIR) photographs are especially useful for camouflage detection, recognition of vegetation and water bodies, and display of special features. Deep, clear water absorbs almost all of the near infrared energy while reflecting somewhat more green and red light. If the water is deep and free of suspended sediment or any organic matter, it will appear black. If there is substantial suspended sediment, it may appear in relatively dark shades of blue and green. Objects Normal Color Color Infrared Snow white white Clouds white white Sky (high oblique) blue blue Clear water blue or green black or dark blue Silty water red or brown light blue or green Deciduous foliage green bright red 5

6 Coniferous foliage green brownish red Aquatic vegetation green pink Citrus trees Healthy green red Previsual stress green pink Late stage of stress yellow white Defoliated trees gray blue or green Artifical turf Dry green blue Wet green black Red sandstone red yellow or green Red sandstone (moist) medium gray dark gray Asphalt black black Concrete gray steel gray Damp ground slightly darker distinctly darker Shadows bluish with details black 5) Multiband photography Taking simultaneous photos in different portions of the spectrum. For example, fourband photography might include separate b/w photographs in blue, green, red, and shortinfrared bands. Environmental factors for Aerial Photograph Acquisition 1. Seasonal consideration For topographic mapping, photographs are usually taken either in the spring or the fall, when deciduous foliage is absent (leaf-off), and the ground is essentially free of snow. For vegetation related application, aerial photographs need to be taken during the growing season. 2. Time of day (Sun Angle) The ideal time for most applications is when the Sun is between 30 and 52 degree above the horizon within two hours of solar noon. Avoid hotspot (sunspot) condition. 3. Weather Avoid humid, wind, and cloud condition. Digitizing Aerial Photograph Films In order for an aerial photograph to be processed in a computer, the photo must first be scanned to create a digital image file. The photo print or transparency is run through a scanner to convert visible images to digital files. Scanning imagery at spot sizes <12 µm may result in noisy digitized data, because the spot size approaches the dimension of the film s silver halide crystals. Most desktop scanners are designed for 8.5x14in originals, but most aerial photographs are 9x9in. 6

7 Aerial photographs normally have fiducial marks around the edges. The positions of these marks are crucial for orthorectification. It is essential that the scanner used is large enough to include the fiducial marks in the scanned image. Two factors affect the resolution of the resulting digital image: 1)The scale at which the aerial photograph was taken, which is determined by aircraft altitude above ground and focal length of the camera during photo acquisition. 2) The Dots Per Inch (DPI) of scanner used to scan the aerial photo. A pixel size of 1 meter is adequate for many applications. The following table shows the relationship between scale of aerial photography, DPI resolution of scanner, and file size: Photo scale: Km. across: Pixel size in meters # of pixels color B & W 150 dpi scan Mb 2 Mb 300 dpi scan Mb 7 Mb 600 dpi scan Mb 28 Mb 1200 dpi scan Mb 111 Mb 2400 dpi scan Mb 445 Mb Digitized NAPP Data-DOQQs In the US, airphoto images are available as Digital Orthophoto Quarter Quadrangles, known as DOQQs. DOQQ images have already been orthorectified. A digital orthophoto is a digital image of an aerial photograph in which displacements caused by the camera and the terrain have already been removed. Therefore it combines the image characteristics of a photograph with the geometric qualities of a map. A DOQ image typically covers one-quarter of a 1:24,000 scale USGS topographic map plus a little overlap. The photography is acquired at an altitude of 20,000 ft above-ground level with a 6 in focal-length metric camera, at about 1:40,000 scale. Using a spot size of 15µm preserves the 27 resolvable line per millimeter (1p/mm) spatial resolution in the original NAPP photography. This generates DOQQ data with a ground spatial resolution of 1 m. The resolution of the image is quite high after scanning at one square meter per pixel. As a result of this fine cell size, these images consume large quantities of disk space. A DOQQ is a subset of a DOQ, in that it refers to a Digital Orthophoto Quarter Quad. There are four DOQQs in a DOQ. Principles for Photointerpretation Photo Interpretation The act of examining aerial photographs/images for the purpose of identifying objects and judging their significance. Because airphoto interpretation often involves a considerable amount of subjective judgment, it is commonly referred to as an art rather than a science. Actually it is both. A large store of information, a sound general background in the earth and life science. 7

8 Experience and imagination. Relatively easy in recognizing features or conditions in oblique photographs, but a vertical or near vertical view can be quite confusing. Recognition elements are often used synergistically to arrive at a logical conclusion. There are recently been a resurgence in the art and science of visual photointerpretation due to new digital remote sensing systems providing higher and higher spatial resolution imagery. SPOT and IRS-C panchromatic imagery is often photo-interpreted and used as a base map in GIS projects. The demand for experienced photointerpreters will increase as next-generation satellite systems like IKONOS acquired 1mx1m imagery Photointerpretation Purposes 1) Detection/Identification The primary task of the interpreter is the detection and identification of objects, features, phenomena and processes. The interpreter conveys his or her response by labeling. 2) Measurement As opposed to detection and identification, the making of measurements is primarily quantitative. Techniques used by interpreters typically are not as precise as those employed by photogrammetrists who use sophisticated instruments in making their measurements. 3) Problem Solving Interpreters are often required to identify objects from a study of associated objects that they can identify; or to identify object complexes from an analysis of their component objects. Analysts may also be asked to examine an image, which depicts the effect of some process and suggest a possible or probable cause. Recognition Elements of Image Interpretation Tone/Color (first-order primitive element) Tone can be defined as each distinguishable gray variation from white to black. Color may be defined as each distinguishable variation on an image produced by a multitude of combinations of hue, value and chroma. Tone or color relates to the reflective characteristics of objects within the photographic spectrum. The wavelength region of sensitivity is a function of film type and filtration. Tonal contrasts in black-and-white photographs and hue, chroma, and value qualities in color photographs provide important clues for object identification. While a human interpreter may only be able to distinguish between ten and twenty shades of gray; interpreters can distinguish many more colors. Some authors state that interpreters can distinguish at least 100 times more variations of color on color photography than shades of gray on black and white photography. For example, healthy vegetation reflects much of the incident near-infrared energy, appearing light tone in black-and-white near IR photography and red in color near IR aerial photography. As incident blue and green light penetrates the water column farther than red and near infrared energy and are reflected off the sandy bottom, therefore, the blue and green band images of aerial photographs provides subsurface detail. Second order elements (Geometric Arrangements of Primitive elements) 8

9 1) Size The size of an object is one of its most distinguishing characteristics. The most commonly measured parameters are length, width, perimeter, area and occasionally volume. Measuring the size of an unknown object allows the interpreter to rule out many possible alternatives. Measuring the size of a few well-known objects in an image such as car length, road and railroad width, size of typical single-family house in the area allows you to understand the size of unknown features in the image and eventually to identify them. It is risky to measure the precise length, perimeter, and area of objects on unrectified aerial photography. The absolute and relative size of objects can be important in discrimination of objects and features (cars vs. trucks or buses; single family vs. multi-family residences, brush vs. trees, etc.). In the use of size as a diagnostic characteristic both the relative and absolute sizes of objects can be important. Size can also be used in judging the significance of objects and features (size of trees related to board feet which may be cut; size of agricultural fields related to water use in arid areas, or amount of fertilizers used; size of runways gives an indication of the types of aircraft that can be accommodated). 2) Shape Shape describes the external form or configuration of an image object. Cultural objects tend to have straight edges and geometrical shapes and distinct boundaries, whereas natural features tend to toward irregular shapes with irregular boundaries. The shape of objects/features can provide diagnostic clues that aid identification. Cultural features with distinct shapes: Pentagon building in Washington DC, airport runways, shopping malls, cloverleaf interchanges, center-pivot irrigation systems. Natural features with distinctive shapes: sand dunes, volcanic cinder cones, alluvial fans, meander floodplain. Man-made features have straight edges while natural features tend not to. Roads can have right angle turns, railroads do not. Numerous shapes: linear, curvilinear, circular, elliptical, radial, square, rectangle, triangular, hexagonal, star, elongated, and amorphous. Second order elements (Spatial Arrangement of Tone/Color) It must be noted here that pattern is scale dependent. An orchard on a low altitude aerial photo may be an area of rough texture on a high altitude air photo or an area of medium or rough texture on a satellite image. This pattern may be thought of as being scale dependent. 1) Texture The visual impression of coarseness (roughness) or smoothness caused by the variability or uniformity of image tone or color is known as texture. It represents the frequency of change and arrangement of tones. This is a micro structure of image pixels. It is produced by an aggregate of characteristics too small to be detected individually, such as tree leaves and leaf shadows. 9

10 Smooth (fine) textures are associated with cropland (plants at about the same height), bare fields, and calm bodies of water; Coarse (rough) textures are associated with forestland (mature tree crowns) and young lava flows, while grass is medium. It s often possible to distinguish between features with similar reflectance characteristics based upon their textural differences. Texture, just like object size, is directly correlated with photo scale. Thus, a given feature may have a coarse texture in a low-altitude photograph, and smooth texture in a high-altitude photograph. 2) Pattern Pattern is the overall spatial arrangement of related image objects. The repetition of certain forms is characteristics of many human objects and some natural features. In comparison with texture, pattern is a macro image characteristic. It is the regular arrangement of objects that can be diagnostic of features on the landscape. The orderly arrangement of trees in an orchard or grove usually a grid pattern versus the random distribution of trees in a forest. Likewise, the network or grid of streets in a subdivision or urban area can aid identification and aid in problem solving such as the growth patterns of a city. Pattern can also be very important in geologic or geomorphologic analysis. Drainage pattern can tell the trained observer a great deal about the lithology and structural patterns in an area. Dendritic drainage patterns develop on flat bedded sediments; radial on/over domes; linear or trellis in areas with faults or other structural controls. Patterns can be described as random, systematic, circular, centripetal, oval, curvilinear, linear, radiating, rectangular, hexagonal, pentagonal, octagonal, etc. Third-order (Locational or Positional) Elements 1) Site The location of object in relation to its environment is called the site factor and is important for recognizing many cultural and natural features. How objects are distributed with respect to geographical locations and associated human and natural environment. The physical characteristics of a site might include elevation, slope, aspect, and type of surface vegetation cover and associated soil type. Socioeconomic site characteristics might include the value of the land, the land-tenure system, zoning codes, etc. For example citrus groves in Florida s central-ridge district are often sited on hillsides to avoid cold-air drainage to low-lying area. In addition, many types of natural vegetation are characteristically confined to specific locations such as swamps, marshes, and stream banks, or to sites differing in elevation and aspect. Thermal and nuclear plants are often found near major sources of surface water. Aspect, topography, geology, soil, vegetation and cultural features on the landscape are distinctive factors that the interpreter should use when examining a site. Just as some vegetation grows in swamps others grow on sandy ridges. Agricultural crops may like certain conditions. Man made features may also be found on rivers (e.g. power plant) or on a hill top (observatory or radar facility). 2) Association Some objects are so commonly associated with one another so that identification of one tends to indicate or confirm the existence of another. 10

11 Association is one of the most helpful clues for identifying cultural features that comprise aggregate components. Smoke stacks, step buildings, cooling ponds, transformer yards, coal piles, railroad tracks = coal fired power plant. Arid terrain, basin bottom location, highly reflective surface, sparse vegetation = playa. Association is one of the most helpful clues in identifying man made installations. Aluminum manufacture requires large amounts of electrical energy. Absence of a power supply may rule out this industry. Cement plants have rotary kilns. Schools at different levels typically have characteristic playing fields, parking lots, and clusters of buildings in urban areas. Large farm silos typically indicate the presence of livestock. Third order -- Interpreted from lower order elements 1) Height The ability to visually appreciate and measure the height (elevation) or depth (bathymetry) of an object or landform is one of the most diagnostic elements of image interpretation. Height can add significant information in many types of interpretation tasks; particularly those that deal with the analysis of man-made features. How tall a tree is can tell something about board feet. How deep an excavation is can tell something about the amount of material that was removed (in some mining operations excavators are paid on the basis of material removed as determined by photogrammetric analysis). Stereoscopic parallax is measurable when the same object is viewed from two different vantage points along a flightline. Stereoscopic method is the optimum approach to visually appreciating the three dimensionality of the terrain and for extracting accurate x,y,and z values of image objects. In monoscopic images, there are visual cues that we can use to appreciate the height or depth of an object. Any objects such as a building or utility pole that protrudes above the local datum will exhibit radial relief displacement outward from the principal point of a typical aerial photograph. In addition, all objects protruding above the local datum also cast a shadow that provides diagnostic height information. The optimum method of obtaining bathymetric measurements is to use a sonar remote sensing device that sends out a sound pulse and measure how long it takes for the sound tom pass through the water column and bounce off the bottom, and be recorded by the sensor. 2) Shadow Most remote sensor is collected within 2 hours of solar noon to avoid extensive shadows in the imagery. Shadow cast by oblique illumination are important in photo interpretation because their shapes provide profile views of certain features that can facilitate their identification. Features often recognizable by the shadows include water towers, electrical-transmittion towers, oil-storage tanks, bridges, and various species of trees. Shadows are particularly helpful if the objects are small or lack tonal or color contrasts with their surroundings. Low sun-angle (early morning or late afternoon) photographs accentuate minor surface irregularities. 11

12 Shadows can provide clues about the height of an object when the image interpreter does not have stereoscopic imagery in hands. When interpreting imagery with substantial shadows, it is a good practice to orient the imagery so that the shadows fall toward the interpreter. This keeps the analyst from experiencing pseudoscopic illusion where low points appear high and vice versa. Shadows can also inhibit interpretation. Aids to Image Interpretation 1) Collateral Material A review of existing source material concerning a given area, process; type of facility, or object, can aid in the interpretation of remotely sensed data. Collateral material has also been called in the literature ancillary data, and site scientific literature. Collateral material may come in the form of text, tables, maps, graphs, or image data/information (metadata). Census data, a map of flora of a given area, a land use map of an area, meteorological statistics, or agricultural crop reports can all be used in support of a given interpretation. Basically, collateral material represents data/information that an interpreter may use to aid in his/her accomplishment of a given analysis task. Two classes of collateral data deserve special mention here. These are photo/image interpretation keys and field verification. 2) Image Analysis Keys and convergence of evidence A photo/image interpretation key is a set of guidelines used to assist interpreters in rapidly identifying photo/image features. As a general rule, keys are more easily constructed and used for the identification of manmade objects and features than for natural vegetation and landforms. For analysis of natural features, training and field experience are often essential to achieve consistent results. Basically, an interpretation key helps the interpreter organize the information present in image form and guides him/her to the correct identification of unknown objects. 3) Field Verification Field verification is typically conducted to assist in the analysis of the data to be analyzed. Essentially, this is familiarizing the interpreter with the area or type of feature or object to be interpreted. 4) Stereoscopic Viewing most stereoscopic viewing for interpretation purposes is done from vertical or near vertical aerial photography acquired by conventional aerial camera systems. 5) Use of Multiple Images Multi-Band, multi-scale, multi-displinary, Multi-Date, Multi-Polarization, Multi- Direction, Multi-Enhancement Reading Assignment: Chapter 4 & 5 in Jensen, J.R Remote Sensing of the Environment: An Earth Resource Perspective. Upper Saddle River, NJ, Prentice Hall. 544 pp. 12