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2024Lillesand_MaRemoteSense3-airborne (1).pptx
2024Lillesand_MaRemoteSense3-airborne (1).pptx
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Introduction to Remote Sensing Objectives Terminology of aerial photographs Displaying multispectral images 1
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Introduction to Remote Sensing Terminology of aerial Photographs Vantage point Flight height Scale View angle Ground coverage 2
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Camera systems Camera systems are passive optical sensors that use a lens (B) to acquire a snapshot of an area (A), and to form an image at the focal plane C. 3
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Vantage point Vantage point = Camera orientation The orientation of the camera relative to the ground during acquisition. • vertical • oblique 4
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Vertical aerial photography A vertical photograph is obtained when the camera’s optical axis is within 3° of being vertical to the Earth surface. optical axis less distortion Used to create planimetric, topographic, and orthophotomaps, and DEMs using fundamental photogrammetric principles. 5
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Oblique aerial photography An oblique aerial photograph is obtained if the camera’s optical axis deviates more than 3° from vertical. optical axis more distortion Useful for covering very large areas in a single image and for depicting terrain relief and scale. 7
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Flight Height (Altitude) Altitude of the camera above ground level (AGL), for a given focal length – the higher the altitude, the larger the area – the lower the altitude, the smaller the area f H1 H2 10
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Scale Photo Scale (S) = Distance on Photograph Distance on the ground For an aerial photograph, scale is: S = camera focal length (f) flying height above terrain(H) Scale can be denoted as Verbal scale: 1 cm equals 1 km Representative fraction: 1/100,000, or Ratio scale:1:100,000 Graphic (bar) scale: 40 0 40 80 Miles 11
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View Angle The field of view of a camera f1 > f2 > f3 a1 < a2 < a3 Giving the same flight height, viewing angle is determined by the focal length. The smaller the focal length, the larger the view angle 12
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Ground Coverage The actual area on the ground that one air photograph covers. Determined by the altitude and the camera’s viewing angle. 13
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Introduction to Remote Sensing The advantage and disadvantage of aerial photography (film) Advantages: • High resolution (ground) • Flexibility • High geometric reliability • Relatively inexpensive Disadvantages: • Day light exposure (10:00 am- 2:00 pm) required • Poorer contrast at shorter wavelengths • Film non-reusable • Inconvenient • Inefficient for digital analysis 14
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Panchromatic imaging With panchromatic imaging, the sensor is a single channel detector sensitive to radiation in a broad wavelength range. Where the wavelength range coincides with the visible range, the resulting image resembles a "black-and- white" photograph. 15
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Multispectral Imaging With multispectral, or multiband data, there are several layers, or values for each pixel, each representing a different “channel/band” or reflectance in a wavelength spectrum. Each “band” or “channel” is sensitive to radiation within a different band of wavelength, through use of different filters. 16
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Displaying multispectral bands separately These bands can be looked at separately. One band from a multispectral image would be displayed as a grayscale image, with each pixel represented by a grayscale value. blue green red 17
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Displaying multispectral bands (2) These bands can be combined to make “composite” images. Composite images are usually displayed by assigning three of the bands to the red, green and blue channels and displaying them additively. 18
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Color types additive primaries: red, green, and blue subtractive primaries: cyan, magenta, and yellow Light mixing Dye mixing follows this follows this Additive and subtractive colors Blue+Green+Red = White Yellow+Magenta+Cyan = Black Displaying image is based on additive mixing of primary colors, whereas color photography is based on subtractive mixing of complementary colors. 19
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True color composite Bands are assigned to color channels in such a way that colors in the image roughly correspond with the colors in the real world. Often assigned red to red, green to green and blue to blue can result in this. Band Composite Output = Color Guns = Band Combination = 3 2 1 (LANDSAT) BLUE GREEN RED NEAR IR SHORT MID- LONGWAVE IR WAVE IR WAVE IR Landsat TM Band 1 2 3 4 5 7 6 20
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False color composite Shows colors that don’t really exist in that location. An example is color infrared composite, where green band is assigned to blue display channel, red is assigned to green and Near IR is assigned to red. Band Combination = 4 3 2 (LANDSAT) NEAR IR SHORT MID- LONGWAVE IR BLUE GREEN RED WAVE IR WAVE IR 21 Landsat TM Band 1 2 3 4 5 7 6
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False color composite Band Composite Output = Color Guns = Band Combination = 4 3 2 (LANDSAT) NEAR IR SHORT MID- LONGWAVE IR BLUE GREEN RED WAVE IR WAVE IR 22 Landsat TM Band 1 2 3 4 5 7 6