Introduction to Remote Sensing

1
Introduction to Remote Sensing

• Inel 5995
• Dr. Ramón Vásquez/Prof. Vidya Manian

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Key concepts of remote sensing

• Spatial Differentiation
• observed spectral differences in the energy
  reflected or emitted from features. Eg.,
  differences in colorobjects - multispectral
  remote sensing
• spectral signature - spectral response of a
  feature. Eg., corn has different responses during
  planting, emerging and maturing. Yet useful at
  specific times and places
• Radiometric Differentiation
• detection of differences in brightness of objects
  and features
• contrast at a specific spectral region - R.S.
  instrument should be capable of recording this
  contrast
• sensitivity of instrument, and existing contrast
  in scene are important issues

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• Spatial differentiation
• smallest area that can be separately recorded as
  an entity on an image - spatial detail
• Pixels - distinct, discrete units identifiable on
  the image
• spatial detail influenced by choice of sensor and
  altitude at which image is recorded
• spatial complexity-some landscape can be
  represented at coarse levels, some require finer
  levels
• Geometric transformation
• RS image represents landscape in a specific
  geometric relationship determined by design of RS
  instrument, specific operating conditions,
  terrain relief
• Ideal RS instrument creates accurate
  correspondence between points on ground and
  corresponding representation on image

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• In reality, RS images have positional errors due
  to sensor optics, motion of scanning optics,
  terrain relief, earth curvature.
• These can be removed or reduced but should be
  taken into account
• Interchangeability of pictorial and digital
  formats
• RS outputs are digital arrays representing
  brightnesses of areas of the earths surface
• Pictorial images in digital form - systematically
  dividing the image into tiny areas of equal size
  and shape, then representing the brightness of
  these areas by discrete values

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• RS instrumentation acts as a system
• many components of RS acts as a system, not
  isolated from one another
• Eg., upgrading quality of a camera lens is useful
  when the quality of film is also improved
• Role of the atmosphere
• All energy reaching the RS instrument passes
  through a portion of earths atmosphere
• The suns energy is altered in intensity and
  wavelength by particles and gases in the
  atmosphere. These changes appear degrading image
  quality and reduces accuracy of interpretation

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Electromagnetic radiation

• All objects except those at absolute zero, emit
  EM radiation
• Understanding of EM radiation and its interaction
  with surfaces, the atmosphere and instruments
• Familiar form of EMR - visible light
• The electromagnetic spectrum
• generated by - changes in energy levels of
  electrons, acceleration of charge, decay of
  radioactive substances, thermal motion of atoms
  and molecules
• Source - nuclear reaction in sun, passes
  atmosphere, reaches earth. Some reflected upwards
  from the surface - this forms basis for
  photographs and similar images
• some absorbed at the surface of the earth and
  re-radiated as thermal energy (can also be used
  for RS images - differ from aerial photographs
  formed from reflected energy

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• Man-made radiation generated by imaging radars
  are also used for RS
• Em radiation consists of electrical field
  (E)-varies in magnitude in a direction
  perpendicular to direction of propagation. A
  magnetic field (H), at right angles to E,
  propagated in phase with E
• Has three properties
• 1. Wavelength distance from one wave crest to
  next (measurement units in Table 2.1)
• Frequency number of crests passing a fixed point
  in a given period (Hertz, one cycle per second
  and multiples of the hertz, Table 2.2)
• Amplitude height of each peak. Measured as
  energy levels (spectral irradiance expressed in
  watts per square meter per micrometer, as energy
  level per wavelength in interval)

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• Speed of electromagnetic energy ( c) is constant
  at 299,893 km per second.
• c ?? wavelength (?), frequency (?)
• characteristic of EM energy can be specified
  using either freq. (Hz, kHz, MHz) or wavelength
  (Angstrom units, microns, micrometers ( 1
  millionth of a meter ?m, nanometers, millimeters)
  
• Major divisions of the EM spectrum
• arbitrarily defined (Table 2.3)- there are no
  sharp breaks as in Fig. 2.3. (subdivisions for
  convenience, may vary)
• The optical spectrum from 0.13 to 15

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• Speed of electromagnetic energy ( c) is constant
  at 299,893 km per second.
• c ?? wavelength (?), frequency (?)
• characteristic of EM energy can be specified
  using either freq. (Hz, kHz, MHz) or wavelength
  (Angstrom units, microns, micrometers ( 1
  millionth of a meter ?m, nanometers, millimeters)
  
• Major divisions of the EM spectrum
• arbitrarily defined (Table 2.3)- there are no
  sharp breaks as in Fig. 2.3. (subdivisions for
  convenience, may vary)
• The optical spectrum from 0.13 to 15 ?m, those
  wavelengths that can be reflected and refracted
  with lenses and mirrors
• The reflective spectrum from 0,38 to 3.0 ?m,
  portion of solar spectrum used directly for RS

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• Ultraviolet spectrum
• For RS begin with UV region (short-wavelength
  region-limit of human vision)
• near UV (UV-A) (0.32-0.40 ?m), far UV (UV-B)
  (0.32-0.40 ?m) and extreme UV (UV-C) (lt0.28 ?m)
• Near UV induce fluorescence (emission of visible
  radiation) in some materials
• UV is largely scattered by earths atmosphere,
  not generally used in RS
• Visible Spectrum
• Isaac Newton - optical light can be divided using
  prisms, or, in our time, diffraction gratings
  into 3 segments - additive primaries 0.4-0.5
  ?m-blue, 0.5-0.6 ?m - green, 0.6-0.7 ?m-red
• all other colors formed by mixing the 3
  primaries equal proportions form white light

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• A blueobject reflects blue light, etc.
• Intermediate colors - when object reflects 2 or
  more of the additive primaries yellow (red and
  green) - useful for exposure of photographic
  films
• colors in films, paintings formed by combinations
  of 3 subtractive primaries defines colors of
  pigments and dyes
• Fig. 2.4 - Each of the 3 subtractive primaries
  absorbs a third of the visible spectrum Yellow
  absorbs blue light and reflects red and green
• Mixture of equal proportions of pigments of the 3
  subtractive primaries yields black (complete
  absorption of visible spectrum)
• The Infrared spectrum
• from 0.72-15 ?m, 40 times as wide as visible
  region
• Near IR and mid IR radiation - closest to
  visible, behave analogous to visible, can use
  films, filters and cameras

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• Far IR - wavelengths well beyond the visible
  extending to ?wave region. Near IR is
  essentially solar reflected from earths surface,
  far IR is emitted by the earth. It consists of
  heator thermal energy, also called emitted
  infrared.
• Microwave energy
• Longest wavelengths used in RS are from about 1
  mm to 1 m. Shortest wavelengths in this range
  have much in common with the thermal energy of
  the far infrared
• Longest wavelengths merge into the radio
  wavelengths (commercial broadcasts)
• Radiation Laws
• Isaac Newton dual nature of light light is a
  stream of minuscule particles (corpuscles) that
  travel in straight lines
• Max Planck discovered that EM energy is absorbed
  and emitted in discrete units called quanta, or
  photons. A constant (h) relate frequency (?) to
  radiant energy (Q) Qh?

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Aerial photography

• Images formed as light passed through a pinhole
  opening in a dark enclosure
• camera obscura-dark chamber-project image onto
  a screen
• Developed to popular cameras after development of
  photographic emulsions
• Aerial photography-most reliable and widely used
  source for landscape information production of
  topographic maps
• Aerial camera-1) a lens to focus light on the
  film, 2) light-sensitive film to record image, 3)
  shutter that controls entry of light into the
  camera, and 4) camera body-light-tight enclosure
  to hold film, lens and shutter
• Aerial cameras also include film magazine,
  drive mechanism, and lens cone

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• Lens - formed from a glass disk ground into a
  shape with nonparallel curved surfaces
• Change in optical density causes refraction of
  light rays the sizes, shapes, arrangements, and
  compositions of lenses control bending of light,
  maintain color balance and minimize optical
  distortions.
• Quality of lens-quality of glass, precision with
  camera
• Imperfections-spherical aberration (not a problem
  for modern cameras)
• Aerial cameras use compound lenses, formed from
  many separate lenses to correct for errors in
  single components
• Optical axis joins centers of curvature of the
  two sides of the lens
• Image principle plane-passing through the center
  of the lens is the center of refraction within
  the lens, intersects with the optical axis at the
  nodal point

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• Parallel light rays reflected from an object at
  infinite distance, pass through lens and focus at
  the principal focal point ( image formation of
  distant object)
• Plane passing through the focal point parallel to
  the image principal plane is the focal plane
• For aerial photographs scenes are at large
  distances, focus can be fixed at infinity, with
  no need to change focus of the lens
• Focal length-distance from the center of the lens
  to the focal point (in inches or mm.)
• Focal length not identical at all wavelengths
  source of chromatic aberration ( corrected by
  lens design, else individual colors of an image
  are out of focus in photo)
• Radiation used to form the image is brought to a
  common focal point
• Field stop controls field of view of a lens a
  mask positioned just in front of the focal plane

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• Aperture stop positioned near center of a
  compound lens consists of a mask with a circular
  opening of adjustable diameter
• Controls the intensity of light at the focal
  plane controls only brightness of image without
  changing its size ( FOV)
• Relative aperture f Focal length / Aperture
  size
• Same unit of length for focal length and aperture
  size, f is the f number
• Large f number aperture opening is small
  relative to focal length
• Small f number opening is large relative to
  focal length
• Specification of aperture as f4has meaning for
  cameras of all sizes one fourth of focal length
  for any size camera
• Standard sequence f1, f1.4, f2.8, f4, f5.6, f8,
  f11, f16, f32, f64,. Changes amount of light by
  a factor of 2 as the f stop is changed by one
  position
• Change from f2 to f2.8 halves the amount of light
  entering the camera, from f11 to f8 doubles the
  amount of light a given lens uses only a portion
  of the range of apertures mentioned above

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• The Shutter controls the length of time film is
  exposed to light are metal blades positioned
  between elements of the lens forming intralens
  shutters
• Focal plane shutter consists of a metal or
  fabric curtain positioned just in front of the
  film, near the focal plane curtain has number of
  slits
• Shutter speed selected by operator produces
  desired exposure
• Between-the-lens shutter is preferred in aerial
  cameras
• This intralensshutter subjects the entire
  negative to illumination simultaneously and
  presents a clearly defined perspective allows
  use of image negative as basis for precise
  measurements
• The Film Magazine light tight container holds
  supply of film includes a supply spool, holds
  100s of feet of unexposed film, a take-up spool
  to accept exposed film

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• The Lens cone- supports the lens and filters and
  holds them in correct position
• It is detachable to allow use of different lenses
• Manufacturer alights lens with other components
  for geometric accuracy of photos
• Common focal length for typical aerial cameras
  150mm(6in.), 300 mm, 450 mm
• The Drive Mechanism advances the film after
  each exposure a vacuum platen holds film during
  exposure
• Vacuum platen (aerial cameras) a flat plate
  positioned at the focal plane a vacuum pump
  draws air through small holes in the plate to
  hold the film flat and stationary during exposure
• Vacuum is released after exposure allows film to
  advance for next exposure

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• Kinds of aerial cameras civilian metric or
  cartographic cameras-less geometric and optical
  error high quality images
• Calibrated at laboratories
• Knowledge of internal geometry (focal length,
  flatness of focal plane, etc) enables
  photogrammetrists to make accurate measurements
• Strip cameras film is moved in front of a fixed
  slit that serves as a shutter speed of film
  movement coordinated with speed and altitude of
  aircraft image is a long continuous strip
• Panoramic cameras records a very wide field of
  view
• Lens with a narrow FOV scans across a wide strip
  of land, image formed by side-to-side motion of
  lens as aircraft moves forward
• Photographs show long narrow strip of terrain
  extending perpendicular to flight track from
  horizon to horizon
• Have serious geometric distortions (forward
  motion of aircraft and side-to-side scan of lens)
  require correction before use
• Useful due to large areas, but only central part
  are suitable for detailed interpretation why
  ??

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• Black-and-White Aerial Films 1700s and 1800s
  photosensitive chemicals (silver nitrate)-darkens
  when exposed to light.
• First practical means of recording the image a
  silver coated metal plate treated with iodine
  vapor
• 1839 date for beginning of photography
• Early long exposure times, required bright
  light, recorded on metal or glass plates heavy
  and fragile
• Aerial photography taken using balloons or
  kites for elevation of camera no standard
  equipment
• George Eastman modern lightweight photographic
  equipment roll film (Kodak camera in 1888)-mass
  production and standardization
• Modern films sensitive to both visible and
  nonvisible portions of spectrum and represent
  reflectances in much more specific spectral
  regions

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• Major components film base, or support, (thin
  40 to 100?m), flexible, transparent material that
  holds a light sensitive coating (polyester film
  flexible lightweight strips, strong for winding
  and unwinding)
• Base should resist change in size as temp. and
  humidity vary precise image size is needed to
  measure distances in images
• Base-coated with a photographic emulsion (silver
  nitrate)-light changes it to metallic silver
  (dark) darker with increased exposure
• Modern emulsions have small crystals of silver
  halide (silver bromide 95 and silver iodide 5)
  suspended in a gelatin matrix (5 ?m thick)
• Gelatin holds crystals in suspension, allows
  spreading evenly on base (also it is transparent
  and porous absorbs halogen gases allows
  photographic chemicals to contact crystals)

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• Silver halide crystals small irregular in shape
  with sharp edges, favor interception of photons
  that pass into the emulsion
• finer size-gtfiner detail that can be recorded
• coarse grains-gt less detail but greater
  sensitivity to light spatial resolution of film
  inversely related to its speed
• Recent from Kodak-film emulsions with flat grains
  increased surface area
• grains are parallel to film surface, expose large
  surface areas to light, increasing speed without
  decreasing resolution color prints only
• Protective super-coat (from scratches) film
  emulsion coated with thin layer of clear gelatin
• Dust and moisture could affect use cotton
  gloves when handling film !!
• Subbing layer below emulsion ensure emulsion
  adheres to base back side of base has an
  antihalation backing
• absorbs light passing through emulsion and base
  to prevent reflection back to emulsion
• anticurling agent counteracts curling effect of
  emulsion that coats upper side of film)

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• If absent images of bright objects surrounded
  by halos due to reflection!!
• Shutter opens allows light strikes silver
  halide crystals (small area contains many
  thousands of crystals) changes to metallic
  silver, more light more crystals affected
• Image is only latent processing is required
• Development bathing exposed film in alkaline
  chemicals that reduces exposed silver halide
  grains
• Developer - Minimally affected crystals
  completely changed to Ag (silver) amplifying
  pattern
• Areas exposed to most intense light have greater
  density of Ag in final image
• Acid stop allows exact control of time film is
  in contact with developer to counteract effect of
  alkaline developer
• Fixer applied to dissolve, remove unexposed
  silver halide grains
• RESULTING IMAGE NEGATIVE (brightness reversed
  from original values in scene)

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• Film speed measure of sensitivity of emulsion
  to light fast film low intensity of light for
  proper exposure slow film - more light - gt
  aperture opened wider (or longer exposure time)
• DIN and ASA ratings assess speed of films for
  hand-held cameras
• Aerial films aerial film speed (AFS) and aerial
  exposure index (AEI)
• Contrast range of gray tones recorded by film
• High contrast largely in black and white
• Low contrast representation largely in grays
  with few really dark and bright tones (
  preferable why ??)
• Fine grained emulsion have low contrast (slower
  films higher spatial resolution and low contrast
  than coarser-grained fast films)
• Emulsions on photographic papers -gt higher
  contrast than emulsions on films (interpreters
  prefer film transparencies)

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• Spectral sensitivity records spectral region to
  which film is sensitive
• Figure spectral sensitivity curve for Kodak
  Tri-X Aerographic Film 2403 features of B/W
  films
• Sensitive throughout visible spectrum and UV
  radiation
• Due to scattering of shorter (UV and blue)
  wavelengths, filters are used with B/W aerial
  films
• Figure B/W view of scene in accordance with our
  view of panchromatic film sensitive to visible
  spectrum (HVS)
• Orthochromatic films with sensitivity in blue,
  green and peak sensitivity in green
• Figure spectral sensitivity curve for Kodak
  Infrared Aerographic Film 2424, B/W IR film
• Desirable to exclude visible radiation film used
  with a deep red filter that blocks visible
  radiation but allows IR radiation to pass
• Figure image by IR B/W film different from the
  visible spectrum image
• Eg., living vegetation is much brighter in near
  IR portion than in visible portion (hence are
  bright white on B/W IR image)

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• The Characteristic Curve in the negative
  pattern of dark and light related to patterns of
  metallic silver formed in processed film (bright
  scenes are darker)
• Intermediate shades of brightness as crystals in
  emulsion are much smaller than the human eye can
  resolve
• Shades of gray variations in abundance of tiny
  grains of silver in processed film
• Crystal present (black) or
• absent (dark),
• shades of gray (variation in abundance of
  crystals proportional to brightness of original
  scene)
• Shine light intensity Io,
• through a small area of the negative fraction
  of a mm in diameter,
• brightness of light measured on other side I is
  a measure of darkness of that region of the film,
  I is lt Io
• Opacity I/Io
• Density darkness of film log10 (opacity)

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• Effect of light on emulsion of a film product
  of intensity ,i (brightnessequivalent to
  irradiance) and time t.
• E i x t
• Used to compensate for use of a fast shutter
  speed by opening the aperture to allow more light
  to enter
• Figure Characteristic curve relationship
  between brightness in scene and density on the
  film
• (or relationship between density of a negative
  and log10 of exposure) has a typical S-shape
• Toe lower part is curved
• Straight-line segment center part (is of
  interest)
• Shoulder upper portion is curved
• If exposure is increased by a certain amount,
  curve allows prediction of corresponding increase
  in image density
• Image scientists with suitable controls, can
  use measurements from photography to learn about
  brightnesses in scene
• Figure difference in slope translates a given
  brightness range (BR) into different density
  ranges (DR) in final processed image

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• Steep slope small range in exposure -gt big
  range in density (is a high contrast image-
  displays large range in brightness)
• Shallow slope range in scene brightness -gtsmall
  range in image brightness low contrast
• Toe and shoulder no consistent relationship
  for very high and very low exposures, film will
  not produce predictable densities compared to
  straight line segment
• Used for artistic effect in photographs(!!)
  scientists avoid use of these image measurements
  based on very high or very low image densities
  have unknown relationship to brightness in scene
• Photographic films used with nonphotographic
  sensors knowledge of characteristic curve
  important in RS
• Sensors record large range of brightnesses may
  exceed capability of film Very dark areas or
  very bright areas or both will be represented in
  nonlinear part of the curve only a small portion
  showing actual brightness
• Hence, DIGITAL IMAGE ANALYSES that does not rely
  on film images are ADVANTAGEOUS

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• Optical distortions- caused by inferior camera
  lens, malfunction, minor significance in modern
  photography
• Tilt -caused by displacement of focal plane from
  a truly horizontal position due to aircraft
  motion
• Isocenter focus of tilt, located at or near
  principal point
• Image areas on the upper side of tilt are
  displaced further away from ground than is the
  isocenter (smaller scales than nominal scale)
  image areas on the lower side of tilt are
  displaced down (scales larger than nominal scale)
• For measurements select distances as lines that
  pass close to the principal point errors caused
  by upward tilt compensate for errors caused by
  downward tilt (eliminates large errors in areas
  further from principal point)
• Relief displacement - important source of
  positional error in vertical aerial photography
• Objects positioned directly beneath the center of
  the camera lens photographed so that only top of
  object is visible
• For other objects top and sides are visible (lean
  outward from central perspective of camera lens

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• Difference in apparent location due to height
  (relief)-source of positional error
• Amount of displacement depends on (1) height of
  object (2) distance of object from nadir
• Relief displacement is basis of measurement of
  height of objects
• Coverage by multiple photographs
• Series of flight lines provides complete
  coverage of a specific region
• Each flight line consists of individual
  frames-camera operator view the area to be
  photographed through a viewfinder attached to the
  camera
• Drift planes course deflected by crosswind,
  position of photos drifted
• Crab caused by correction of flight path to
  compensate for drift without change in
  orientation of camera

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• If forward overlap is 50 or more, the image of
  the principal point of one photograph is visible
  on the next photograph in flight line conjugate
  principal points
• In coverage of large areas - several parallel
  strips of photography each strip is called
  flight line
• Sidelap between adjacent flight line is about 5
  to 15
• As pilots and other crew members collect
  photographs, there may still be gaps (holidays)
  due to equipment malfunction, navigation errors,
  cloud cover
• Number of photos Length of flight line / (gd of
  photo) x (1 overlap)
• gd ground distance represented on a single
  frame (same units as length of flight line)
• If flight line 33 mi.,
• each photograph represent 3.4 mi. on a side and
  forward overlap is 0.60, then 33/3.4 x
  (1-0.60) 33/(1.36)
• 24.26 or about 25 photographs are required

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• Stereoscopic Parallax difference in appearances
  of objects due to change in perspective
• two photographs of the same area taken from
  different camera positions -gt displacement of
  images of objects from one image to the other
• Distant objects display little or no observable
  parallax
• Used for measuring distance or height
• Overlapping aerial photographs record parallax
  due to shift in position of camera as aircraft
  motion carries camera forward between successive
  exposures.
• If forward overlap 50, entire ground area can
  be viewed in stereo using 3 adjacent frames (a
  stereo triplet)
• Displacement due to stereo parallax is always
  parallel to flight line
• Estimation of height of objects - tops of tall
  objects, nearer to camera show more displacement
  than do shorter objects

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• Measurement of stereoscopic parallax
• X distance between two principal points
• Y distance between separate images of the base
  of the object as represented on the two images
• P X Y
• B top-to-top distance
• A base-to-base distances
• dp A - B
• H Flying height of aircraft
• h Height of object
• h H x dp / P dp
• In practice, parallax measurements made using
  parallax wedge, or parallax bar devices that
  permit accurate measurements of small amounts of
  parallax

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• Mosaics join series of vertical aerial
  photographs that show adjacent regions on ground
  to form a mosaic
• Uncontrolled mosaics placing photos together to
  provide continuous coverage of an area, without
  concern for preservation of consistent scale and
  positional relationships (rough alignment at
  edges) used as aerial index
• Aerial index for identifying particular
  photographs needed for a specific purpose small
  scale overview of a particular region
• Cannot be used for measurements of distance or
  area lack geometric accuracy
• Controlled mosaic individual photographs
  assembled in a manner that preserves correct
  positional relationships between the features
  they represent
• Region near principal point most accurate
  region is cut out and used for mosaic
• Locational control is provided by ground survey
  or from accurate maps (controlled mosaic more
  expensive)

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• Orthophotos and Orthophotomaps aerial
  photographs are not planimetric maps have
  geometric errors- effects of relief displacement
• Stereoscopic photographs used to generate a
  corrected form of aerial photograph known as
  orthophotos without error tilt and relief
  displacement
• Orthophotoscope project a corrected version of
  a small portion of an image exposes each small
  section individually in a manner that corrects
  for the elevation of that small section
• Scans an entire image piece by piece to generate
  corrected version of entire image
• Projection orientation is adjusted to correct for
  tilt
• Instrument continuously varies projection
  distance to correct for relief displacement
• Orthophotomaps used as maps, show correct
  planimetric position and preserve consistent
  scale throughout image

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• Orthophotomaps can be compiled more quickly and
  cheaply than usual topographic maps
• Photogrammetry- science of making accurate
  measurements from photographs, applies principles
  of optics and knowledge of interior geometry of
  camera and its orientation to reconstruct
  dimensions and positions of objects within
  photograph
• Analysis of stereo aerial photography estimate
  topographic elevation (by estimating stereo
  parallax for any array of points within a region
  ) for topographic mapping
• Analytical stereoplotters instrument designed
  in 1920s, reconstruct orientations of photographs
  at the time taken. Operators view image in
  stereo and by visually maintaining constant
  parallax, trace lines of uniform elevation
• Quality of information depends on quality of
  photography, accuracy of data, operators skill
• Now possible to automatic match of corresponding
  points on stereo pairs and thereby identify lines
  of uniform parallax with limited operator
  assistance

37

• Digital photography - electronically scanned to
  record patterns of blacks, grays and whites as
  digital values represent brightness of point
  within image
• Bypass digitization step replace film in focal
  plane with an array of light-sensitive detectors
  that directly record image in digital form
• Softcopy photogrammetry extend automation of
  photographic process to conduct analysis in
  digital domain
• GPS global positioning systems acquire
  accurate positional information and use of data
  recorded from the aircrafts navigational system
  to record the orientations of photographs
• reconstruct geometry of image using above
• Digital or soft copy photogrammetry does not
  require the physical (hardcopy) form of
  photograph
• Image is used as input for a series of
  mathematical models that reconstruct orientation
  of each image to create planimetrically correct
  representations
• Advantage of speed and accuracy and also creates
  output data easily integrated into other
  production and analytical systems including GISs

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• Sources of aerial photography can be acquired
  by (1) user, (2) purchased from organizations
  (archival imagery)
• Produced upon request by specialized firms
• Customers individuals, governmental agencies or
  other businesses that use them
• Photography customized to meet specific needs of
  customers date, scale, film and coverage
• do-it-yourselfaerial photography many small
  cameras (small format 35 mm) are suitable,
  charter aircraft (low cost)
• At low altitude and with clear atmosphere
  ordinary films produce satisfactory results
• Use a high-wing aircraft to ensure clear view of
  landscape
• If camera can use filters, possible to use other
  films (IR or color IR) aim camera away from sun
  !!

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• EROS Data Center (EDC), Sioux Falls, South
  Dakota, operated by USGS repository of aerial
  photographs and satellite images acquired by
  NASA, USGS and other agencies computerized data
  base
• Earth Science Information Centers (ESICs) maps
  and aerial photographs information about federal
  programs and agencies collect data pertaining to
  maps and photographs held by state and local
  governments Reston, Virginia
• Aerial Photography Summary Record System (APSRS)
  maintained by ESIC computer-based information
  system recording detailed information about
  aerial photography help by numerous federal,
  state and private organizations
• National Aerial Photography Program (NAPP) USGS
  manages NAPP
• Flight lines are oriented N-S direction centered
  on each of 4 quadrants systematically positioned
  within USGS 7.5 minute quadrangles, with full
  stereoscopic coverage at 60 forward overlap and
  27 sidelap
• 10 frames acquired in 2 flight lines, provide
  full stereoscopic coverage of each quadrangle
• Camera of 6 in. focal length is used at flying
  altitude of 20,000 ft above terrain, provide
  coverage at 140,000 Black--White or Color film
  is used