Chapter 3 Photogrammetry

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Chapter 3Photogrammetry
Geography 4260Remote Sensing
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• Photogrammetry is the science and technology of
  obtaining spatial measurements and geometrically
  reliable derived products such as maps from
  photographs or digital imagery.
• Photogrammetry was developed around hardcopy
  photographic negatives and prints, but has been
  extended to digital or softcopy photogrammetry.

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• A primary application of photogrammetry is the
  generation of topographic maps from aerial
  photography, but many other applications are
  possible including the production of
  geometrically accurate orthophotographs from
  aerial photos and satellite imagery.

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• Photogrammetry includes
• Determining image scale and making measurements
  of distances from images,
• Making area measurements from images,
• Quantifying the effects of relief displacement
  on vertical aerial images,
• Determination of object heights from image
  relief displacement measurements,
• Determination of object heights and terrain
  elevations by measurements of image parallax

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• Use of ground control points,
• Mapping from aerial images, and
• Flight mission planning necessary to obtain
  cost-effective quality imagery.

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• The use of the term images instead of
  photographs is intentional. Although
  photogrammetry was developed around aerial
  photography, the techniques are applicable to
  scanned photos and other forms of digital
  imagery.
• The notes on the remainder of this section
  generally refer to photographs, but are equally
  applicable to images of all types.

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• Type of Aerial Photographs

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• Aerial photos fall into two basic categories
• Vertical aerial photos, and
• Oblique aerial photos which include the
  subcategories of
• Low oblique, and
• High oblique.

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• Oblique photos are generally less suited for
  photogrammetric purposes than vertical air photos
  because keeping the lens axis as vertical as
  possible minimizes the geometric distortions that
  are found in all aerial photos.
• Although keeping the lens axis exactly vertical
  is impossible, photos acquired when the axis is
  less than 3 from the vertical are considered
  vertical air photos and are suitable for
  photogrammetric purposes.

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• Acquiring Vertical Air Photos

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• Vertical air photos are acquired along generally
  straight flight lines or flight strips over the
  ground surface.
• The line on the ground surface directly below the
  aircrafts path is the nadir line. If the lens
  axis of the camera is perfectly vertical, the
  principal point on a photograph would be
  coincident with the nadir point. However, this is
  generally not the case due to aircraft tilt and
  pitch.

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• Air photos are acquired at intervals along the
  flight line, normally with about 55 to 65 percent
  endlap between every adjacent pair of photos.

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• Adjacent flight lines are arranged so that there
  is also about 30 percent sidelap.

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• Complete stereo coverage requires at least 50
  endlap and that the photos in adjacent flight
  lines at least abut one another.

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• The larger percentages are used to ensure stereo
  coverage and to allow features in the area of
  sidelap to be shown optimally.

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• Each adjacent pair of images along a flight line
  forms a stereopair. The objects seen in the
  endlap area of a stereopair are viewed from
  different angles at the moments of exposure.

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• When stereopairs are viewed through a
  stereoscope, the viewer sees a three-dimensional
  stereomodel of within the area of overlap.

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• Using aerial stereopairs allows us greatly
  exaggerate the stereo effect by effectively
  separating our eyes by the air distance between
  exposure.

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• The ground distance between adjacent photo
  stations is referred to as the air base.
• The ratio of air base to flying height determines
  the amount of vertical exaggeration seen in the
  stereomodel.

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• How can vertical exaggeration be increased?
• How can it be decreased?

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• The Geometry of Vertical Air Photos

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• Aerial cameras image the ground surface on a flat
  plane that is above the lens at a distance equal
  to the focal length of the lens (f).
• The focal plane is perpendicular to the lens axis.

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• The lens axis is oriented with 3 of the nadir
  point which is where a vertical line extending
  from the center of the lens system intersects the
  ground surface.

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• The intersection of the lens axis with the ground
  surface defines the location of the principal
  point of the photograph.

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• The ground position of this point in the image is
  defined as the ground principal point.
• It is rarely exactly coincident with the nadir
  point because of aircraft tilt.

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• Light rays from objects on the ground pass
  through the lens and focus an image of the ground
  surface on the film plane.

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• Each part of the scene is exposed to light
  collected through the whole aperture, not just
  through the center of the lens.

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• The second plane shown below the lens is the
  print plane.

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• Straight lines drawn from objects on the ground
  surface through the center of the lens pass
  through the print plane and determine the
  positions of objects in the negative used to
  produce the print.

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• Because most positive prints used for
  photogrammetry are contact prints, the print
  plane lies below the center of the lens at a
  distance equal to the focal length of the lens.

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• A Cartesian coordinate system can be defined for
  a positive print by connecting opposite fiducial
  marks with straight lines.

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• The x axis of the coordinate system is
  arbitrarily assigned to the fiducial line most
  nearly parallel to the flight line, and the
  origin of the coordinate system is located at the
  principal point of the photo.

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• X coordinates increase in the direction of flight
  and y coordinates increase 90 to the left
  (counterclockwise) of the x axis.

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• Once the coordinate system is defined, the
  location of any point in the image can be
  specified by its x,y coordinate pair.

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• In softcopy photogrammetry, the row and column
  positions of objects can be converted into
  photocoordinates through computer conversion
  algorithms, most commonly an affine coordinate
  transformation.
• This process is similar to fitting a digitized
  map into a mapping coordinate system with a GIS.

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• Photographic Scale

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• Photographic scale is the ratio of a distance in
  an aerial photo to the same distance on the
  ground surface using the same units of
  measurements
• S photo scale photo distance / ground
  distance

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• Photographic scale is usually expressed as a
  representative fraction by dividing both the
  numerator and the denominator of the scale
  expression by the numerator of the expression.
• This reduces the numerator to 1 and the result
  is written as a ratio, e.g. 110,843.

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• What is the local scale of an aerial photography
  if the measured distance between two road
  intersections that are known to be one mile apart
  is 3.52 inches?
• If the photo distance between a water tower and
  one of these road intersections is 0.25 inches,
  how many feet is the tower from the intersection?

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• Because the original measurements are in the same
  units, the scale is dimensionless.
• This makes it possible to determine ground
  distances from photo distances in whatever units
  the photogrammetrist chooses.

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• Photo scales determined by comparing photo
  distances to ground distances are strictly
  applicable only between the pairs of points used
  for the measurements.
• Photo scale commonly varies widely even along a
  line between two points. Therefore, it should be
  determined using known points located as close as
  possible to the end points of the distances we
  wish to estimate.

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• Photo scale can also be determined over areas of
  flat terrain because it is also equal to the
  ratio between lens focal length (f) and flying
  height above the terrain (H)
• Scale f/H
• H is easily determined by subtracting the
  terrain elevation (h) from the aircrafts
  altitude (H).

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• The scale equation on the preceding slide is
  derived from the geometric relationships shown in
  this diagram.

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• What would be the nominal scale of photography
  acquired with a 6 lens over uniform terrain at
  an elevation of 880 above sea level is photos
  are acquired from an aircraft altitude of 6,080?
• What scale would result if a 3.5 lens was used
  instead?
• Which of these lenses would produce the larger
  scale imagery?

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• We can conclude from the relationship between
  scale and flying height above the terrain that
  variations in terrain elevation produce spatial
  variations in the scale of a photograph of the
  terrain.
• Only absolutely vertical air photographs over
  absolutely flat terrain have uniform scales
  across their extents. Even tilts less than the
  nominal 3 tilt allowed for vertical air photos
  produce measurable scale variations.

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• The spatial variations in the scale produced by
  variations in terrain elevations produce
  geometric distortions in aerial photographs that
  are not present in maps.

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• Features shown on maps that have little or no
  scale distortion are in their correct planimetric
  positions relative to one another.
• An aerial photograph, however, distorts both the
  distances and the directions between features.

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• These distortions in aerial photographs are a
  result of the fact that aerial photographs are
  perspective projections while maps are
  orthographic projections.

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• In an orthographic projection, features are
  projected from the Earths surface onto the
  mapping surface at 90 angles.

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• In a perspective projection, features are
  projected from the Earths surface onto the
  mapping surface at angles that converge on a
  point, i.e. the center of the cameras lens
  system.

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• Perspective projection causes objects that are
  closer to the camera to be larger and to be
  displaced radially outward from the principal
  point of the image.
• This relief displacement affects the positions of
  terrain features, but it also causes the vertical
  objects to lean outward from the center of the
  image.

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• Relief displacement is most pronounced on low
  altitude, wide angle images that include tall
  objects near their edges.

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• This image is a reduced resolution version of the
  original.
• Click on the image to view a higher resolution
  version.

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• Measuring Areas on Aerial Photographs

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• If the distortions introduced by relief
  displacement and camera tilt are small enough to
  be ignored, ground distances can be measured on
  an aerial photograph of known scale.
• It is also possible to measure the size of areal
  units from the photo if these distortions are
  negligible.

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• The areas of regular geometric shapes are most
  easily measured from areal photos because linear
  measurements of the dimensions of the features
  can be converted to areal measurements through
  simply geometry.

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• Irregular shapes are more difficult to measure,
  but a variety of tools are available.

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• Most simply, a transparent square grid or dot
  grid can be laid down over the photo.
• The number of grid cells or dots falling within
  the area being measured is converted to an area
  through a knowledge of the scale relationships.

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• If more precise measurements of many features are
  needed, it is more convenient to use a coordinate
  digitizer to outline each area and let an
  image-processing program or GIS calculate the
  area of each feature.

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• The Geometry of Relief Displacement

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• Figure 3.13 illustrates the geometric components
  of relief displacement and leads to a method to
  determine the heights of objects from
  measurements of relief displacement on single
  aerial photographs.

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• The lower plane is a horizontal datum which
  intersects the base of the feature whose height
  is to be determined.
• The upper plane represents a vertical air photo
  of the feature.

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• The photo was taken from photo station L at
  height H above the datum plane.
• The tower is height h above the datum plane.

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• The top of the tower at point A is imaged at
  point a having been radially displaced from its
  true planimetric position which is coincident
  with its base at point A.

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• The photo displace-ment (d) can be measured from
  the photo.
• The distance from the principal point to the top
  of the tower (r) can also be measured from the
  photo.

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• Both d and r can be projected to datum from the
  photo to determine equivalent ground distances.

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• Because triangles AAA and LOA are similar
  triangles, the ratios of their vertical and
  horizontal lengths are identical
• D/h R/H

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• Using photo distances instead of ground distances
  allows us to express the same relationship as
• d/h r/H

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• Rearranging the terms of d/h r/H to solve for d
  yields
• d rh/H

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• d rh/H
• This equation indicates that relief displacement
  is directly proportional to the height of the
  tower and the distance from the ground principal
  point for a given flying height.

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• Determining Object Heights from Relief
  Displacement

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• Solving d rh/H for h yields an equation that
  can be used to measure the height of an object
  from a single vertical air photo if we know the
  flying height of the aircraft above the base of
  the object
• h dH/r

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• This equation (h dH/r) requires that we know
  the flying height of the aircraft above the base
  of the object (or can determine it), but the
  other two variables are easily measured on the
  photo.

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• This accuracy of the method depends on the
  assumptions that were used to derive it
• The lens axis must be truly vertical,
• The aircraft altitude must be precisely known,
• The terrain elevation at the base of the object
  must be precisely known,
• The objects base and top must both be clearly
  visible, and
• The position of the photos principal point must
  be accurately determined.

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• If these assumptions are true, then very accurate
  height measurements are possible from single air
  photos.
• However, significant camera tilt at the moment of
  exposure or imprecise values for any of the
  required measurements introduce errors that
  produce imprecise results.

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• Image Parallax

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• If stereo images are available, then the parallax
  that was introduced by viewing objects from two
  different locations can be used to determine both
  object heights and the locations of objects in a
  ground coordinate system.

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• Figure 3.15 illustrates the effects of aircraft
  movement on the positions of objects that are
  visible in both photos of a stereopair.
• Both the absolute and relative photo positions of
  points A and B are affected by parallax.

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• However, parallax displace-ment occurs only
  parallel to the line of flight.
• Although the line of flight normally lies close
  to the x-axis formed by connecting on pair of a
  photos fiducial marks, it is usually not
  coincident.

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• The lack of coincident results primarily from the
  fact that any crosswind forces the pilot to turn
  the nose of the airplane slightly into the wind
  to maintain a given track over the ground, but
  imprecise navigation can also contribute to
  variations in the airplanes path over the ground.

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• The actual path of the aircraft, however, can be
  located in a photograph that is part of a
  stereopair if the photo location of the
  stereopairs other component can be identified in
  the photo.

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• The actual flight paths determined by locating
  the conjugate principal point in both photographs
  can be used to define the x axis of a Cartesian
  coordinate system which can then be used to
  measure parallax (which is parallel to the
  direction of this axis).

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• A points parallax is expressed as
• pa xa xa
• pa parallax of point A,
• xa x coordinate of a on the left photo, and
• xa x coordinate of a on the right photo.

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• Notice that xa has a negative value in this
  situation.
• The signs of coordinates must be used correctly
  to avoid errors when these values are used later
  to determine ground coordinates and elevations.

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• One additional variable must be known to make use
  geometric relationships show in this diagram,
  i.e. the air base (B) which is the distance that
  separates the two exposure stations.

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• B can be determined from GPS positions obtained
  at the time of exposure, or it can be estimated
  as the average distance between the principal
  point and the conjugated point of each photo in
  the stereopair times the average scale factor of
  the photos.

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• The relationships shown in this diagram can be
  used to along with the value of B to derive two
  important equations known as the parallax
  equations
• XA B(xa/pa)
• and
• YA B(ya/pa)

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• The parallax equations allow us to calculate x,y
  coordinate positions in a map coordinate system
  from measurements made on stereopairs.
• XA B(xa/pa)
• and
• YA B(ya/pa)

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• Making Parallax Measurements

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• Parallax displacement results in the apparent
  positions of objects being displaced parallel to
  the flight line and proportionate to the
  elevation. This section deals with measuring the
  total amount of displacement.

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• Although parallax measurements can be made
  directly from the photographs by laying out the
  flightline coordinate system and carefully
  measuring distances with a finely-divided scale,
  it is necessary to determine the x-coordinate of
  each position separately and then take their
  algebraic difference.
• There are other methods available to measure
  parallel that greatly simplify the task.

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• One method is accomplished by taping the two
  photographs of a stereopair to a table in such a
  way that the flight lines of the photos are
  coincident.

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• Under these circumstances x x is equal to D
  d. Because D is fixed, it is then only necessary
  to measure d to determine p.

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• Parallax measurements can be made with any linear
  measuring device, but a parallax bar is
  specifically designed to make very precise
  measurements of parallax.

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• A parallax bar is a finely-calibrated scale with
  a vernier scale that allows very precise
  measurements of the distances separating objects
  on stereopairs.

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• Various other devices based on this principal
  facilitate this process by providing a stereo
  view, magnification, and a floating mark.
• A floating mark is line that appears to intersect
  the surface of a stereo model or spot of light
  that appears to hover above, below, or on the
  surface at the point whose parallax is to be
  determined.

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• This diagram illustrates the stereoscopic
  principals involved.

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• After the images are positioned and D has been
  determined, the user adjusts the apparent
  position of the floating mark so that it
  intersects or lies precisely on the surface at
  the point in question.
• The user can then read the points parallax from
  a finely-calibrated scale or a digital readout.

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• A parallax wedge is another tool designed to be
  used with a stereoscope after the images have
  been aligned.

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• Under a stereoscope, the pairs of converging
  lines on the parallax wedge appear as a single
  sloping line passing through the surface of the
  stereomodel at the location of the point whose
  parallax is to be determined when the wedge is
  properly positioned.

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• The millimeter scales next to each pair of
  converging lines appear as a single scale when
  the photos are positioned under the stereoscope
  as described earlier.
• This allows the user to read a value where the
  line appears to intersect the surface.

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• The difference between values read at the top and
  bottom of the same stereoscopic object is the
  parallax of the object in millimeters.
• The parallax equations can then be used to
  determine the location and height of the object.

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• This particular parallax wedge also includes
  linear scales along the side, a protractor along
  the bottom edge, and a square-dot grid above the
  protractor.
• At a retail price of 11.50, you might also
  expect it to be gold plated.

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• Parallax measurements can also be taken from
  digital images using softcopy photogrammetric
  techniques.
• These techniques have the advantage that a
  computer can compare the pixels within a small
  area to maximize the correlation between pixel
  values within the area and precisely determine
  the parallax of the central pixel.

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• They also have the advantage that a computer
  solve a set of collinearity equations to
  compensate for variations in aircraft altitude,
  pitch, bank and yaw at the moment of exposure for
  each photo.
• This allows the software to recreate the geometry
  of each photosite at the moment of exposure and
  compensate for factors that are difficult (but
  not impossible) to compensate for using manual
  techniques.

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• Ground Control

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• Ground control points are one type of reference
  data.
• Ground control points are locations on the ground
  surface whose coordinates are known in a mapping
  coordinate system (or whose coordinates can be
  determined) and whose location within an aerial
  photograph can be precisely determined.

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• Ground control points can include natural or
  constructed features such as stream junctions or
  the centers of road intersections, or they can be
  permanent survey markers or other surveyed points
  whose position is marked during photo acquisition
  by painted targets or fabric materials that
  contrast with the background around them.

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• Because ground control points can be located in
  both the ground coordinate system and the photo
  coordinate system, they allow other points seen
  only in an air photo stereo model to be assigned
  coordinates in the ground coordinate system.
• Ground control points can be horizontal control
  points, vertical control points, or both.

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• Although it is possible to locate and mark
  surveyed control points prior to an aerial
  photography mission, previously surveyed points
  are few and far between in many areas.
• Therefore, additional points, now including GPS
  control points, are often established and marked
  to increase the density of well-defined control
  points.

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• Virtually all photogrammetric techniques rely on
  accurate ground control. Therefore, identifying
  and marking control points prior to flight is a
  critical stage in flight planning.
• It does no good to locate the ground position of
  a flight line on an aerial photo or on the ground
  unless the ground coordinates of the end points
  of the line can be accurately determined.

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• However, it is also possible to identify points
  in existing photos that can be located on the
  ground and to determine the locations of those by
  GPS or other survey techniques.
• Establishing ground control after the flight
  mission avoids the possibility that a previously
  marked ground control point cant be easily
  located in one or more of the photographs.

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• Methods to simplify ground control by using GPS
  receivers to simultaneously acquire data at a
  single well-defined control point and onboard the
  aircraft are currently being tested.
• These methods require that the aircraft attitude
  at the moment of each exposure be known in
  addition to its three-dimensional location.
  However, that data can be acquired with an
  inertial measurement unit (IMU).

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• An inertial measurement unit is an
  electro-mechanical device that simultaneously
  detects and records changes in acceleration and
  deceleration, including changes in aircraft
  pitch, bank and roll.
• The data from an IMU can be used to determine the
  exact location and orientation of the cameras
  lens axis and film plane at the moment of each
  exposure.

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• If both the location and attitude of the aircraft
  are known, there is no need for ground control
  except at the single location used during the
  flight mission.

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• Mapping with Aerial Photography

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• Photogrammetric mapping applications include the
  production of
• Contour maps through stereoscopic plotting,
• Orthophotos and digital orthophotos,
• Perspective views (three-dimensional block
  diagrams), and
• GIS data layers from aerial photos and digital
  images.

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• Stereoscopic Plotting

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• The primary purpose of stereoscopic plotting is
  the production of contour maps from aerial
  photographs.
• Originally, stereoplotter were precision analog
  instruments designed specifically to generate
  contour lines from stereo images. More modern
  analytical plotters incorporate digital methods.

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• Different types of stereoplotters use different
  projection systems to project stereopairs to
  create a stereomodel of the terrain
• Direct optical projection,
• Mechanical or optical-mechanical projection,
• Projections based on mathematical models
  including analytical plotters and softcopy
  plotters.
• The following slides describe the use of a
  stereoplotter using direct optical projection.

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• A direct optical stereoplotter generates a
  three-dimensional image of the terrain from a
  stereopair of photographs or diapositives.

A diapositive is a positive film image. The use
of a diapositive allows the operator of a
stereoplotter to view a positive stereomodel.
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• Most stereoplotters create a stereomodel by
  projecting diapositives from two separate
  projectors onto a map manuscript.
• The illustration shows a Kelsh plotter, an
  industry standard for many years.

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• The orientation of the diapositives, including
  their separation, tilt direction and tilt angle
  can be independently controlled so that they can
  be oriented in the same manner as the film
  material used to create them was oriented at the
  time of exposure.

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• Their positions of the diapositives can also be
  adjusted so that the locations of control points
  that were placed on the manuscript map coincide
  with the locations of the same control points in
  each image.

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• The stereoplotter operator then uses a viewing
  system that creates the impression of a
  three-dimensional view of the terrain in the
  stereopairs area of overlap.

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• Stereo vision is accomplished by one of several
  methods, all of which allow the operator to see
  only the left image with his left eye and the
  right image with his right eye
• Anaglyphic viewing,
• Viewing with a stereo-image alternator,
• Polarized-platen viewing, and
• Binocular viewing with lenses, mirrors and
  prisms.

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• An anaglyphic viewing system works by projecting
  the monochromatic images through
  complementary-colored filters, usually cyan and
  red, and viewing the image through eyeglass
  lenses of the same colors.

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• The cyan filter blocks red light and the red
  filter blocks cyan light. As a result, the
  operator can only see the cyan image through the
  cyan lens of the eyeglasses and the red image
  through the red lens.

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• A stereo-image alternator works by alternately
  projecting the left and right images in rapid
  succession while simultaneously blocking the
  right or left eye of the viewer through a shutter
  system.
• A binocular system combines two separate views
  through a system of mirrors and prisms that allow
  the viewer to see a stereo view through a pair of
  lenses.

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• With softcopy stereoplotters, a computer image of
  a digitized air photo or an image produced by
  digital methods is viewed in stereo on a computer
  screen.
• The end result of each of these viewing systems
  is the same -- a stereo view of the original
  scene.

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• While viewing the stereomodel, the operator can
  use a measurement tool that projects two spots of
  light onto the stereomodel whose spacing can be
  adjusted by the operator.

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• The operators stereo vision causes these two
  spots to merge into one so that the operator sees
  a single point of light which appears to rest
  directly on the models surface or to float above
  or below the surface.

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• If the point of light is not precisely on the
  surface, the operator can adjust the spacing
  between the two points so that the stereo point
  moves up or down until it appears to rest on the
  surface.

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• With the point on the surface, the elevation of
  the surface can be read from a scale on the
  measuring device.
• If the map manuscript is then moved horizontally,
  its horizontal position in the stereomodel
  appears to shift.

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• By shifting the manuscript while keeping the
  point on the surface, the operator can trace a
  horizontal line throughout the stereomodel, thus
  producing a contour at that elevation on the
  manuscript map.

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• Optical and optical-mechanical stereoplotters
  allow the user to see the separate components of
  a stereomodel through a binocular system to
  create a stereomodel rather than projecting
  images onto a surface.

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• These types of stereoplotters reduce the
  distortions that are inherent in the projection
  process and are therefore capable of higher
  accuracy.

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• Analytical stereoplotters also allow the user to
  see the separate components of a stereomodel
  through a binocular system, but they determine
  the positions of features through mathematical
  simulation rather than an optical system.

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• Analytical stereoplotters use hardcopy images,
  but the operator inputs the locations of the
  fiducial marks and other control points on the
  photo.

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• The computer then uses these data to orient the
  photos, after which the operator can determine
  the ground coordinates of additional points seen
  in the photos.

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• These more modern systems further reduce
  distortions and are therefore capable of even
  higher accuracy than optical and
  optical-mechanical stereoplotters.

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• Softcopy stereoplotters use the same principles
  used with older stereoplotters, but they use
  digital images rather than hardcopy negatives,
  diapositives or paper prints.

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• Photogrammetric Workstations

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• Photogrammetric workstations are rapidly
  replacing stereoplotters because they are capable
  of performing functions in addition to generating
  contour lines .

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• Photogrammetric workstations integrate the
  hardware and software necessary to perform
  analytical stereoplotting, but they can also
  create
• Digital elevation models (DEMs),
• Orthophotographs,
• Perspective views, and
• Fly-through of perspective views.
• Photogrammetric workstations can also capture
  data to be input to geographic information
  systems.

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• This is a perspective view created by draping an
  digitized aerial photograph over a perspective
  view created with a DEM of the same area.

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• Orthophotographs

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• Orthophotographs are photographic images that
  have been modified to eliminate the geometric
  distortions that are inherent in the perspective
  views obtained with aerial cameras.
• The process of creating an orthophotograph is
  known as differential rectification and can be
  performed by either analog or digital techniques.

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• Orthophotograph lack topographic information, but
  they have a constant scale and features are
  located in their correct relative positions.

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• If the geometric relationships between the image
  coordinate system and a real world coordinate
  system are known, a digital orthophoto can be
  displayed in a mapping coordinate system.

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• An orthophotomap is an orthophoto overlaid with a
  grid associated with a mapping coordinate system.

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• Topographic orthophotomaps overlay topographic
  contours onto an orthophotograph so that
  elevations can be read from a geometrically-correc
  t image of the Earths surface.

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• The relief displacement that makes stereoscopic
  viewing of normal stereopairs possible is removed
  during the creation of an orthophoto.
• However, the orthophoto production process
  (described in the text) creates a digital
  elevation model that can be used to introduce
  distortions into an orthophoto to create a
  stereomate that can be used with the original
  orthophoto to produce a stereomodel.

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• The existence of a stereomate makes it possible
  to have the best of both worlds a geometrically
  correct orthophoto and the ability to generate a
  stereomodel.
• Orthophotos are a particularly valuable data
  source for geographic information systems.

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• Next Chapter 3
• Visual Image Interpretation