Photogrammetry

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Photogrammetry

• CE 426

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Introduction

• Definition of Photogrammetry the art, science,
  and technology of obtaining information about
  physical objects and the environment by
  photographic and electromagnetic images.

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Basic Information

• Mapping from aerial photos is the best mapping
  procedure yet developed for most large projects.
• Used successfully for maps varying in scale from
  11,000,000 1120 with contour intervals as
  small as 1 foot.
• Topographic mapping is the most common form.
  U.S.G.S updated and done this way.
• Used to reconstruct a scaled 3-dimensional
  optical model of the lands surface using a
  stereoplotter.

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Basic Information

• Uses Aerial photos
• Aid geological investigations, soil surveys,
  land surveys, tax mapping, reconnaissance and
  military intelligence, urban and regional
  development, transportation system
  investigations, quantity estimates, shore
  erosion, etc.
• Mathematical methods have been developed to make
  precise 3-dimensional measurements from photos.
• Phototriangulation 3-dimensional positioning of
  survey stations.

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Basic Information Continued

• Photo has been used to take geometric
  measurements of human bodies, artificial human
  hearts, large radio telescopes, ships, dams,
  buildings and very accurate reproductions.
• In general it is not economical for small
  projects the cost break even point is somewhere
  between 30 100 acres depending on the situation.

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Basic Information

• Photogrammetry can not be used successfully over
  the following types of terrain.
• Desert or plains areas, sandy beaches, and snow
  the photograph as uniform shades with little
  texture.
• Deep canyons or high buildings that conceal
  ground surface.
• Areas covered by dense forest.

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2 Basic Categories

• Metrical photogrammetry obtaining measurements
  from photos from which ground positions,
  elevations, distances, areas, and volumes can be
  computed and topographic or planimetric maps can
  be made.
• Photo interpretation evaluation of existing
  features in a qualitative manner.

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Types of Photogrammetry

• Aerial series of photographs of an area of
  terrain in sequence using a precision camera.
• Terrestrial photos taken from a fixed and
  usually known position on or near the ground with
  the camera axis horizontal or nearly so.
• Close range camera close to object being
  observed. Most often used when direct measurement
  is impractical.

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History

• The first use of photogrammetry was by Arago, a
  French geodesist, in 1840. This included
  topographic and terrestrial.
• The first aerial photogrammetry was by the French
  in 1849 using kites and balloons.
• Laussedat (French) father of photogrammetry.
• 1st in N. America Deville, Surveyor General of
  Canada.
• U.S.G.S. adopted photogrammetry as mapping
  process in 1894 mapping border between Canada
  and Alaska.

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History

• Airplanes brought great change to photogrammetry.
• 1st used in 1913.
• Used extensively in WWI photo interpretation.
• Used in WWII mapping for recon and
  intelligence.
• WWII 1960 used often, expensive and accuracy
  problems for engineering design.
• After mid 60s advent of computer and plotting
  has made photogrammetric mapping accurate and
  affordable.

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Photogrammetry for Engineering

• Defined Photogrammetry is the process of
  measuring images on a photograph.
• Modern photogrammetry also uses radar imaging,
  radiant electromagnetic energy detection and
  x-ray imaging called remote sensing.

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Basic Categories of Photogrammetric Interpretation

• Metrical Photogrammetry obtaining measurements
  from photos from which ground positions,
  elevations, distances, areas and volumes can be
  computed and topographic or planimetric maps can
  be made.
• Photo interpretation evaluation of existing
  features in a qualitative manner timber stands,
  water pollution, soils, geological formations,
  crops, and military interpretation.

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Geometry of Photographs

• Orthographic projection each point projected
  normal to reference plane.
• Perspective projection each point projected
  through a central point, due to points being at
  different elevations, they look 3 dimensional.
• Principal point (center of photo) located at
  the intersection of lines joining the Fiducial
  points.

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• To perform computations, one must know
• H height above datum from which photos taken.
• f focal length of camera lens either in
• in or mm.
• Items on photo
• Fiducial points
• Date
• Roll and Photo

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Scale of a Vertical Photo

• S or
• f focal length 6 or 152.4 mm is common
• H height of plane above ground
• h height (elevation) of ground
• H height of place above datum altimeter
  reading (2 error)

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Scale of a Vertical Photo

• Datum Scale the scale which would be effective
  over entire photo if all points were projected
  downward to datum.
• SD
• Average Scale for photo planning
• SAV.
• Average elevation can be determined for USGS
  topo maps, etc.

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Relief Displacement

• Relief Displacement exists because photos are a
  perspective projection.
• Use this to determine the height of object
• h
• h height of object
• d radial distance to top of object-radial
  distance to bottom of object.
• r radial distance to top of object.

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Planning and Executing Photo Project

• Basic Overall Process
• Photography obtain suitable photos.
• Control obtain sufficient control through field
  surveys and/or extension by photographic methods.
• Map Compilation plotting of planimetric and/or
  topographic features.
• Map Completion map editing and special field
  surveys.
• Final Map Drafting

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Elements of Planning

• Conversion of requirements to project specs.
• Factors
• Purpose of photogrammetry
• Majority of projects for engineering involves
  making topographic map in a stereoscopic plotting
  unit.
• Wide angle photography (152mm focal length) is
  required for topographic mapping because it
  provides better vertical accuracy.
• If area is heavily wooded, use f210mm (standard
  angle) to allow more visibility through trees.
• Generally 60 overlap with 15-30 sidelap.
• Orientation of flightlines is dictated more by
  economy than geometric considerations.

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Elements of Planning

• Photos for mosaics should be flown as high as
  possible.
• Reduces relief displacement.
• Orthophotos similar to topo maps, however,
  should be taken normal to ground topo.
• Photo Scale somewhat dependent on type of
  plotter.
• Essentially can be dependent on type of plotter
  you need to see and dividing it by the resolving
  power of the photo equipment.
• Also affected by map accuracy and area
  configuration.

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Elements of Planning

• Allowed scale variation.
• Variation caused by difference in ground
  elevation and flying height.
• Longer focal length reduces scale variation.
• If flying height remains constant and ground
  elevation increases the area covered by photo
  becomes less.
• Overlap becomes less
• Viewfinder needed to control overlap and
  flightline spacing, thus eliminating possible
  gaps.
• Relief displacement
• Affects mosaics most.
• Large amount of relief displacement will make it
  difficult to form continuous picture desired in
  mosaics.

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Elements of Planning

• Relief displacement decreases as flying height
  increases, the focal length must also be
  increased.
• Relief displacement has no adverse affect on map
  making with stereo.
• With greater relief displacement, elevations can
  be measured and plotted more accurately.
• Tilt
• Amount in direction of flight (y tilt).
• Will cause overlap to be greater on one end than
  other.
• Amount normal direction of flight (x tilt).
• Will increase sidelap on one side and decrease on
  other.
• Y tilt corrected by viewfinder.
• X tilt corrected by increasing planned sidelap.

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Elements of Planning

• Crab and Drift
• Crab angle formed between flightline and edges
  of photo in direction of flight and caused by not
  having focal plane square with direction of
  flight at time of exposure.
• Corrected by rotation of camera on vertical axis
  through viewfinder.
• Reduces coverage, but sidelap compensates.
• Drift plane not staying on flightline.
• Most common cause of re-flights and gaps.

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Elements of Planning

• Flying height determined after sidelap and
  overlap determined.
• Factors affecting
• Desired scale, relief displacement, and tilt.
• Precision of equipment used.
• Greater precision, greater possible flying
  height.
• By doubling flying height, ground coverage
  increased 4 times, thus less ground control and
  fewer photos.
• Vertical accuracy most important in topographic
  mapping.
• Flying height is related to contour interval
  desired.
• Relationship called C-factor (precision factor)
• Flying height desired contour interval x
  C-factor
• C-factor is the value used to compute flying
  height which will produce photos satisfactory to
  obtain the desired vertical accuracy of the maps.

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Elements of Planning

• Direction or orientation of terrain
• Arrange to fly along ridges, not across.
• Gathering material and people.
• Existing photos, maps, survey data, instruments
  and personnel.
• Determine specifications and conditions for
  operation.
• Preparing final plans.
• Scheduling
• Surveying instructions
• Cost estimating and replanning.

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Flight Design

• Considerations
• Project boundaries
• Existing and planned control
• Time schedule
• Final product needed
• Optimum flying season
• Found cover conditions
• Objectives
• Determine optimum conditions for spacing of
  photos along flightlines.
• Number and spacing of fligtlines to cover area.
• Plan must account for allowable deviations.
• Distance between flightlines on fllightway.

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Flight Design

• Flight Patterns
• Totally dependent on overlap and sidelap.
• Under ideal conditions with 9x 9 photo with 6
  focal length, and overlap of 57, and sidelap of
  13 will provide maximum stereo coverage with no
  gaps.
• If additional safety factor desired, overlap can
  be increased to 70-75 and sidelap can be
  increased to 50.

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Computation of Flight Plan

• Data required to compute flight map lines, time
  interval between exposures, and amount of film
  needed.
• Focal length of camera.
• Flying height above datum or photo scale for
  certain elevation.
• Size of photo.
• Size of area to be photographed.
• Positions of outer flight lines with respect to
  boundary.
• Overlap.
• Sidelap.
• Scale of flight map.
• Ground speed of aircraft.

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Example

• Area 15 miles N-S 8.5 miles E-W
• Photos 9 x 9
• Save tobe 112000 _at_ 700 above elevation
• Overlap 60
• Sidelap 35
• Ground speed of plane 150 mph
• Flight lines to be laid out N-S on a map _at_ a
  scale of 162500
• Outer flight lines coincide with E W boundary

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• Flying Height
• 12000 above 700 or 12700 above sea level
• Ground Distance Between Flight lines since
  sidelap is 35, photo distance between lines is
  65 of 95.85
• Number of flight lines
• Total width 8.5 miles x 5280 44880
• flight lines (Round up)
• Adjust ground distance between flight lines
• Spacing of flight lines on flight map
• 5610 on map _at_ 162500 scale

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• Ground Distance Between Exposures with 60
  overlap gain on
• each photo is 40
• 40 of 9 3.60 ground distance is