Air Photos

Air Photos
- Remote Sensing
  - definition
    - acquiring information about objects
    - without coming into contact
    - instead use reflected EMR ("sun's energy") or our own energy source
    - e.g. eye sight, camera/detector (images: photos, satellite, radar, LiDAR, etc)
  - energy source (signal source)
    - passive - 'existing light energy' (photos, satellite images)
    - active - transmit/ receive energy (RADAR, LiDAR)
  - camera/ detector location
    - satellites - space
    - aircraft
  - spectral range of images
    - broad-band - "range of light we see" (panchromatic & colour film)
    - multi-band - 2+ distinct bands (R-G-B, IR, UV, etc.) - satellite images
    - narrow-band - transmits & receives a limited freq. (radar. LiDAR)
- EMR
  - = "energy" that travels in the form of a wave emitted by the sun (stars)
  - EMR has a broad spectrum (link for reading)
    - gamma rays (short)
    - X-rays
    - UV
    - visible spectrum
    - infrared (reflected & thermal)
    - microwaves
    - radio (long)
  - atmospheric window
    - portion of EMR that passes through atmosphere to the ground
    - UV, visible spectrum, IR (reflected & thermal), radio waves
  - interactions of EMR with matter
    - reflection
    - transmission
    - absorption
    - refraction
  - spectral signature
    - bands that get reflected ("what we see")
    - "unique combination" become signatures
- Activities (uses)
  - photogrammetry
    - obtain reliable measures & make maps
    - measures are
      - distance
      - direction
      - area
      - height (displacement)
      - elevation of the ground
  - photo interpretation
    - ID objects and determine their significance
    - examples
      - timber (vegetation) types
      - habitat suitability
      - land use
      - landforms & soils
  - orienteering
    - use map/ photos to navigate and find features
      - photogrammetry to measure directions & dist. to "get around"
      - photo interp. to orient photo to real world, ID features, confirm location
- Types of air photos
  - oblique
    - at an angle
    - low
    - high
    - greater area covered
    - "natural" - terrain/objects easy to recognize
  - vertical (<3 degrees)
    - "straight down"
    - "uniform scale" (... not really)
      - measures possible
      - map-like, use for navigation
    - features not "masked" by neighbouring objects ... but forest canopy
    - can be used to make maps
- Lights! Camera! Action!
  - EMR (review)
    - atmospheric window = transmittance of EMR = what's available to us
    - interactions (we capture reflection)
  - Camera
    - large (f = 30cm), neg. = 23cm
    - panchromatic (B&W) or colour or 'extra' (to include a bit of UV or near IR)
    - photo number
      - roll number
        
        focal length
        
        'company' or gov't (e.g. BC)
        
        (optional "C" for colour
        
        year (may be missing on older photos)
        
        roll of film (for that yr)
      - individual photo #
      - e.g. 30BCC05012 - 109
        
        30 cm focal length
        
        BC gov't
        
        C for colour
        
        05 - 2005
        
        012 - the 12th film roll that yr
        
        109 - the 109th photo on that flight
  - Action
    - flight line
      - 60% endlap (for stereo)
      - 20-30% sidelap (ensures no missed area)
    - problems
      - drift
      - crab
      - tilt (turbulance or banking)
- Air photo geometry & scale
  - perspective vs. orthographic projection
  - geometry: similar triangles
  - 3D diagram showing ground, negative and positive
  - scale
    - calculation (ratios)
      - PD / GD (= map dist / real world dist.)
      - f / H (focal length / height of camera above ground)
    - scale varies with
      - H
        
        dist. from lens to ground
        
        greater height, the smaller the scale
      - f
        
        dist. from lens to negative
        
        greater the f, the larger the scale
    - point scale vs. ave. scale of photo
      - if elev varies in a photo ...
      - ... then H varies ...
      - ... and scale varies too
      - Can also just average the H to get ave scale
    - altitude vs. height above ground
      - flying altitude is above MSL, usu. in feet
      - H is flying height above the ground
      - the difference is elevation, thus ...
      - H + elev = flying alt.
  - questions
    - handy conversions: 1 ft = 30.48cm, 1 m = 3.28ft
    - scale? PD = 9.7 cm, GD (i.e. real world) = 1,117 m
    - scale? f = 15.2 cm, alt. = 10,000 ft, elev = 800 m
    - H(ft)? scale 1:15,000, f = 6"
    - E(m)? alt.= 23,000ft, f= 305mm, scale 1: 20,000
- Stereo Viewing
  - vision
    - reflected light from object -> eye -> nerve -> brain
    - monocular vs. binocular
    - stereoscopy
      - use of binocular vision
      - overlapping pair of air photos
      - form a 3D image in our brain (gives depth & distance)
      - conditions
        
        stereo pair
        
        properly aligned
        
        stereoscope (separates view)
  - stereoscopes
    - lens
      - cheap
      - portable (brief case)
      - 2x's magnification
    - mirror
      - no overlap
      - entire stereo area viewable
      - room to work
      - no lens distortion
  - Proper Set-Up
    - select stereo pair (or a triplet)
    - locate fiducial marks
    - locate PP on each photo (hey! what's a "PP"?)
    - locate CPP on each photo (visual assessment)
    - (orient shadows towards you)
    - align photos with straight edge - recreates flight line
    - adjust photo distance - match dist. to stereoscope (inter-pupil dist.)
    - align stereoscope with flight line
    - fine tune photo distance