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Copyright © John Lindsay, 2015


The Earth From Space

Photogrammetry Part 1

John Lindsay
Fall 2015


Jensen Chapter 6


  • Photogrammetry is the art and science of making accurate measurements by means of aerial photography.

  • Focused on gaining quantitative information from images

  • Analog vs. digital photogrammetry

Photogrammetric measurements

  • From a single vertical image:
    • Photo scale
    • Object height, length, area, perimeter
    • Object grayscale tone or colour

  • From multiple overlapping images:
    • Precise planimetric (x, y) feature location
    • Precise location height (z)
    • Digital elevation models (DEMs)
    • Bathymetric models
Single image geometry (source: Jensen, 2006)

Images are not maps

Image projections (Modified From: Paine and Kiser, 2012)

Photo centers

  • Principal Point
    • Lens distortion is radial from PP

  • Nadir
    • Topographic displacement is radial from nadir

  • Isocenter
    • Tilt displacement radiates from the isocenter

  • On a vertical photo, all three are coincident!
Image tilt and photo centers

Distortion and Displacements

  • Distortion is a shift in object positions in the image that alters the perspective of the image (i.e. the central projection)

  • Displacement is a shift in object positions that does not affect the perspective.

  • Main distortions and displacements:
    • Lens distortion, Tilt displacement, and Topographic (relief) displacement

Lens distortion

  • Caused by imperfections in the geometry of the lens.

  • Causes objects in image to appear closer or farther from the PP than they actually are.

  • Solved by calibration of the lens

  • Only needed for most precise mapping as expensive cameras come with near-perfect lenses (expensive!)

Tilt displacement

Tilt displacements (source: Jensen, 2006)

Tilt displacement

Tilt displacements (Modified From: Paine and Kiser, 2012)

Topographic displacement

  • Most serious displacement, especially in areas of high relief.

  • Corrected by orthorectification.

  • Can be useful for measuring heights and stereoscopic viewing.

Topographic displacement

Topographic displacements (From: Lillesand et al., 2012; modified by Mersey)

Topographic displacement

Topographic displacements (From: Lillesand et al., 2012; modified by Mersey)

Image scale

  • The scale of an aerial photograph expresses the relationship between distance on the image and actual ground distance.

  • The larger the scale of a photograph, the more detail can be resolved; however, more images will be needed to cover a given ground area.

Image scale

  • Scale can be expressed by:
    • Scale= Image Distance / Ground Distance

  • verbal expression

  • bar or graphic scale

  • representative fraction (RF)

Image scale

Small scale1:24,000
Medium scale1:10,000 to 1:24,000
Large scale1:1000 to 1:10,000
Very large scale1:1000 and larger

Photo-scale Reciprocal (PSR)

  • The inverse of the RF

  • PSR is unitless

  • Example: if RF is 1:24,000 then PSR = 24,000

  • That is, the ground distance is 24,000 × the image distance