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Copyright © John Lindsay, 2015

Readings

Jensen Chapter 6

Photogrammetry

• 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

Images are not maps

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!

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 displacement

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 displacement

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

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