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Copyright © John Lindsay, 2015

Scale Calculation: Method 1

• Measure the distance between two points on the image and divide by the corresponding ground distance between the same two points.

Scale Calculation: Method 1

A working example:

• Measured a photo dist (PD) = 0.15 m
• Measured a ground dist (GD) = 5000 m
• Scale = PD / GD = 1 / PSR
• PSR = GD / PD = 5000 m / 0.15 m
• PSR = 33,333
• Scale (as a RF) = 1 : 33,333

A working example:

• What if you don't have the original photo but do know the grid resolution (m / pixel)?


• Ground dist = grid resolution × #pixels


• (m / pixel) × pixel = m

Scale Calculation: Method 2

• Divide the focal length (f) by the flying height of the aircraft above the terrain, H.


• H is calculated by subtracting the ground elevation (E) from the altimeter reading (A), which indicates flying height above sea level.

Scale Calculation: Method 2

Elevation and photo scale

Flying height and photo scale

Focal length and photo scale

Measuring heights

Methods for measuring vertical heights

• Single-photo methods


• Stereo-pair or parallax methods (two images required)

Single-photo method 1:
Sun-angle shadow method

Single-photo method 2:
Proportional Shadow Length method

Single-photo method 2:
Proportional Shadow Length method

Single-photo method 2:
Proportional Shadow Length method

• The shadow must fall on level ground


• The full shadow length must be visible including its beginning (which is often obscured by the object itself!)

Single-photo method 3:
Relief-displacement method

Single-photo method 3:
Relief-displacement method