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## GEOG*2420

John Lindsay
Fall 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.

(From: Mersey)

### 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.

(From: Mersey)

(From: Mersey)

(From: Mersey)

(From: Mersey)

### Methods for measuring vertical heights

• Single-photo methods

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

### Single-photo method 2: Proportional Shadow Length method

(From: Jensen, 2007)

### 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

(From: Jensen, 2007)
d = relief displacement
r = radial distance from top of object to the principal point
d is related to r, flying height (H) and the height of the object (h).

### Single-photo method 3: Relief-displacement method

(From: Jensen, 2007)