Aerial Photography and Photogrammetry

Interpretation and Measurement

Aerial Photographs

• Photographs taken from a platform, usually an airplane, flying above the earth’s surface.

• Can be taken from space (by astronauts) but usually taken from within the atmosphere

Types of aerial photos

• Vertical air photos• Vertical (on nadir) or nearly vertical angle to the

local ground surface (90° ± 3°)

• Oblique air photos• Tilted away from vertical

– High-oblique• Shows the surface, the horizon, and a portion of

sky

– Low-oblique• Shows only the surface

Information on air photos

• Date• Mission details• Roll + film number• Altitude (not always)• Fiducial marks

pp

Analog vs. Digital

• Analog: Film– Emulsions react with light– B&W, Color, Color infrared– Prints, negatives

• Digital: Electronic storing device– Calibrated sensors– Digital values

Film vs. Digital

• Film uses grains of silver chloride embedded in gel rather than pixels.– Silver chloride turns to silver (opaque) when

exposed to light. Creates a negative.– Must pass light through negative to create a

positive (print).• Typical B&W film sensitive to UV through

red wavelengths (panchromatic)• Some films sensitive to infrared (IR film)

Characteristic Curve

• Film records radiance as tone – the more light (radiance) that hits the film, the more grains of silver chloride are converted to silver

• The relationship between radiance and tone is captured in the characteristic curve of the film.

Image courtesy Sprawls Educational Foundation www.sprawls.org.

The Characteristic Curve

Products

• Film-based cameras• Type

– B&W, color, color infrared, panchromatic• Format

– 9 x 9, 9 x 18, …• Media

– Negative, positive (transparent or opaque) prints

photo type

Photo Geometry

• Because airplanes are subject to turbulence, photo geometry can be less predictable than for satellite data

Flight characteristics that affect air photo geometry

• Flight line orientation (overlap, etc.)• Airplane movement caused by turbulence• Height, air speed, etc.

Flight Paths

Flight paths

12345

6 7 8 9 10

Side overlap Fore and aft overlap

Types of Distortion Caused by Aircraft

• Roll• Pitch• Yaw

Distortion caused by roll, pitch and yaw

Photointerpretation

• Identifying features on the ground by using information depicted in air photos or satellite data– Shape– Size– Pattern– Shadow– Tone, color (or gray shade)– Texture– Context (Association)

Shape

Size

Pattern

Shadow

Color

Texture

Association or Context

Photointerpretation

• Keys can be used to standardize interp.• Photointerpretation is both a science and

an art

Photogrammetry

• Technique of obtaining reliable measurements of objects from their photographic images– Heights of objects– Areas– Lengths– Density– Etc.

Photographic scale

• Relationship between the linear distance on a vertical photograph and the corresponding actual distance on the ground

• Scale is expressed as ‘representative fraction (RF)’ between linear measurements on photo (the numerator) and corresponding distance on the ground (the denominator)

Photo scale –> ground distance

• Example • 1/24,000 or 1:24,000• 1 unit on photo = 24,000 units on ground• 1 cm = 24,000 cm• 1 mm = 24,000 mm• 1 inch = 24,000 inches• 1 inch = 24,000 in / 12 in/ft = 2,000 ft

Scale

• Scale is the ratio of the measured length of an object on an image to its real length on the ground– Always expressed as a ratio (e.g. 1:24,000)

• Small scale photo covers large area on ground• Small scale photo has less detail• Large scale photo cover small area on ground• Large scale photo has considerable detail

Scale: Photo-Ground distance

• Scale (RF) when given a photo measurement (PD) and the corresponding ground measurement (GD)

PDGDRF

1

Scale: Photo-Ground distance

• Distance between two points– On the ground = 1200 m– In the photo = 5 cm

000,24:124000

1

05.01200

1

mm

RF

Important facts

• Scale is not uniform within a photo– Pitch, roll, yaw– Terrain– Used for vertical airphotos only

• Average or Nominal scale

Photogrammetry Summary

• You can calculate many characteristics of ground properties (e.g., building heights, shrub density) from aerial photographs if you know the scale and can use simple geometry and logic.