1. Aerial Photography and Photogrammetry
Interpretation and Measurement

2. 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

3. Types of aerial photos Vertical air photos Oblique 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

9. Information on air photos
Date
Mission details
Roll + film number
Altitude (not always)
Fiducial marks

10. pp

11. Analog vs. Digital Analog: Film Digital: Electronic storing device
Emulsions react with light
B&W, Color, Color infrared
Prints, negatives
Digital: Electronic storing device
Calibrated sensors
Digital values

12. 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)

13. 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.

14. The Characteristic Curve
Image courtesy Sprawls Educational Foundation

16. Products Film-based cameras Type Format Media
B&W, color, color infrared, panchromatic
Format
9 x 9, 9 x 18, …
Media
Negative, positive (transparent or opaque) prints

17. photo type

18. Photo Geometry
Because airplanes are subject to turbulence, photo geometry can be less predictable than for satellite data

19. Flight characteristics that affect air photo geometry
Flight line orientation (overlap, etc.)
Airplane movement caused by turbulence
Height, air speed, etc.

20. Flight Paths

21. Flight paths 1 2 3 4 5 6 7 8 9 10
Side overlap
Fore and aft overlap

22. Types of Distortion Caused by Aircraft
Roll
Pitch
Yaw

23. Distortion caused by roll, pitch and yaw

24. 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)

25. Shape

26. Size

27. Pattern

28. Shadow

29. Color

30. Texture

31. Association or Context

32. Photointerpretation Keys can be used to standardize interp.
Photointerpretation is both a science and an art

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

34. 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)

35. 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

36. 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

38. Scale: Photo-Ground distance
Scale (RF) when given a photo measurement (PD) and the corresponding ground measurement (GD)

39. Scale: Photo-Ground distance
Distance between two points
On the ground = 1200 m
In the photo = 5 cm

40. Important facts Scale is not uniform within a photo
Pitch, roll, yaw
Terrain
Used for vertical airphotos only
Average or Nominal scale

41. 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.