Aerial Photography

1. Aerial Photography

2. Objectives • Discuss factors affecting the quality of aerial photography, including: • Vantage point • Camera • Filtration • Film emulsion • Briefly discuss the planning of aerial photography missions, including: time of day, weather, and flightline layout

3. Vantage Points • Three vantage points: • Vertical • Camera’s optical axis  3 from vertical • Low-oblique • Camera’s optical axis > 3 from vertical • Horizon not visible • High-oblique • Camera’s optical axis > 3 from vertical • Horizon visible • “Right” vantage point depends on project goals Oblique

4. Vertical Aerial Photography Jensen 2000 Goosenecks, San Juan River, UT

5. Low-Oblique Aerial Photography Jensen 2000 Golden Gate Bridge, San Francisco, CA

6. High-Oblique Aerial Photography Jensen 2000 Golden Gate Bridge, San Francisco, CA

7. Vertical Aerial Photography • Planimetric (x, y location) base maps • Topographic (z-elevation a.s.l.) base maps • Digital elevation models (DEMs) • Orthophotomaps

8. Aerial Camera Components • Retina= Film at film/focal plane • Lens • Used to focus reflected light from real world onto retina/film • Iris + Eyelid= Lens Aperture + Shutter • Control amount of light reaching the retina/film • Exposure

9. Focal Plane & Focal Lens • Focal Plane/ Film Plane • Area in which film is held flat during an exposure • Focal Length • Distance from lens’ rear nodal point to film plane when a camera is focused at infinity • Equals image distance for a far subject • Fixed focal length • Non-zoom lenses • E.g., 88 mm, 305 mm • Changing focal length • Zoom lenses • E.g., 24-85 mm Focal length increases / lens gets longer as one zooms on something closer than infinity

10. ƒ/stop • ƒ/stop • Indicates relative speed of the camera lens system • Examples: • ƒ/stop for camera with 80 mm focal length and a maximum lens diameter (full aperture) of 40 mm = f/2 • ƒ/stop for camera with 80 mm focal length and a minimum lens diameter of 3.6 mm = f/22 • Lens opening of camera with 80 mm focal length and ƒ/stop of 4 = 20 mm

11. Focusing Ring Distance Scale Aperture Ring & Scale ƒ/stop • The smaller the ƒ-rating, the greater the aperture, the greater the amount of light admitted through the lens per unit of time, and the faster the lens

12. ƒ/stop • Each lens opening transmits one-half as much light as the preceding lens opening (e.g., ƒ/5.6 transmits one-half as much light as ƒ/4) • “Stop up” (ƒ/5.6 to ƒ/4)  decrease of aperture by factor 2 • “Stop down” (ƒ/4 to ƒ/5.6)  increase of aperture by factor 2 • Lens speed • ƒ-number of the maximum effective lens diameter at full aperture

13. Shutter Speed • Shutter speed= Exposure time • Length of time the shutter is open • May range from “bulb” (length of time the photo-grapher pushes the exposure button) to 1/4000 sec.

14. ƒ/stop & Shutter Speed • Task of aerial photographer: • Ensure that the film emulsion receives the right amount of light during an exposure • How? --- Select the correct relationship between: • ƒ/stop • Shutter speed • Increase amount of light reaching the film plane: • Increase size of ƒ/stop (e.g., from ƒ/5.6 to ƒ/4), or • Increase exposure time (e.g., from 1/200 to 1/100) • Decrease amount of light reaching the film plane: • Decrease size of ƒ/stop (e.g., from ƒ/4 to ƒ/5.6), or • Decrease exposure time (e.g., from 1/100 to 1/200)

15. Types of Aerial Cameras • Single-lens mapping (metric) cameras • Most commonly used cameras to obtain air photos • Provide highest geometric and radiometric quality • Multiple-lens (multiple-band) cameras • Provide simultaneously acquired photographs in several bands of the EM spectrum • Digital cameras • Panoramic cameras • Miscellaneous cameras • Analog vs. digital

16. Camera System Components • Vibration isolation elements • Film platen pressure plate • Drive unit • Control unit • Film feed controls • Film take-up reel • Film magazine • Unexposed film supply reel • Drive unit • Forward-motion compensation device • Mount • Exchangeable universal shutter • Lens cone assembly • Lens

17. Camera System Components Profile view of a metric camera + system components

18. Angular fields of View • Lens cone assembly with multiple-element lens • Focused at infinity • Lenses with various angular fields of view • Narrow: < 60 • Normal: 60 – 75 • Wide-angle: 75 – 100 • Super-wide-angle: > 100  • Projects undistorted images of the real world onto the film plane

19. Angular Fields of View • Note relationship b/w flight altitude, angle of view, and ground area recorded on film

20. Intervalometer & IMC • Intervalometer • Used to expose film at specific time intervals to generate proper amount of overlap (stereoscopic) coverage • Time interval depends on (a) aircraft altitude above-ground-level and (b) aircraft speed • Image motion compensation (IMC) • Depends on velocity, altitude, and lens focal length With IMC Without IMC

21. Air Photo Annotation Annotation: • Grayscale step wedge(proper exposure?) • Notepad (critical mission notes?) • Altimeter • White cross-hair fiducial marks • Clock • Lens cone serial number • Focal length in mm • Project frame number • Mission name and date • Navigation data • Typical film characteristics • Film rolls • 24 cm (9.5 in.) wide • > 30–150 m (100–500 ft) long • Individual exposures: 2323 cm (99 in.)

22. Multiple-Lens (-Band) Cameras • Simultaneously takes photos of the same geographic area in several EMS regions (bands) • Each camera has unique film +/- filter combination

23. Airborne Digital Cameras • Sensor in digital cameras (CCD/CMOS replaces film) • Charge-coupled device (CCD) • Complimentary Metal Oxide Semiconductor (CMOS) • Sensor detectors convert light into electrons that can be measured and converted into radiometric intensity value • Light photons cause electrical charge that is directly related to the amount of incident radiant energy • This analog signal is sampled electronically and converted into digital brightness values [8-bit: 0-255; 12-bit: 0-4096] • BVs obtained from A-to-D conversion may be stored and read by computer systems • Based on linear arrays or area arrays of detectors

24. Aerial Photography Filtration • Sir Isaac Newton, Opticks, 1704

25. Color Theory • White light –all colors of the visible spectrum • Black – absence of colors of the visible spectrum • Primary colors = red, green, blue • Complementary colors = yellow, magenta, cyan • Additive color theory • Describes what happens when primary colors are combined • Used, e.g., to display images on TV and PC monitors • Subtractive color theory • Describes what happens when white light is passed through complementary color filter(s) • Used, e.g., in work with filters and to create colors in developed color negatives + color positive s

26. Color Theory Additive Subtractive

27. Types, Sizes, and Colors of Filters • Filters • Filter out / Subtract certain wavelengths of light before they reach the film plane and expose the film • Examples: • Red filter absorbs blue and green light and transmits red light • Yellow filter absorbs blue light and transmits red and green light

28. Common Filters Used in AP • Haze Filter • In natural color aerial photography • Eliminates much of the scattering of UV radiation caused by atmospheric haze • Absorbs light < 400 nm • Yellow (minus-blue) Filter • In color-infrared aerial photography • Eliminates much of the scattered blue light • Absorbs light < 500 nm • Band-pass filtering • Process whereby a certain camera film/ filter combination is chosen to selectively record a very specific band of reflected EM energy on film

29. Common Filters Used in AP • Polarizing Filter • Passes the vibration of a light ray in just one plane at a time • Filter out unwanted reflections from, e.g., water bodies

30. Aerial Photography Films • Sensitometry: science of measuring the sensitivity of photographic materials • Cross sections of films: • Emulsion layer containing light-sensitive silver halide crystals in a gelatin suspension • Base/ support material that may be transparent (acetate film/glass) or opaque (paper) • Anti-halation layer that absorbs light that passes through the emulsion and the base to prevent reflection back to the emulsion • Sensitivity of photographic emulsion: • Function of size, shape, and number of silver halide crystals per unit area and the wavelengths of light to which the grains are sensitive

31. Aerial Photography Films

32. Black & White Photography • Three sensitivities: • Orthochromatic emulsions: • Sensitive to blue and green light • Panchromatic emulsions: • Sensitive to ultraviolet, blue, green, and red light • Near infrared emulsions: • Sensitive to blue, green, red, and near-infrared light • B&W positive print: •  Shows highly reflective areas in light tones (e.g., sandy beach) and highly absorptive areas in dark tones (e.g., ocean)

33. Normal Color Photography • Records energy in the region from 0.4 – 0.7 mm (RGB) • Three emulsion layers • Depicts terrain in the same hues (colors) as our eyes perceive it • Haze filter removes UV light Spectral sensitivity of dye layer of normal color film Normal color film development, etc

34. Color-Infrared Photography • Records energy in the region from ~ 0.3 – 0.9 mm (ultraviolet, R, G, B, near-infrared) • Three emulsion layers • Dyes are “offset by one” Spectral sensitivity of dye layer of color-infrared film Color-infrared film development, etc

35. Natural Color vs. Color-Infrared Natural color Color-infrared

36. Comparison of Air Photo Films B&Wpanchromatic B&Wnear-infrared Color-infrared Natural color

37. Planning Air Photo Missions • Time of Day: • Within 2 hours of noon (Sun angle ~ 30° – 52°) • Bad: low sun angle, high sun angle • Weather: • Low amount of water vapor and particulates in air • Bad: humidity, smog, clouds, wind • Flightline layout: • Determine: photo scale, base map scale, size and corner coordinates of study area, overlap of photo frames, average sidelap of frames, film format, camera focal-length • Calculate: flight altitude AGL, number of flightlines, map distance b/w flightlines, ground distance b/w exposures, total number of exposures

38. ? Any questions ?