A Guide to Aerial & Satellite Photography Basics

1. A Basic Introduction to Aerial & Satellite Photography ~~~~~~~~~~ Rev. Ronald J. Wasowski, C.S.C. Associate Professor of Environmental Science University of Portland Portland, Oregon 8 October 2015

2. Aerial & Satellite Photography • Air photos were the first RS images • Lighter-than-air craft • H2 & hot-air balloons • Boston Harbor by J. W. Black on 13 October 1860 • Kites: Civil War reconnaissance • Dirigibles / Blimps • Heavier-than-air craft • Airplanes • Spacecraft • Unmanned • Manned

3. J.W. Black: Boston (13 Oct. 1860)

4. Pigeon-Mounted Cameras

5. A Different Pigeon Camera Design

6. Light Matter Interactions – 1 • Two categories of EMR scattering • Surface scattering • Solid/liquid: Continents, oceans… • Liquid/gaseous: Clouds… • Atmospheric scattering • Caused by multiple EMR/atom interactions • Selective scattering: l’s ~ Particle size • Shorter l’s scattered more than longer l’s • Non-selective scattering: l’s « Particle size • All l’s scattered equally

7. Light  Matter Interactions – 2 • Effects of atmospheric scattering • Reduction of contrast ratio • Actual contrast ratio: 5 / 2 = 2.5 • Observed contrast ratio: (5 + 4) / (2 + 4) = 1.5 • Loss of color fidelity by of blue light • Actual color: R = 127 G = 63 B = 63 • Observed color: R = 127 G = 63 B = 190 • Handling atmospheric scattering • Filter out problematic wavelengths • Lose spectral but gain spatial data

8. Basics of Film Technology • Film emulsion • Tiny crystals of silver halide • Large grains • Little spatial information: ~ 2x actual grain size • Detail measure in line pairs per mm • Short exposures possible: ~ n–2D grain size • Exposure proportional to r2 • Small grains • Detailed spatial information: ~ 2x actual grain size • Long exposures: required: ~ n–2D grain size

9. Imaging Using Film Technology • Produce a visible image on film • Focus an image at the film plane • Expose film located at the film plane • A latent image is produced • Develop: Convert halide into metallic silver • Negative image: Bright areas  dark silver • Fix: Remove undeveloped halide • Print: Make a negative of a negative • Paper • Transparencies

10. Problems With Film Technology • Black-and-white (B/W) • Sensitivity varies for different l’s • Non-linear response to brightness variations • Toe, straight-line & shoulder • Non-linear response to development variables • Developer temperature, concentration, agitation… Shoulder Straight-line Response Toe Exposure

11. The Characteristic Curve of Film

12. 12 min 15 min 20 min 9 min Developing Time Affects Film

13. Normal contrast film High contrast film Contrast and Characteristic Curves

14. More Film Technology Problems • Natural color (Color) • R + G + B layers + embedded filters • Three superimposed non-linear-response layers • Inherent difficulties with color fidelity • Color InfraRed (CIR) • Grnl’s => Blu; Redl’s => Grn; IRl’s=> Red • Need to filter out blue light • Yellow filter over the lens • Yellow filter embedded in the film emulsion • Light meters not sensitive to reflected infrared • Use specialized light meters

15. An Aerial Camera

16. An Aerial Multispectral Camera IR1 Red Control Unit Blue Green

17. Aerial Photograph Characteristics • Factors that affect system resolution • Atmospheric scattering • Atmospheric absorption • Platform vibration • Platform motion • Forward motion • Roll (wingtips U/D), pitch (nose U/D) & yaw • Exposure time • Resolving power of lenses • Resolving power of films

18. Aerial Photograph Characteristics • Ground resolution • System resolution • Lens focal length • f = 6.0” standard • ~ 12.5” film diagonal: f / w = 0.48 • 50 mm lens on 35 mm camera: f / w = 1.16 • Resolution degrades toward edges • Flying height • 12,000’ standard • Photogrammetric scale: 1:24,000 w/6” focal length • Engineering scale:1: 2,000 (inches: feet)

19. Aerial Photograph Characteristics • Critical locations on aerial photographs • Fiducial marks: Corners & edge centers • Intersect at the… • Principal point: Optical center of the photo • Nadir: Line to Earth’s center • Seldom coincides with the principal point • Orthophotos… • Superimpose nadir & principal point • Appear to look straight down everywhere in the photo

20. Fiducial Marks

21. Principal Point

22. Optical Axis

23. Pocket Stereoscope

24. Full-Size Stereoscope

25. Stereo Aerial Photography • Relief displacement • Features appear to lean • High features lean away from principal point • Low features lean toward principal point • Makes stereo viewing possible • Photogrammetry possible: Topographic maps

26. Viewing Stereo Aerial Photos • Critical characteristics • Minimum 10% sidelap & 60% endlap • Stereoscopes • Pocket: Small sections of aerial photos • Mirror: Full-frame aerial photos • Computer: Digitized aerial photos • Special display / eyepiece systems • Vertical exaggeration • Vertical scale seems larger than horizontal scale • Determined primarily by air base: Percent overlap • Base / Height ratio: Height usually constant • Less overlap yields more vertical exaggeration

27. Photomosaics • Large-area coverage with small photos • Index images of aerial photography coverage • Detailed coverage • Critical issues • Photographic scale variation • Flying height • Center to edge • Brightness, contrast, color balance variations • Minimum 10% sidelap & endlap • Insure complete coverage

28. Photomosaic Brightness Changes Cut line Cut line

29. Photomosaic Feature Movement Cut line Cut line

30. Photomosaics • Orthophotography • Model geometry = Real-world geometry • Very complicated mechanical/optical systems • Digital orthophotography • Digitize aerial photographs • Combine with an elevation model • Derived from stereo analysis of digitized photos • Digital Terrain Models (DTM’s) • Bare Earth • Digital Elevation Models (DEM’s) • Bare Earth + vegetation + structures

31. Low-Sun-Angle Photography • Vertical Topographic mapping • 45° ≤ q ≤ 90° sunlight • 9:00 a.m. ≤ Local Solar Time ≤ 3:00 p.m. • Low-angle Archaeology • Subtle topographic features may be revealed • Exposure issues with rapidly changing light • Morning preferable to evening • Haze, smog & clouds typically develop late

32. Black-and-White Photography • Single-band UV • Ideal for oil slicks on water • Single-band Visible • Used to construct multispectral photos • Panchromatic Minus-blue • Extremely common for topographic mapping • Single-Band Photo IR • Excellent haze penetration • Vegetation very bright • Water & water/land boundaries very obvious

33. Color Science • Additive primary colors: Projected • RedGreenBlue • Subtractive primary colors: Reflected • CyanMagentaYellow • Color dyesremove unwanted colors

34. One Test for Color-Blindness • What numbers do you see below?

35. Natural Color Photography • Negative color film • Develop exposed halide grains • Remove unexposed halide grains • Negative of both brightness & color • Orange mask to control contrast

36. Natural Color Photography • Positive color film “Slide” film • Develop exposed halide grains • Remove metallic silver grains • Develop unexposed halide grains

37. InfraRed Color Photography • False-Color InfraRed (CIR) film • Gl’s => BluRl’s => GrnIRl’s => Red • Human eye is most sensitive to red • Vegetation appears in shades of red • World War II camouflage-detection film • Cut vegetation quickly loses CIR red color • IR-reflective camouflage paints quickly developed

38. Natural & CIR Photos Compared Natural Color (Col) Color InfraRed (CIR)

39. High-Altitude Aerial Photography • NHAP: National High Altitude Photography • One cycle: 1980 to 1986 • Simultaneous photographs @ 40,000’ • Pan B/W @ 1:80,000 scale AND… • CIR @ 1:58,000 scale

40. Nat.High-AltitudeAerialPhotography Boston Harbor (Panchromatic) NHAP

41. Nat.High-AltitudeAerialPhotography Boston Harbor (CIR) NHAP

42. Nat.High-AltitudeAerialPhotography

43. Nat’l Aerial Photography Program

44. High-Altitude Aerial Photography • NAPP: National Aerial Photography Prog. • Continuing program • First cycle: 1987 to 1991 • Second cycle: 1992 to 1996 • Third cycle: 1997 to 2003 • Fourth cycle: 2004 to 2007 • Simultaneous photographs @ 20,000’ • Pan B/W @ 1:40,000 scale OR… • CIR @ 1:40,000 scale • Quarter-quad centered • Production of digital orthophotoquads (DOQ’s)

45. USGS Quadrangle: Quarter-Quad

46. Sources of Aerial Photography • EROS Data Center • http://eros.usgs.gov/aerial-photography • Microsoft TerraServer • http://www.terraserver.com/view.asp?tid=142 • GloVis • http://glovis.usgs.gov/ • Collection … Aerial …

47. New Technology • Scan existing aerial photographs • Drum scanners • Flatbed scanners • Sheet-fed scanners • DVD-R (recordable) disk drives • Drives ~ $ 20.00 • Disks ~ $ 0.30

48. Benefits of Digital Aerial Cameras • Improved radiometric resolution • ≥ 12 bits / pixel (≥ 4096) brightness range • Simultaneous Pan, Col & CIR acquisition • Pan sharpened Col & CIR images • No film processing & scanning • Reduced monetary & temporal costs • No spectral degradation due to scanning • Ability to perform multispectral classification • Accelerated digital workflow • Rapid post-disaster response

49. Digital Aerial Camera Sensors • Linear sensor arrays • Comparable to flatbed scanners “Pushbroom” • One linear array per spectral band • Platform forward motion produces the image • Poor geometry with critical need for correction • Area sensor arrays • Comparable to ordinary digital cameras “Frame” • One pixel array per spectral band • Electronic shutter produces the image • Excellent geometry with minimal need for correction

50. Linear Array Digital Cameras • Jena • Jena Airborne Scanner (JAS 150s) • Variable exposure time from 1.25 ms to 10.112 ms • Leica Geosystems • Airborne Digital Sensor (ADS40) • SH51 Multispectral; panchromatic stereo imagery • SH52 CIR + Color + panchromatic stereo imagery • Wehrli • 3-DAS-1 System • Three camera systems • Nadir (0°), forward (+26°) & backward (–16°) flight line stereo