Acquisition of Aerial Photographs Lecture 8 prepared by R. Lathrop 9/99 Updated 9/03 with reference to material in Avery & Berlin 5th edition

Aerial Photographic Sources National High Altitude Photography (NHAP): ( ) 1:58,000 CIR or 1:80,000 Pan National Aerial Photography Program (NAPP): (since 1987) 1:40,000 CIR NASA high altitude photography: (since 1964) 1:60,000-1:120,000 PAN, COLOR, CIR These images are archived by the Eros Data Center as part of the USGS Global Land Information System. To search archive

Aerial Photographic Sources USDA:(since 1955): mainly PAN of 1:20,000-1:40,000. These photos are archived by the Aerial Photography Field Office htm National Archives and Records Administration archives older (pre- 1950’s) aerial photography ordr.html

Aerial Photographic Sources National Ocean Survey (NOS) coastal photography: (since 1945), color, scales of 1;10, :50,000 The photos are used for a variety of geo- positioning applications, which include delineating the shoreline for Nautical Chart creation, measuring water depths, mapping seabed characteristics, and locating obstructions to marine and air navigation.

Contract Photography Existing aerial photographs may be unsuitable for certain projects Special-purpose photography - may be contracted through commercial aerial survey firms

Contracting Photography Considerations Camera focal length Camera format size Photo scale  ground coverage and resolution desired Film/filter Overlap/sidelap Photo Alignment/tilt Seasonal considerations Time-of-Day considerations/ cloud cover

Seasonal considerations Cloud free conditions, ideally < 10% Leaf-off: spring/fall when deciduous tree leaves are off and ground free of snow used for topographic/soils mapping, terrain/landform interpretation Leaf-on: summer when deciduous trees are leafed out or late fall when various tree species may be identified by foliage color used for vegetation analyses

Scale Considerations What is the minimum mapping unit or size of smallest object that you want resolved and mapped? What is the ground coverage desired for an individual photo? How large of a study area to be covered? 3 considerations involve trade-offs

Time-of-day considerations Quantity of light determined by solar elevation angle no shadows: +- 2 hrs around solar noon shadows desired: early or late day Spectral quality: possibility of sun/hot spots causing image saturation

Flight Alignment Flight lines are planned to be parallel Usually in a N-S or E-W direction. For maximum aircraft efficiency, they should be parallel to the long axis of the study area (minimize aircraft turns). Crab or drift should be minimized Tilt, 2-3 o for any single photo, average < 1 o for entire project

Example: Flight planning for aerial photography of submerged aquatic vegetation Color film gives better water depth penetration

Example: Flight planning for aerial photography of submerged aquatic vegetation Other considerations Scales of 1:12,000 to 1:24,000 needed Time of year: late spring-early summer Time of day: sun angles o, generally early morningto reduce wind/surface waves Tides: +- 2 hours of lowest tide

Example: Flight planning for aerial photography of submerged aquatic vegetation GeoVantage Digital Camera 4 bands: Blue, Green, Red, NIR Pixel Array Size: mm Focal Length: 12mm Field of View: 28.1 o crossrange, 21.1 o along range Easily mounted on wheel strut Coordinated acquisition with Inertial Measurement Unit to determine precise geodetic positioning to provide for georegistration and orthorectification

Example: Flight planning for aerial photography of submerged aquatic vegetation What Flying Height (m) needed to resolve individual SAV beds of 1m wide x 10 m long (0.001 ha in size)? General Rule of Thumb: GSD at a minimum of ½ the size of smallest feature. In this case need, GSD of 0.5m. GSD = array element size * H’. focal length Example: array element size = mm f = 12 mm GSD = 0.5mH’ = ? H’ = 0.5m * 12 mm / mm = 1290 m

Example: Flight planning for aerial photography of submerged aquatic vegetation What will be the image width(m)? FOV = 28.1 o H’ = 1290m

Example: Flight planning for aerial photography of submerged aquatic vegetation What will be the image width(m)? Remember your basic trigonometry? Tan = opposite / adjacent Tan FOV/2 = (1/2 image width)/H’ Image width = 2 * tan14.05 * 1290m = 2 * * 1290m = 645 m FOV = 28.1 o H’ = 1290m opp adj

Example: Flight Planning Mission parameters Study area: 20 km E-W & 35 km N-S Elevation of study area: 500 m above sea level Desired Photo scale: 1:25,000 Film format: 23 x 23 cm or 0.23 x 0.23 m Focal length: 152 mm or m Overlap: 60% Sidelap: 30% From Avery & Berlin, 5th ed. pp

Example: Flight planning Flight altitude RF = f / H or H = RF d * f H = (25,000) (0.152 m) = 3,800 m above terrain Flight altitude = 3,800 m = 4,300 m above sea level

Example: Flight planning Ground distance Ground distance coverage of a single photo RF = PD / GDor GD = RF d * PD GD = 25,000 * 0.23 m = 5,750 m

Example: Flight planning Number of flight lines NL = [W / (GD)(S g )] + 2 where W = width of study area GD = ground distance of single photo S g = sidelap gain (100 - % sidelap) expressed as a decimal fraction 2 = extra flight lines (1 per side) NL = [20 km / (5.75 km)(0.7)] + 2 = = 6.97 = 7 (always round up)

Example: Flight planning Number of photos per flight line NP = [L / (GD)(O g )] + 4 where L = length of flight line GD = ground distance of single photo O g = overlap gain (100 - % overlap) expressed as a decimal fraction 4 = extra photos (2 per end of flight line) NP = [35 km / (5.75 km)(0.4)] + 4 = = 19.2 = 20 (always round up)

Example: Flight planning Total number of photos Number of flight lines x number of photos per flight lineor NP x NL TN = NP x NL = 7 x 20 = 140 photos