Acquisition of Aerial Photographs

Lecture 8prepared by R. Lathrop 9/99

Updated 9/03with reference to material in Avery &

Berlin 5th edition

Aerial Photographic Sources• National High Altitude Photography (NHAP): (1980-1987)

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

• http://edcsns17.cr.usgs.gov/EarthExplorer/

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 http://www.fsa.usda.gov/dam/APFO/airfto.htm

• National Archives and Records Administration archives older (pre- 1950’s) aerial photography http://www.nara.gov/research/ordering/mapordr.html

Aerial Photographic Sources• National Ocean Survey (NOS) coastal photography:

(since 1945), color, scales of 1;10,000 - 1:50,000• 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.

• http://mapfinder.nos.noaa.gov

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 spotscausing 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 , 2-3o for any single photo, average < 1o 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 15-30o, generally early morning to 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: 0.00465mm• Focal Length: 12mm• Field of View: 28.1o crossrange, 21.1o 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 = 0.00465mm f = 12 mm GSD = 0.5m H’ = ?

• H’ = 0.5m * 12 mm / 0.00465mm = 1290 m

Example: Flight planning for aerial photography of submerged aquatic vegetation

• What will be the image width(m)? FOV = 28.1o

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 * 0.250 * 1290m

= 645 m

FOV = 28.1o

H’ = 1290m

opp

adj

Example: Flight PlanningMission 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 0.152 m

• Overlap: 60%

• Sidelap: 30%

From Avery & Berlin, 5th ed. pp 101-102

Example: Flight planningFlight altitude

• RF = f / H or H = RFd * f

• H = (25,000) (0.152 m) = 3,800 m above terrain

• Flight altitude = 3,800 m + 500 = 4,300 m above sea level

Example: Flight planningGround distance

• Ground distance coverage of a single photoRF = PD / GD or GD = RFd * PD

• GD = 25,000 * 0.23 m = 5,750 m

Example: Flight planningNumber of flight lines

• NL = [W / (GD)(Sg)] + 2where W = width of study areaGD = ground distance of single photo Sg = 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 = 4.97 + 2 = 6.97 = 7 (always round up)

Example: Flight planningNumber of photos per flight line

• NP = [L / (GD)(Og)] + 4 where L = length of flight line GD = ground distance of single photo Og = 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= 15.2 + 4 = 19.2 = 20 (always round up)

Example: Flight planningTotal number of photos

• Number of flight lines x number of photos per flight line or NP x NL

• TN = NP x NL = 7 x 20 = 140 photos