Aerial Photography

Film Processing
Our photo lab processing is directly overseen by a manufacturer-certified manager. Quality, accuracy, and resolution shall be checked based on the above requirements, while exposure and contrast shall be checked with densitometry. Rejected imagery shall be re-flown and reviewed. Special care shall be taken to ensure that film is properly handled and not subjected to extreme changes in humidity and temperatures.

Scanning
All imagery is scanned at a resolution maximizing readability, with one of worlds most leading edge a roll-feed photogrammetric scanner. A look up table (LUT) shall be created from a test scan of each flight line by adjusting histograms from each flight line. The LUT from each test scan shall be applied to the flight line after the imagery is checked for detail, aesthetics, contrast, realism, tonal balance, digital dodging and color quality. All imagery shall be and equalized automatically to establish uniform quality throughout the entire project. Final scans shall be checked to eliminate any tonal anomalies caused by shadows, hot-spots, time, date and weather.

Flight
All photography shall meet the following conditions:

• Flight height shall not exceed an 8.3 x-factor and 1800 c-factor, where detail and clarity support horizontal and vertical accuracy requirements.
• Acquisition shall avoid atmospheric (conditions haze, clouds, overcast, snow, dust, precipitation, and smoke) and a sun angle, optimize contrast with minimal deep shadows, of less than 30 degrees.
• Photography shall be collected in stereo, close to 60% forward overlap (at least two photos beyond the project limits) and 30% sidelap (with sufficient side coverage).
• Camera optical axis shall be near vertical, and not exceed from nadir 3 degrees or 4 degrees between consecutive exposures. Camera orientation shall be with the flight line, where crab shall not exceed 3 degrees between consecutive exposures.
• The mapping flight will be at 1:3600, which is 1800’ above ground and is an 1800 c-factor of 1’ contours. This 1:3600 photography is also a 7.5x enlargement factor for 1”=40’ mapping. A higher flight at 1:9600 will be used for the digital image base which will yield an 8x enlargement factor for 1”=100’ orthorectified imagery. This meets and exceeds standards.

These parameters are documented on the prints provided, and will be re-flown if they do not meet the stated standards.

Basic Photogrammetric Math Principles
The following are a few terms in photogrammetry used in flight planning to control and define the accuracy of a project.

C-Factor defines the flight height based on the contour interval
X-Factor is the enlargement factor that defines the flight height based off the target scale.

PRS (Photo Ratio Scale) = PS (Photo Scale)/12”
Neat Model = 6.5” width by 3.6” gain
AGL (above ground level) = PRS (feet) * Focal Length (feet)
X-factor = PS / Target Scale (limit 8.3x)
C-factor = AGL / CI (target contour interval) (better than 1800)
Reliability = AGL/10,000 (in respect to the ground)
Repeatability = AGL / 20,000 (in respect to the photography)

Example
1:3600 PRS, ½’ focal length, 1”=40’ Mapping, 1’ CI

PRS (Photo Ratio Scale) = 1:3600 / 12” = 1”=300’
Neat Model
Width = 6.5” * 300” = 1950’
Gain = 3.6” * 300” = 1080’
AGL (above ground level) = 1:3600 * ½’ = 1800’
X-factor = 1”=300’/1”=40’ = 300’/40’ = 7.5x
C-factor = 1800’ / 1’ci = 1800
Reliability = 1800’ / 10,000 = 0.18’
Repeatability = 1800’ / 20,000 = 0.09’

Example
1:7200 PRS, ½’ focal length, 1”=100’ Mapping, 2’ CI

PRS (Photo Ratio Scale) = 1:7200 / 12” = 1”=600’
Neat Model
Width = 6.5” * 300” = 3900’
Gain = 3.6” * 300” = 2160’
AGL (above ground level) = 1:7200 * ½’ = 3600’
X-factor = 1”=300’/1”=40’ = 300’/40’ = 6x
C-factor = 1800’ / 1’ci = 1800
Reliability = 1800’ / 10,000 = 0.36’
Repeatability = 1800’ / 20,000 = 0.18’