Principles of Aerial Photography
- Film: air photo missions are flown using black and white film, color, infrared, and false-color infrared film are sometimes used for special projects.
- Focal length: the distance from the middle of the camera lens to the focal plane (i.e. the film). As focal length increases, image distortion decreases. The focal length is precisely measured when the camera is calibrated
- Scale: the ratio of the distance between two points on a photo to the actual distance between the same two points on the ground(ie. 1 unit on the photo equals “x” units on the ground). If a 1 1800 ft stretch of highway covers 1 inch on photo, the scale is calculated as follows:
- Another method used to determine the scale of a photo is to find the ratio between the camera’s focal length and the plane’s altitude above the ground being photographed.
If a camera’s focal length is 153mm, and the plane’s altitude Above Ground Level (AGL) is 10483 feet, using the same equation as above, the scale would be :
Scale may be expressed three ways:
- Unit Equivalent
- Representative Fraction
A photographic scale of 1 inch on the photograph represents 1800 feet on the ground would be expressed as follows:
- Unit Equivalent – 1 inch = 1800 feet
- Representative Fraction – 1/21600 (1800 feet = 21600 inches)
- Ratio – 1:21600
Expressions of scale:
- Large Scale – Larger-scale photos (e.g. 1/25 000) cover small areas in greater detail. A large scale photo simply means that ground features are at a larger, more detailed size. The area of ground coverage that is seen on the photo is less than at smaller scales.
- Small Scale – Smaller-scale photos (e.g. 1/50 000) cover large areas in less detail. A small scale photo simply means that ground features are at a smaller, less detailed size. The area of ground coverage that is seen on the photo is greater than at larger scales.
- Fiducial marks: small registration marks exposed on the edges of a photograph. The distances between fiducial marks are precisely measured when a camera is calibrated. This information is used by Cartographers and Photogrammetrists for analyzing and interpreting geographic information provided by the aerial photograph.
- Overlap: is the amount by which one photograph includes the area covered by another photograph, and is expressed as a percentage. The photo survey is designed to acquire 60 per cent forward overlap (between photos along the same flight line) and 30 per cent lateral overlap (between photos on adjacent flight lines).
- Ground Sample Distance (GSD) In a digital photograph, the distance between pixel centers measured on the ground. For example, in an image with a one-foot GSD, adjacent pixels image locations are 1 foot apart on the ground. As the GSD decreases the sampling rate increases to cover the same area on the ground, thereby increasing image resolution.
- Ground Control Points (GCP) Locations on the earth which have been accurately surveyed for both their elevation and GPS location. GCP’s allow images to be georeferenced to a higher degree of accuracy.
Selecting image scale or GSD
Correct selection is fundamental to achieving specified mapping accuracy
In film imaging, scale is the primary parameter. The selected camera lens focal length (f) affects imaging altitude (H) where :
Image Scale = f/H
During digital conversion, various scanner pixel may be selected to achieve specified GSD.
For example, 20cm GSD equivalent can be achieved by :
scanning 1:20,000 scale imagery at 10u pixel size, or
scanning 1:10,000 scale imagery at 20u pixel size.
In digital imaging, sensor native pixel size (Ps) is fixed. Specified GSD is achieved only by varying the image altitude. Image scale is simply:
Image Scale = Ps/GSD
Since Image Scale = f/H also, therfore f/H = Ps/GSD. Imaging altitude (H) can be derived from :
H = GSD x f/Ps
For Dig-Camera #2: f= 135 mm, Ps=9.0 µm. Therefore to achieve a 20 cm GSD:
H = 20 cm × 135 mm/9.0 µm = 3000 m (9843 feet AGL)