1. Aerial Images

2. Aerial Images
Aerial imagery has been a primary source of geographic data
Started as soon as portable cameras were invented
Became practical with invention of airplane
Photogrammetry – science of measuring geometry from images

12. Aerial Image Attributes
Image scale – relative distance on image to distance on ground
Images contain distortions typically not found on maps
Non-uniform scale across image
Usually specify an average scale

13. Aerial Image Attributes
Extent – Area covered by image
Depends on…
Physical size of recording media
Lens system
Flying height

14. Aerial Image Attributes
Image resolution – smallest object that can be reliably detected on an image
Typically resolution is measured between two highly contrasting colors
Resolution depends on film grain in analog cameras and sensor density in digital cameras
Resolution is often tested using alternating patterns of black and white lines

15. Photo Test Pattern
Higher Resolution
Lower resolution

16. Widely used types of Film
Panchromatic – black and white
Inexpensive and wide exposure range
True Color – sensitive to light across visible spectrum
Requires three emulsions

17. Widely used types of Film
Infrared – Sensitive to visible and infrared wavelengths
Sensitive to difference in brightness – high contrast
Used widely for vegetation mapping

18. Digital Aerial Cameras
Quickly replacing film cameras
Eliminates conversion
Instead of film, use Charge Coupled Device (CCD) or similar element
CCD is a rectangular array of pixels that respond to light.
Multiple CCDs may be used with a single or multiple lens