Aerial Images

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

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

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

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

Photo Test Pattern Higher Resolution Lower resolution

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

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

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