You should be able to:

1. list and describe the choices you must make in selecting an aerial photo

2. list and describe potential problems with an aerial photo

A. There are some basic photo parameters you need to consider when selecting from existing aerial images or ordering custom images

1. the desired scale – which is determined by focal length and height. The larger the scale the more images (and dollars) are needed to cover the same area.

a. remember scale is determined by the focal length of the camera and the height above ground

2. desired resolution – what is the smallest feature you need to be able to detect?

3. desired overlap – greater overlap allows better stereo viewing and assures complete coverage

4. color / tone – do you need color images, color infrared or black and white?

a. trade offs include poorer resolution on color film, ability to detect vegetation type

5. season – do you need l leaves on to measure forest cover? leaves off to see what’s below the trees? spring or fall for tree species identification?

B. There are some problems that may occur on an aerial photo that you need to look out for

1. lens distortion – the image may be distorted by the lens, especially toward the edges with a short focal length (wide angle) lens

2. tilt – if the camera is not looking straight down, the scale will vary across the image

3. elevation changes – in hilly terrain the scale will be larger on the tops of hills and smaller in valleys

4. flight track – the actual path of the airplane may differ from the desired track, affecting overlap and photo orientation

5. crab – to maintain a straight flight path, the plane may have to fly at an angle into the wind so images will be rotated relative to the flight path (edges of adjacent photos won’t line up)