I am not very experienced in photography and I get confused with the specifications of the cameras. Can someone give me an answer to the following problem: Say I have a digital camera at a distance d from the ground(actually the camera will be mounted on a helicopter). How do I measure the total picture size on ground? (which spec. from the camera determine this) and how do I mesaure the pixel size on ground? Thank you

-- Patricia Yague (pyague@insa.org), January 19, 2000

Answers

If you are at 1000 feet and you have an angle of view of about 45 degrees, you will cover and area about 800 feet on one side. If you have an 1100X1600 pixel camera one pixesl will correspond to about 6 inches or 4 pixels per sq. foot. Shoot film, get it scanned, more latitude than digital as well much more resolution especially considering how much it costs to rent the helicopter. 35 mmm is just adequate for aerials, you do better to rent a medium format camera load it with 400 iso print film shoot at as fast a shutter speed as you can get (make sure the camera is not touching any part of the helicopter because of the vibration) shoot at the middle of the day for maximum contrast, use a polarizing filter. If you have any questions e mail me, I have done a little bit of aerial photography

-- jonathan ratzlaff (jonathanr@clrtech.bc.ca), January 19, 2000.

If you need a precise answer, then find out (a) the lens focal length, (b) the number of pixels along one side, and (c) the size of the sensor along that side. Suppose these are 6mm, 1100, and 8mm. Now suppose your altitude is 300 metres (300,000mm).

If x is the distance along the ground that corresponds to the 8mm on the image, then x/300,000 = 8/6. So x = 300,000 * 8 / 6. x = 400,000. The ground distance is 400 metres. Each pixel covers 400,000 / 1100, or 364mm.

As Jonathan says, films are more often used for this, and the larger the format, the better.

-- Alan Gibson (Alan.Gibson@technologist.com), January 20, 2000.