“Open Aerial Map is an open collection of aerial photographs, collected into a single coherent view of the world.”
It is run by our good friend Chris Schmidt, and it rocks.
Chris has posted imagery of Where Camp 2008 taken by Pict Earth. Pict Earth is what happens when RC hobbyists become geowankers, or vice versa. They have a number of stock radio control planes mounted with cameras and GPS units. It is very cool.
This image is centered on the registration tent at Where Camp.
Aerial imagery precision is referred to by the resolution. If you say you have ‘2 meter imagery’ it means that you have 1 pixel of image data for every 2 meters of the subject. Other things being equal, if you are closer to the subject you have higher resolution (more pixels per meter), and if you are higher you have fewer pixels.
The resolution of an image is simply the number of pixels in the image divided by the area covered by the image. Resolution is just a fancy word for ’scale.’ You could say ‘100 pixels = 1km’ which would mean 10 meter imagery.
You can calculate the area covered by an image if you know the distance to the subject, the size of the camera sensor (or film plane), and the focal length of the lens. The focal length of common lenses is given in the 35mm film equivalent. First calculate the (horizontal) angle or field of view:
Angle = 2 * ArcTan(35/2f)
Where f is the focal length of the lens. The ‘35′ is the width of a 35mm negative (35 mm film is 35 x 24mm).
As a rule of thumb a 50mm lens has a 40 degree angle of view, and the angle of view is inversely related to the focal length. A 100 mm lens has a 20 degree angle of view, 200 mm is 10, 400mm is 5 degrees, and a 25mm lens is 80%. (For 50mm and larger lenses the actual field of view is about 3% higher - but 3% seems pretty good for a rule of thumb!).
You can determine the subject width from the angle of view and the distance:
subject width = distance * sin(angle of view)
Shooting a 50 mm lens from 100 meters away you get just under 64 meters of subject in each shot.
To get pixels per / meter:
pixel resolution per meter = number of pixels / number of meters
With my 8 megapixel Canon S5IS (3264 pixels horizontal) shooting a 50mm lens at 100 meters I get about 50 pixels per meter of subject, or 2 cm resolution.
As a (rougher) rule of thumb, doubling the focal length of the lens doubles the resolution. This is about a 10% overstatement - a 400 mm lens is about 360 pixels per meter, rather than the 400 of that rule of thum.
In order to continue my theme of ‘all Gigapan all of the time’ I have documented some of these notes on another page.
