What is an orthophoto?
An orthophoto (also known as a orthophotograph) is an aerial image that has been geometrically corrected (ortho rectified) so that the image is uniform from edge to edge. Orthophotos are corrected to remove terrain effects (what happens when you take a 3-D surface and make it into a 2-D product) and distortions that result from the camera's lens and the angle the photo was taken from the plane. The goal of ortho rectification is to create an image where distance measurements are the same across the entire image.
A digital orthophoto typically has a geographic reference to the Earth, such as a UTM or State Plane coordinates, so each pixel in the photo can be accurately located. Many of the digital aerial photographs available through GIS clearinghouses are orthophotos.