Poor man's orthorecification?

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I am working on a project in which I need images I am collecting from a quadcopter to be the have the same scale thoughout (40 pixels in the center of the image = 4 feet and 40 pixels on the edge of the image = 4 feet).

Perfect accuracy is not essential, but I do know the approximate altitude of the quadcopter taking the images. Additionally I know the field of view of the camera I am using. I know methods for orthorectifying images can get involved and require 3d models and such, but is there an easy way to do this by using the information I know and assuming the ground is flat?