My problem is to find at what height we are when we take a photo of an image of ground when we are in aircraft

actually this my project what we are doing is for an unmanned aircraft if the communication link is failed during landing then there is no way to land it. so what we are doing is we will take the photo of the run way or ground and we have to calculate distance between the run way and the where we are and what height we are. this is our problem plz some one help me .