Remember that GIS uses a common coordinate system to analyze geographic data of different types.  The aerial photographs, however, were just simple digital images and did not have geographic positions, so it was necessary to first georeference the photographs before they could be used by GIS.  Georeferencing is the assignment of a coordinate position to a digital image.
	We downloaded existing data layers of the roads and water bodies of the Togus Pond watershed from the Maine Office of GIS, which were already positioned in a coordinate system.  Then, we assigned control points by matching points in the existing layers with their corresponding points on the photographs.  Obvious road intersections and shoreline points were used as the control points, and the assignment of each point caused the photograph to warp to fit into the assigned position.
	The image on the right is an example of one of the 2002 photographs after it was georeferenced to fit the roads layer.