DOQQ

DOQQ - Digital Orthophoto Quarter Quad - aerial photo that is geometrically corrected so that the scale is uniform.  An orthophotograph can be used to measure true distance as it is an accurate representation of the Earths surface.

http://www.utexas.edu/depts/grg/hudson/grg360g/EGIS/final_project/spring04_projects/web_justin_reynolds/Lake%20Travis.htm

http://www.utexas.edu/depts/grg/hudson/grg360g/EGIS/final_project/spring04_projects/web_justin_reynolds/Lake%20Travis_files/blue_Travis_doqq.jpg