E. Neat Model

1. Endlap and Sidelap

To see a 3D model requires overlap between successive photographs, Figures E-1 and E-2. 

Figure E-1 Endlap - Flight

Figure E-2 Endlap - Photos

B is the airbase, the distance between successive exposure points.

How much overlap is needed? For most mapping photography 60% or more is used so the principal points of both photos (o₁ and o₂) are in the overlap area. Including both principal points helps identify the actual flight path.

What about using 100% overlap? Not possible unless the camera is in a stationary helicopter or drone. If it is, then a 3D model isn't possible because the perspective doesn't change. The greater the overlap, the more photos needed to cover the same area. Maximum overlap is limited to about 75%.

Most aerial photography is flown in strips so their overlap is referred to as endlap (photos are end-to-end).

Adjacent strips are also overlapped but not to the same extent. Sidelap, Figures E-3 and E-4, is typically 15-30%.

Figure E-3 Sidelap - Flight

Figure E-4 Sidelap - Photos

  W is the distance between adjacent flight lines.