Aerial imagery taken from a camera positioned directly vertical relative to the ground, allowing the observer to see only the roofs of tall buildings, or, imagery which has been mathematically adjusted using photogrammetry techniques to achieve the same perspective.  Orthophoto are precisely orthogonal, or at a right angle to the camera.  The result is that the imagery is uniformly scaled and can be used to measure distances and angles in the same way as a planimetric map.