Quality of Images

Geometric Quality of Aerial Images All aerial images contain geometric distortion Orthographic – plots position of objects after being projected onto a common plane Equivalent to looking vertically downward onto a scene from an infinite height

Most aerial photos do not provide an orthographic view…they provide a perspective view, which is a geometrically distorted image of the Earth’s surface.

Aerial Image Distortion Six primary sources Source Amount of Error Terrain Major Camera Tilt Film Deformation Minor Camera Lens Atmospheric Bending Other Camera Errors

Terrain Distortion Terrain variation is often largest source of geometric distortion Causes relief displacement

Displaced location of building corner X True location building corner Example Location of tall objects are displaced away from the center of the photograph Center of photograph X Displaced location of building corner X True location building corner

Terrain Distortions Terrain distortions are radial Relief distortions affect angles and distances on an aerial image Scale is not constant on aerial images A vertical aerial image taken over varied terrain is not orthographic

Camera Tilt Distortion Camera tilt is when the optical axis points in a non-vertical angle Results in complex perspective convergence

Camera Tilt Distortion Camera mounting systems are devised to keep optical axis pointing directly below Tilt is characterized by three angles of rotation Pitch Roll Yaw

Other Errors Radial Lens Displacement – imperfections in camera lens surface Systematic errors Ex. Deformed plattern - geometric distortion Atmospheric Distortion – bending of light

Geometric Correction Photogrammetry provides tools needed to remove geometric distortion from photos Correction process requires measurement of image coordinates and their corresponding ground coordinates End result: orthographic image -an image with terrain and perspective distortions removed