What does orthorectification mean?

Orthorectification is the process of correcting the positioning of imagery for topography. If topography is not corrected for, then the imagery is referenced to a near-planar surface such as mean sea level. In areas with significant topography, this means that parts of the image may be displaced horizontally from their true positions by significant amounts. Therefore it is generally the case that most non-specialist users will require imagery that is at least coarse orthorectified, by which we mean that the image is rectified to an off-the-shelf model of topography which removes gross errors in positioning where topography is significant. Precision orthorectification involves a higher standard of orthorectification, and may be needed for particular applications where the positioning of the imagery is critical to its value. In this case, custom orthorectification can be applied using higher quality off the shelf elevation model, processing of stereo imagery and/or the use of ground control points.