A typical video projection system displays rectangular images on a flat screen. The physical characteristics mandate that the projector be placed as close to perpendicular to the screen as possible. However, many venues force the projector to be dislocated to inappropriate "throwing distances" or viewing angles or show misalignments. The reasons may be a general traffic flow, but also for safety reasons. Even more problems occur for any non-planar surfaces like in planetariums and domes. The image appears distorted. Optical and geometric correction techniques must be employed to prevent these distortions. This means, that the projection itself has to be modified such that the topographic relief of the screen is inherent to the transformation process. The removement of the distortions is called orthorectification.

In principle, the method of orthorectification is the same as obtaining a planimetric map from spherical coordinates and vice versa. A common example is the Mercator transform of the geographic system of the Earth (latitudes and longitudes) to Cartesian coordinates. But also other models exist. The geometric correction includes three steps:

1. Assigning a coordinate system such that each data point associates with a specific location on the screen.
2. Transformation of the raster via mathematical polynomials. These will shift, scale and rotate the raster.
3. After the transformation all locations are pointed to new positions. Comparing the actual location of the map coordinate to the transformed positions gives the so-called residual. Rectifiying it is the final step making the projection conform.

Digital images often contain geometrical distortions that arise from the curvature of the screen, platform motions, tilts and other effects.