The 3D point residual is generated when the location of the associated 3D point recorded in the C3D file is calculated, and records the average accuracy distance of the point, calculated by the photogrammetry software and recorded in POINT:UNITS that documents the intersection of the rays used to generate the 3D point locations.  Low residual numbers indicate that the 3D point locations are more accurate when these numbers are derived from the measured 2D vector data used for the photogrammetry reconstruction and will always be absolute, non-zero values.

Residual values of zero indicate that the point was not directly derived from measurements, i.e. the associated 3D coordinates were estimated by interpolation, affected by filtering, or simulated from a software model.  Negative residual values indicate that the stored 3D point is probably invalid.  It is recommended that all residual calculation methods are fully documented.