Projection bearings are bearings as calculated between the coordinates of the instrument and target station in a projection. These are not true observations. Usually the only time projection bearings will be used is for reprocessing data recorded on survey plans.
The data file must specify the coordinate system in which the projection is defined using the #projection command. This specifies one of codes in the coordinate system definition file.
By default SNAP can only calculate projection bearings if the projection coordinate system uses the same datum as the network coordinate system. However this can be relaxed using the proj_bearing_use_datum configuration command in the SNAP command file.
Projection bearings may be subject to an associated bearing orientation error. This is a constant angular offset (measured in seconds) which may be applied to, or calculated for, a group of azimuth observations. For example if several survey plans are being processed each may have an associated bearing orientation error.
The projection bearings are formatted in input files either as degrees, minutes, and seconds, in the HP format, or as decimal degrees as define by the #hp_angles, #deg_angles, and #dms_angles data definition commands. The error is in seconds, or in degrees if the #deg_angles option is used.
The errors and residuals of bearings are expressed in seconds in the SNAP outputs.
The #bearing_orientation_error command