Zenith distance data (or vertical angles) are expressed as the angle measured between the gravitation vertical direction (upwards) and the target.
They should be entered without refraction and curvature corrections applied (these corrections are done by SNAP). SNAP can be used to calculate one or more refraction coefficients to best fit the data.
The zenith distance observations 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 errors of the observations are expressed in seconds, or in degrees if the #deg_angles format is used.
The errors and residuals of zenith distances are expressed in seconds in the SNAP output files.