Columbus supports keywords that allow you to indicate which coordinate components are
to be fixed, refraction settings and deflection of the vertical settings. Defined 
refraction or deflection values apply to all applicable stations and observations that 
follow in the file. You can turn them off at any time by setting them to zero.




Example 1: 

Fixing station components for adjustment. To set station Point Lookout to be fixed in
3D, do the following:

! Fix northing coordinate (latitude or north).
_LAT_NORTH_FIX; Point Lookout


! Fix easting coordinate (longitude or east).
_LON_EAST_FIX; Point Lookout


! Fix height coordinate.
_HGT_FIX; Point Lookout




Example 2:

! Set Refraction correction for Zenith Angle observations that follow in the file.
!              Constant
_REFRACTION;   0.5

! Define some observations with zenith angles.
_OBS_AZ_SET;  A; D; 0.0; 0.001; 93.0; 25.0; 106.25; 0.05; 0; 0
_OBS_HA_SET;  A; B; D; 90.0000; 20.0; 88.0000; 25.0; 93.7500; 0.050; 0; 0
_OBS_ZEN; 101; 102; 93.3030; 15; 0; 0

! Now, turn Zenith Angle constant off or change to another value for zenith observations.
! that follow.
_REFRACTION;  0


During a 3D adjustment, the actual refraction correction (for each zenith angle) is 
calculated from the constant using the following equation:

RC = (K x D) / (2 x R)

Where:

RC = refraction correction in radians
K  = zenith refraction constant (0.0 - 0.5)
D  = chord (slope distance) between At and TO stations
R  = radius of earth + ellipsoidal height + instrument height at occupied station

zenith (corrected) = zenith (uncorrected) + RC




Example 3:

! Set deflection of the vertical for stations (_STA_COORD records) that follow in the file.
!           Value (in seconds)
_DEFL_NS;   15.0
_DEFL_EW;  -10.0


! Now, turn off
_DEFL_NS;   0
_DEFL_EW;   0


! To set a deflections to zero (where 0.0 is the actual deflection), use a small number.
_DEFL_NS;   0.0001
_DEFL_EW;   0.0001