fix [automatically] [type] [all] [station_list]
Defines which stations are to be constrained to be fixed in the adjustment. That is their coordinates are not calculated but are treated as being exactly correct. By default all stations are adjusted if there are observations to them. The parameters are:
automatically
if present specifies that the constraint is only applied it appears to be necessary. See below for more details.
type
specifies which coordinates are to be fixed. It can be one of "horizontal", "vertical", or "3d". If the type is omitted then all the station coordinates are held fixed.
all
may be included if you want to fix all stations. You can then use the free command to select a few stations that are to be adjusted. (This is useful if you only want to adjust a few stations).
station_list
is a list of stations to be held fixed separated by blank spaces.
Here are some examples of the fix command
fix all
fix automatically all
fix 1 5 23
fix horizontal 1A BM3 50-55
fix vertical 3 4 5A
fix 3d order=1 order=2 order=3
Automatically constraining stations is not guaranteed to be sufficient for a solution. The constraints are applied based upon the types of observations to the station. Snap tests for observations which depend on horizontal position (such as bearing, ellipsoidal distance) or on height (such as levelling, slope distances). It will only constrain a station if there are no observations to the station that are influenced by the constraint (this also means that there are no observations providing the levelling or slope distance). Note that this is only based on observations that are being used. The constraints added may mean that residuals of rejected observations are misleading, as they are based on constrained rather than calculated coordinates. types.
Note that "fix automatically 3d" will fix vertically or horizontally as necessary. It will never fix a station both ways - stations for which neither horizontal or vertical coordinates can be calculated will be rejected.
Use the command list problem_definition command to see a list of stations automatically fixed.