SNAP station offset file format

A station offset file defines the history of station offsets for selected stations as a time series. Station offsets may be used to define deformation affecting a station which is not included in the deformation model, or offsets such as CORS station antenna height changes which are not included in data files. Offsets for stations are defined as a function of time.

The offsets of each stations may be either relative to the deformation model used in the adjustment, or to the datum on which it is based. In the latter case the deformation model is ignored of the station. More than one offset can be defined for each station - all the offsets that apply at a given date are added together.

Note that where the offset replaces a deformation model then it must be defined in terms of the same reference frame as that underlying the deformation model. For example if it is replacing the NZGD2000 deformation model then it must define the station time series in terms of ITRF96.

Each line in the station offset file defines one offset, except that blank lines and lines starting with "!" are ignored.

Each offset line is formatted as:

code [coordtype] [offset_type] [mode] type data

The coordtype keyword can be one of:

• "enu" - the default value, offsets are specified as east/north/up components
• "height" - the offset is just a vertical component
• "xyz" - the offset is specified in terms of geocentric XYZ coordinates

The offset_type keyword can be:

• "offset" - the default value, offsets are relative to the deformation model
• "deformation" - the offset replaces the deformation model for the station.

The mode keyword can be:

• "step" - the default value, the data will consist of a date, followed by the coordinate offset. The offset will apply to all station coordinates after that date.`
• "time_series" - the data consists of a series of values offset date offset ... date offset. The dates must be in sequence. The first offset applies before the first date, and the last offset after the last date. Between two dates the offset is interpolated linearly between the values after those dates.
• "velocity" - the model data consists of a date and velocity components. The offset at any date is given by applying the velocity relative to the specified date.