A station coordinate file is a data file containing a list of all the survey stations to be used in an adjustment. Each station is denoted by a code, and has a position. A station may also have a short text description (usually the name of the stations), a station coordinate order, geoid information, and other user defined mark classifications (attributes) such as mark type.
The coordinate file also defines the coordinate system to which the station positions are referenced. The coordinate system is defined by a coordinate system code such as "NZGD2000". The codes and coordinate systems they represent are define in the coordinate system definition file
The coordinate system may be a projection coordinate system, with positions defined in terms of easting, northing, and height, a geodetic (or geographic) system with positions defined in terms of longitude, latitude, and height, or a geocentric system with positions defined in terms of X, Y, and Z coordinates relative to the centre of mass of the earth.
The geoid information for each station can include both geoid height (undulation) and east and north deflection of the vertical. It can be specified explicitly for each station, or implicitly by defining a geoid, or vertical datum, that will be used to calculate these values for each station.
A vertical datum can be selected as part of the coordinate system code. The coordinate system definition file defines a list of vertical datums, such as NZVD2016. The vertical datum code is appended to the coordinate system code to define the station coordinate system. For example the station file coordinate system could be NZGD2000/NZVD2016, meaning the coordinates are in terms of NZGD2000 using geoid information (geoid height and deflection of the vertical) from the NZVD2016 reference surface.
The height coordinate in the station coordinate file is assumed to be an ellipsoidal height if no vertical datum is specified, or an orthometric height if there is a vertical datum. However this can be overridden by options in the station coordinate file. However the height coordinate can be explicitly set in the coordinate file to be ellipsoidal or orthometric.
Note that if the coordinate system defines a vertical datum and also explicit geoid information for each station then the vertical datum geoid heights will be ignored. When the geoid height is recalculated by snap programs such as snapgeoid then either the ellipsoidal or orthometric height must be changed. The height supplied in the coordinate file will be unaltered (that is, if the coordinate file contains ellipsoidal heights then the orthometric height will be changed, and vice versa).
The coordinate system code can also select an alternative reference frame for a projection coordinate system by appending the reference frame code in brackets. For example "EDENTM2000(NZGD2000_20140201)" uses the EDENTM2000 tranverse mercator projection on version 20140201 of NZGD2000.
For network adjustments using snap the station positions need only be approximate except for the fixed stations in the adjustment, as snap will calculate improved coordinates using the survey observations. One of the outputs of snap is a revised coordinate file containing the adjusted coordinates.
Approximate station coordinates can be generated from observation data using the program dat2site.
The station coordinate data may be modified by snapconv (which changes the coordinate system) and snapgeoid (which adds or removes geoid information) as well as by snap itself.
The SNAP programs can use two basic formats for coordinate files - either the SNAP format, or a generic CSV delimited text format. The CSV format requires a format definition file to specify how the station information is organised in the CSV file.
Currently several of the SNAP utility programs only support SNAP format coordinate files. In particular updated coordinate files generated by the programs are always in SNAP format.