During most import routines in TerrSet, you are given the opportunity to document the reference system and the X and Y coordinates. A default reference system of “plane” is often inserted by default. If you do not know the correct reference system or coordinates, leave the defaults. However, a plane reference system cannot be projected and must be resampled if you intend to use these data with other data that do contain a proper reference system.
The plane coordinate system is known as a 'local' grid referencing system. A grid referencing system can be thought of very simply as a systematic way in which the plane coordinates of the map sheet can be related back to the geodetic coordinates of measured earth positions. This plane coordinate system uses an arbitrary plane system whereby the geodetic and projection parameters are unknown.
This lack of parameter set makes a plane reference system unable to be directly projected. If the reference system is unknown, then you will need to use the module RESAMPLE to properly transform the data to a known reference system with non-local coordinates.
Note that in some cases after an import, the reference system may be documented as plane, but the X and Y coordinates may be in their correct units and range. In this case, searching the metadata that accompanied the raw data may determine the correct reference system. In this case, simply edit the reference system using the Metadata panel in TerrSet Explorer for those data.