The Data | Coordinate System | Assign Coordinate System command links a data file to a specific coordinate system. Once the coordinate system is defined for the data file, a Golden Software Georeference .GSR2 file is created. This file contains all the relevant projection information that Surfer needs to load the data in the proper projection.
When a .GRD file is created using the Grids | New Grid | Grid Data command, the .GSR2 file for the data is read. The projection information can be saved with the grid file using the Spatial References options. It is recommended to check the GS Reference (Version 2) file if you intend to use the grid file in Surfer, as the GSR2 retains all of the information needed. The grid has the same coordinate system as the original data file, but the .GSR2 is required to define the coordinate system. When a map is created from either the data file or the .GRD file, the .GSR2 file is read and the map layer automatically has the correct coordinate system.
Assign Coordinate System Dialog
New Projected Coordinates
Define Coordinate System
Custom Datum Definitions
Introduction to Map Projections