The Grids | Edit | Assign Coordinate System command links a grid file to a specific coordinate system. Once the coordinate system is defined for the grid file, a Golden Software Georeference .GSR2 file is created. This file contains all the relevant projection information that Surfer needs to load the grid in the proper projection.
Use the Data | Coordinate System | Assign Coordinate System command to link a data file to a .GSR2 file. 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.
If the data file did not have an assigned coordinate system, or the grid file was provided directly from another source, the Grids | Edit | Assign Coordinate System can be used to link the grid with a .GSR2 file.
When any grid operation is performed on the referenced grid file, the Export Options dialog appears with the option to create a new .GSR2 file for the new output grid. For instance, if you use the Grids | Edit | Filter command and smooth the grid, the output smoothed grid will contain a .GSR2 file with the same coordinate system information as the original grid. When a map is created from the smoothed grid file, the .GSR2 file is read and the map layer automatically has the correct coordinate system. For commands that use multiple files, such as Grids | Edit | Assign NoData or Grids | Resize | Mosaic, all files should be in the same reference system, otherwise unexpected results may occur. The .GSR2 file in these cases is assigned by the first .GRD file specified.
Assign Coordinate System Dialog
Define Coordinate System
Custom Datum Definitions
Introduction to Map Projections
Assign Coordinate System - Worksheet
Map Coordinate System