The Edit-Undo Menu Option

This menu option will undo the last major edit that you 
have made.  As you edit a raster map, the raster map cell 
values that you change are stored in an "undo buffer" so 
that you may recover them if you make a mistake.  When 
you use the Edit-Undo menu option all of the raster map 
cell values in this undo buffer are restored to their
original values. 

The undo buffer is cleared each time that you use the 
@ref("View-Redraw",xgre/view_redraw) menu option and when you exit from the editor.  
It is also cleared when you use the @ref("Edit-Polygon-Region",xgre/edit_poly)
and @ref("Edit-Box-Region",xgre/edit_box) menu options.

You should clear the edit buffer periodically.  Otherwise, 
when you use Edit-Undo too many things may be "undone."  
Once you have made a number of edits and you are reasonably 
sure that they are acceptable, you should use the View-Redraw 
to clear the undo buffer.




