Index: trunk/SDToolBox/output_messages.py
===================================================================
diff -u -r59 -r60
--- trunk/SDToolBox/output_messages.py	(.../output_messages.py)	(revision 59)
+++ trunk/SDToolBox/output_messages.py	(.../output_messages.py)	(revision 60)
@@ -21,3 +21,5 @@
 error_all_arguments_required = 'All arguments are required for resampling.'
 error_max_lon_not_set = 'Max longitud needs to be set.'
 error_needs_to_be_in_subclass = 'Needs to be implemented in subclasses.'
+error_no_gradient_was_calculated = 'No gradient was calculated, ' + \
+                                    ' please calculate the gradient on the xarray before proceeding'