add a input file command #output_dir to specify the directory of output files other than the directory of the input files.

docs/source/input.rst

@@ -153,6 +153,18 @@ Allows you to control the amount of information displayed on screen when gprMax
 
 where ``c1`` can be either y (yes) or n (no) which turns on or off the messages on the screen. The default value is y. When messages are on, gprMax will display on the screen information the translation of space and time values to cell coordinates, iteration number, material parameters etc... This information can be useful for error checking.
 
+#output_dir:
+----------
+
+Allows you to control the directory of output files.  The syntax of the command is:
+
+.. code-block:: none
+
+    #output_dir: str1
+
+where ``str1`` can be either  the absolute path of the output directory or  relative to the directory of the input files. The default value is the same as the directory of the input files. 
+
+
 #num_threads:
 -----------------