The original source was downloaded from http://www.saga-gis.org 
The main upstream author is: Olaf Conrad <oconrad@saga-gis.org>

On two places files were removed to adhere to the DFSG:
- the files triangle.c and triangle.h were removed from libgrid_gridding
  because they are incompatible with the DFSG. The functionality
  could be retained by linking to libqhull
  src/modules/grid/grid_gridding/nn/triangle.[c/h]
- the module fire simulation: src/modules_simulation/fire/*
  it relies on a non DFSG library: libfire  (www.fire.org)

These steps can be reproduced by running debian/rules get-orig-source