Error creating rupture enclosing polygon from area source

[Lars Butler]

Traceback (most recent call last):
  File "bin/openquake", line 467, in <module>
    main()
  File "bin/openquake", line 447, in main
    args.force_inputs, args.exports)
  File "bin/openquake", line 219, in run_hazard
    completed_job = engine2.run_hazard(job, log_level, log_file, exports)
  File "/usr/local/openquake/oq-engine/openquake/engine2.py", line 336, in run_hazard
    return _run_calc(job, log_level, log_file, exports, calc, 'hazard')
  File "/usr/local/openquake/oq-engine/openquake/engine2.py", line 403, in _run_calc
    _do_run_calc(job, exports, calc, job_type)
  File "/usr/local/openquake/oq-engine/openquake/engine2.py", line 476, in _do_run_calc
    calc.pre_execute()
  File "/usr/local/openquake/oq-engine/openquake/calculators/hazard/classical/core.py", line 229, in pre_execute
    self.initialize_sources()
  File "/usr/local/openquake/oq-engine/openquake/calculators/hazard/general.py", line 632, in initialize_sources
    src_db_writer.serialize()
  File "/usr/lib/python2.7/dist-packages/django/db/transaction.py", line 217, in inner
    res = func(*args, **kwargs)
  File "/usr/local/openquake/oq-engine/openquake/input/source.py", line 351, in serialize
    geom = nhlib_src.get_rupture_enclosing_polygon()
  File "/usr/local/openquake/nhlib/nhlib/source/area.py", line 67, in get_rupture_enclosing_polygon
    return self.polygon.dilate(max_rup_radius + dilation)
  File "/usr/local/openquake/nhlib/nhlib/geo/polygon.py", line 149, in dilate
    return type(self)._from_2d(new_2d_polygon, self._projection)
  File "/usr/local/openquake/nhlib/nhlib/geo/polygon.py", line 99, in _from_2d
    xx, yy = numpy.transpose(polygon2d.boundary.coords)
  File "/usr/lib/python2.7/dist-packages/shapely/geometry/base.py", line 486, in coords
    "Multi-part geometries do not provide a coordinate sequence")
NotImplementedError: Multi-part geometries do not provide a coordinate sequence

This error originated from an area source, represented by a polygon which is rather convex. When we tried to dilate the polygon (`get_rupture_enclosing_polygon`), a hole was created in the geometry and resulted in a multipolygon.

There are two possible actions to address this error:

1) Give a more precise error message to the user, explaining which source, including ID and name, caused the error.
2) If the rupture enclosing polygon is a multipolygon, take the exterior of the shape so as to not include the hole. This is valid because the rupture enclosing polygon is only considered in site-to-source distance filtering.

[Lars Butler (lars-butler)]

Patch: https://github.com/gem/nhlib/pull/83