diff --git a/create_polygon.py b/create_polygon.py
index e2c14e4..b88fcf5 100755
--- a/create_polygon.py
+++ b/create_polygon.py
@@ -1,27 +1,26 @@
 #!/bin/python3
 
+import argparse
 import osmapi
 import shapely
 import pickle
-from os.path import exists
-
-
-OSM_KOGGENLAND_ID = 161930
-PICKLE_FILE = 'polygon.pickle'
 
 
 def chunks(lst, n):
-    """Yield successive n-sized chunks from lst."""
+    """Yield successive n-sized chunks from lst.
+    https://stackoverflow.com/a/312464
+    CC BY-SA 4.0
+    """
     for i in range(0, len(lst), n):
         yield lst[i:i + n]
 
 
-def dump_polygon():
-    osm_api = osmapi.OsmApi()
-    koggenland = osm_api.RelationGet(OSM_KOGGENLAND_ID)
-    ways = filter(lambda x: x['type'] == 'way', koggenland['member'])
-    shell_ids = []
-
+def get_shell_ids(osm_api, ways):
+    """
+    Get a list of OSM node IDs
+    that forms the shell of the area's polygon.
+    """
+    shell_ids = list()
     for way in ways:
         way = osm_api.WayGet(way['ref'])
         node_ids = way['nd']
@@ -30,35 +29,52 @@ def dump_polygon():
         for node_id in node_ids[1:]:
             shell_ids.append(node_id)
 
+    return shell_ids
+
+
+def get_node_data(osm_api, shell_ids):
+    """Get the data of OSM nodes as a dict of (id -> data)."""
     node_data = dict()
     for chunk in chunks(shell_ids, 100):
         node_data |= osm_api.NodesGet(chunk)
 
+    return node_data
+
+
+def get_shell(shell_ids, node_data):
+    """Get the shell of the area's polygon as a list of (lat, lon) tuples."""
     shell = list()
     for shell_id in shell_ids:
         node = node_data[shell_id]
         shell.append((node['lat'], node['lon']))
 
-    koggenland_polygon = shapely.Polygon(shell)
-    with open(PICKLE_FILE, 'wb') as f:
-        pickle.dump(koggenland_polygon, f)
+    return shell
 
 
-def main():
-    if not exists(PICKLE_FILE):
-        print('Using OSM API to create polygon...')
-        dump_polygon()
-        print('Dumped polygon to file.')
-    with open(PICKLE_FILE, 'rb') as f:
-        area = pickle.load(f)
-        print('Loaded polygon from file.')
+def main(args):
+    osm_api = osmapi.OsmApi()
+    rel = osm_api.RelationGet(args.osm_relation_id)
+    # Filter out city centre etc.
+    ways = filter(lambda x: x['type'] == 'way', rel['member'])
+    shell_ids = get_shell_ids(osm_api, ways)
+    node_data = get_node_data(osm_api, shell_ids)
+    shell = get_shell(shell_ids, node_data)
+    polygon = shapely.Polygon(shell)
 
-    point_out = shapely.Point(52.68811, 4.91267)
-    point_in = shapely.Point(52.69168, 4.91324)
-
-    print('out', area.contains(point_out))
-    print('in', area.contains(point_in))
+    with open(args.output_file, 'wb') as f:
+        pickle.dump(polygon, f)
 
 
 if __name__ == '__main__':
-    main()
+    parser = argparse.ArgumentParser(description="""
+    Creates a polygon from an OpenStreetMap relation and saves this
+    to a file as a Python pickle.
+    """)
+    parser.add_argument("osm-relation-id",
+                        help="OpenStreetMap relation ID", type=int)
+    parser.add_argument(
+        "-v", "--output-file", help="Output file for the resulting polygon",
+        default="polygon.pickle")
+    args = parser.parse_args()
+
+    main(args)
diff --git a/gc_area.py b/gc_area.py
new file mode 100755
index 0000000..a980ee5
--- /dev/null
+++ b/gc_area.py
@@ -0,0 +1,36 @@
+#!/bin/python3
+
+import argparse
+import gpxpy
+import gpxpy.gpx
+import shapely
+import pickle
+
+
+def main(args):
+    with open(args.polygon_file, 'rb') as f:
+        area = pickle.load(f)
+
+    with open(args.input_file, 'rb') as f:
+        gpx = gpxpy.parse(f)
+
+    gpx.waypoints = list(filter(lambda wp: area.contains(
+        shapely.Point(wp.latitude, wp.longitude)), gpx.waypoints))
+
+    with open(args.output_file, 'w') as f:
+        f.write(gpx.to_xml())
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser(description="""
+    Takes an input GPX file containing
+    geocaches and filters out any waypoints outside a given area.
+    """)
+    parser.add_argument("input-file", help="Input GPX file")
+    parser.add_argument("output-file", help="Output GPX file")
+    parser.add_argument(
+        "-p", "--polygon-file", help="File containing area's polygon",
+        default="polygon.pickle")
+    args = parser.parse_args()
+
+    main(args)
diff --git a/parse_gpx.py b/parse_gpx.py
deleted file mode 100755
index e4e6b93..0000000
--- a/parse_gpx.py
+++ /dev/null
@@ -1,28 +0,0 @@
-#!/bin/python3
-
-import gpxpy
-import gpxpy.gpx
-import shapely
-import pickle
-
-GPX_IN_FILE = 'caches.gpx'
-GPX_OUT_FILE = 'koggenland.gpx'
-PICKLE_FILE = 'polygon.pickle'
-
-
-def main():
-    with open(PICKLE_FILE, 'rb') as f:
-        area = pickle.load(f)
-
-    with open(GPX_IN_FILE, 'rb') as f:
-        gpx = gpxpy.parse(f)
-
-    gpx.waypoints = list(filter(lambda wp: area.contains(
-        shapely.Point(wp.latitude, wp.longitude)), gpx.waypoints))
-
-    with open(GPX_OUT_FILE, 'w') as f:
-        f.write(gpx.to_xml())
-
-
-if __name__ == '__main__':
-    main()