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"""
Filter all objects with a coastline tag.
This example shows how to write objects to a file.
We need to go twice over the file. First read the ways, filter the ones
we are interested in and remember the nodes required. Then, in a second
run all the relevant nodes and ways are written out.
"""
import osmium
import sys
if __name__ == '__main__':
if len(sys.argv) != 3:
print("Usage: python filter_coastlines.py <infile> <outfile>")
sys.exit(-1)
# go through the ways to find all relevant nodes
nodes = set()
# Pre-filter the ways by tags. The less object we need to look at, the better.
way_filter = osmium.filter.KeyFilter('natural')
# only scan the ways of the file
for obj in osmium.FileProcessor(sys.argv[1], osmium.osm.WAY).with_filter(way_filter):
if obj.tags['natural'] == 'coastline':
nodes.update(n.ref for n in obj.nodes)
# go through the file again and write out the data
writer = osmium.SimpleWriter(sys.argv[2])
# This time the pre-filtering should only apply to ways.
way_filter = osmium.filter.KeyFilter('natural').enable_for(osmium.osm.WAY)
# We need nodes and ways in the second pass.
for obj in osmium.FileProcessor(sys.argv[1], osmium.osm.WAY | osmium.osm.NODE) \
.with_filter(way_filter):
if obj.is_node() and obj.id in nodes:
# Strip the object of tags along the way
writer.add_node(obj.replace(tags={}))
elif obj.is_way() and obj.tags['natural'] == 'coastline':
writer.add_way(obj)
writer.close()
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