explodegeopandaslinestringpythonshapely
I am trying to find explode lines tool in QGIS equivalent in shapely or geopandas (which uses shapely anyway). does anyone know this?
note that geopandas.GeoDataFrame.explode() does not work because it converts multilinestring into linestring which is not the issue.
Suppose I have a LineString:
'LINESTRING ((0,0),(0,1),(0,2),(0,3),(0,4))'
How do I explode this into
'LINESTRING ((0,0),(0,1))'
'LINESTRING ((0,1),(0,2))'
'LINESTRING ((0,2),(0,3))'
'LINESTRING ((0,3),(0,4))'
This has been solved in a number of posts. Here's the one from Paul Ramsey that I like the best: http://blog.cleverelephant.ca/2015/02/breaking-linestring-into-segments.html
It boils down to:
WITH segments AS (
SELECT gid, ST_AsText(ST_MakeLine(lag((pt).geom, 1, NULL) OVER (PARTITION BY gid ORDER BY gid, (pt).path), (pt).geom)) AS geom
FROM (SELECT gid, ST_DumpPoints(geom) AS pt FROM lines) as dumps
)
SELECT * FROM segments WHERE geom IS NOT NULL;
This is typically a result of the the borders not fitting perfectly one next to another (and this is very easy to get with floating point coordinates).
As an example, I use the world dataset available in geopandas, and take the unary union of the Africa continent:
import geopandas
gdf = geopandas.read_file(geopandas.datasets.get_path('naturalearth_lowres'))
gdf_africa = gdf[gdf['continent'] == 'Africa']
africa = gdf_africa.geometry.unary_union
# only take main continent, disregard Madagascar
africa = list(africa)[1]
You can see on the following plot that also this union is not perfect:
This is a typical problem to encounter when doing unary unions, so now how to solve this:
One possible solution is to take the exterior of this polygon (so disregarding the interior holes) (taken from this answer), and create a new polygon of it:
from shapely.geometry import Polygon
africa2 = Polygon(africa.exterior)
which gives this result:
which is already much better. There is still a small distortion at the left, because such a 'hole' touched the overall border.
Another solution is to apply a very small buffer to the polygon, and undo it again (taken from this answer)
from shapely.geometry import JOIN_STYLE
eps = 0.00001
africa3 = africa.buffer(eps, 1, join_style=JOIN_STYLE.mitre).buffer(-eps, 1, join_style=JOIN_STYLE.mitre)
which gives:
In this case, this solution looks better from the figure (no remaining distortion on the left), but this method can slightly change the coordinates (although this is often negligible).
Which of the methods works best for you, probably depends on the actual case.
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