As of Shapely version 1.2.14, coordinates are slicable. This looks very similar to GEOSExtractLine, where a subset of the LineString can be extracted.
Here are some examples how you can slice coordinates to extract a new line object:
from shapely.geometry import LineString, Point
# Original LineString used for examples
line = LineString([(30, 50), (60, 100), (100, 70), (150, 120), (190, 80)])
line.length # 235.58873956203155
# Extract a LineString from first two coordinates
LineString(line.coords[:2]).length # 58.309518948453004
# .. and from the last two coordinates
LineString(line.coords[-2:]).length # 56.568542494923804
# Everything but the first and last coordinate
LineString(line.coords[1:-1]).length # 120.71067811865476
# Every second coordinate
LineString(line.coords[::2]).length # 163.35495027417937
# Reversed/flipped
LineString(line.coords[::-1]).length # 235.58873956203155
etc.
As for the general topic of getting GEOS functionality to Shapely, that is indeed a @sgillies answer. Generally, the functionality needs to also be supported in the GEOS C API.
To extract a new LineString using linear referencing methods, here is an add-on function in pure Python. If normalized=True, then uses normalized start/end linear referencing (i.e., within [0.0, 1.0]); otherwise the default is absolute linear referencing (i.e., within [0.0, total length]). Negative end is supported as the reverse distance from the end of the linestring.
def extract_line(line, start, end, normalized=False):
"""Extract LineString from `start` to `end`"""
assert start >= 0, '`start` must be greater than or equal to zero'
if end < 0:
if normalized:
end += 1.0
else:
end += line.length
assert end > start, '`end` must be greater than `start`'
start_c = line.interpolate(start, normalized).coords[0]
end_c = line.interpolate(end, normalized).coords[0]
for start_i, c in enumerate(line.coords):
if c == start_c:
start_c = None
break
if line.project(Point(c), normalized) >= start:
break
for end_i, c in enumerate(line.coords):
if c == end_c:
end_c = None
end_i += 1
break
if line.project(Point(c), normalized) >= end:
break
extracted = line.coords[start_i:end_i]
if start_c:
extracted.insert(0, start_c)
if end_c:
extracted.append(end_c)
return LineString(extracted)
Examples:
# Your example in the question:
extract_line(LineString([(0,0), (10,0)]), 0.5, 1, True).coords[:]
# [(5.0, 0.0), (10.0, 0.0)]
# My example, length of 10.0 removed from each end:
extract_line(line, 10, -10).coords[:]
# [(35.144957554275265, 58.57492925712544), (60.0, 100.0),
# (100.0, 70.0), (150.0, 120.0), (182.92893218813452, 87.071067811865476)]
Best Answer
Haven't tried, but found a tweet that looks promising and leads to a repo even more promising.
Seems like the concept of "buildpack" is a way to get things loaded on Heroku, and someone has made GEOS iavailable as a buildpack. By setting a custom geos-path you can install Shapely via pip and it will find GEOS.