Understanding `gis.geos.MultiLineString.closed` in Django Contrib GIS

gis.geos.MultiLineString

• MultiLineString objects are useful for representing features like rivers, roads, or boundaries that may consist of multiple disconnected line segments.
• Each LineString is an ordered sequence of points defining a linear path.
• Represents a collection of one or more non-intersecting LineString geometries in Django's GEOS integration.

closed Property

• A closed LineString has its first and last points coinciding, forming a complete loop.
• The closed property of a MultiLineString object in django.contrib.gis returns a boolean value indicating whether all individual LineString components within the MultiLineString are closed.

How it Works

1. GEOS checks each LineString within the MultiLineString to see if the first and last points have the same coordinates.
2. If all LineString components are closed, closed returns True. Otherwise, it returns False.

Example

from django.contrib.gis.geos import MultiLineString

# Closed MultiLineString (two closed LineStrings)
closed_mls = MultiLineString([((0, 0), (1, 0), (1, 1), (0, 1), (0, 0)),
                             ((2, 2), (3, 2), (3, 3), (2, 3), (2, 2))])

# Check if closed
if closed_mls.closed:
    print("The MultiLineString is closed.")
else:
    print("The MultiLineString is not closed.")

When to Use closed

• Identify potential errors in data where a MultiLineString might be intended to be closed but isn't.
• Check if a MultiLineString represents a closed boundary or path. This is useful in functions that work with areas or perimeters, as these typically require closed geometries.

• If you need to ensure a single closed loop, consider using MultiPolygon objects, which are collections of closed Polygon geometries.
• closed only checks for closure within individual LineString components. It doesn't guarantee that the entire MultiLineString forms a single, continuous closed loop.

---

Identifying Closed and Non-Closed MultiLineStrings

from django.contrib.gis.geos import MultiLineString

# Closed MultiLineString (two closed LineStrings)
closed_mls = MultiLineString([((0, 0), (1, 0), (1, 1), (0, 1), (0, 0)),
                             ((2, 2), (3, 2), (3, 3), (2, 3), (2, 2))])

# Non-Closed MultiLineString (missing closing point in second LineString)
non_closed_mls = MultiLineString([((0, 0), (1, 0), (1, 1), (0, 1)),
                                 ((2, 2), (3, 2), (3, 4))])  # Missing closing point here

# Check if closed
if closed_mls.closed:
    print("The first MultiLineString is closed.")
else:
    print("The first MultiLineString is not closed (shouldn't happen).")

if non_closed_mls.closed:
    print("The second MultiLineString is closed (incorrect).")
else:
    print("The second MultiLineString is not closed (correct).")

Handling Potential Errors in Data

def process_multiline(mls):
    if not mls.closed:
        print(f"Warning: MultiLineString (ID: {mls.id}) is not closed. Consider fixing the data.")
    # Rest of your processing logic here...

# Example usage
multiline_data = ...  # Load your MultiLineString data
process_multiline(multiline_data)

from django.contrib.gis.geos import MultiPolygon

# Create a MultiPolygon from closed MultiLineStrings
closed_mls = ...  # Your closed MultiLineString (as in example 1)
multipolygon = MultiPolygon(closed_mls)

if multipolygon.closed:
    print("The MultiPolygon is closed (as expected).")

# You can now use multipolygon for area calculations, etc.

---

Custom Function with GEOS Operations

from django.contrib.gis.geos import GEOSGeometry, MultiLineString

def is_multiline_closed(mls):
    """
    Checks if all LineStrings in a MultiLineString are closed.

    Args:
        mls (MultiLineString): The MultiLineString to check.

    Returns:
        bool: True if all LineStrings are closed, False otherwise.
    """

    for line in mls:
        if not GEOSGeometry(line).isClosed():  # Use GEOSGeometry for GEOS methods
            return False
    return True

# Example usage
my_multiline = ...  # Your MultiLineString
if is_multiline_closed(my_multiline):
    print("The MultiLineString is closed.")
else:
    print("The MultiLineString is not closed.")

Looping and Point Comparison

You can iterate through each LineString in the MultiLineString and compare the first and last points. However, this approach might be less efficient for large datasets compared to using GEOS methods directly.

def is_multiline_closed(mls):
    """
    Checks if all LineStrings in a MultiLineString are closed by comparing first and last points.

    Args:
        mls (MultiLineString): The MultiLineString to check.

    Returns:
        bool: True if all LineStrings are closed, False otherwise.
    """

    for line in mls:
        first_point = line[0]
        last_point = line[-1]
        if first_point != last_point:
            return False
    return True

# Example usage (same as above)

Working with MultiPolygon

If your primary concern is ensuring a single closed loop, consider converting the MultiLineString to a MultiPolygon if possible. By definition, all Polygon geometries within a MultiPolygon are closed. This approach might not be feasible for all scenarios, but it could be an option for specific use cases.