ST_SharedPaths¶

Introduction: Returns the paths shared by two lineal geometries, grouped by traversal direction.

Format: ST_SharedPaths(A: geometry, B: geometry)

Return type: Geometry

This function supports Snowflake GEOMETRY values. It is not available for GEOGRAPHY values. Both inputs must be a LineString or MultiLineString. The result is a GeometryCollection containing exactly two MultiLineString elements. The first contains paths traversed in the same direction by both inputs; the second contains paths traversed in opposite directions. Coordinates in both elements follow the direction of A.

For non-simple inputs that traverse the same path more than once, direction follows the first matching source traversal. Consequently, swapping A and B can change which result element contains a path.

Snowflake's native GEOMETRY bridge passes values to Sedona as GeoJSON, which does not carry SRID metadata. Consequently, this variant cannot reject mixed input SRIDs or preserve a matching SRID; its result has SRID 0. The legacy binary geometry interface preserves matching SRIDs. A non-empty result retains Z only when at least one input has Z and every coordinate of every returned shared path resolves to a finite Z from the input geometries. If any returned coordinate does not resolve to a finite source Z, the entire result is two-dimensional. M values are always dropped. A result with no shared paths is also two-dimensional.

When the inputs do not share a path, including when they intersect only at points, both elements are empty MultiLineString geometries.

Visual examples¶

Same and opposite directions¶

Input B first follows A, leaves it, and later traverses another part of A backwards. The single result therefore populates both direction buckets. Notice that the path in element 1 is reoriented to follow A, even though B traverses it in the opposite direction.

Same-direction and opposite-direction paths returned together by ST_SharedPaths

Multipart input and noded output¶

Input A can be a MultiLineString. In this example, B shares part of two different components of A. The vertex at (90 161) belongs to B and splits the shared diagonal into two paths in element 0.

ST_SharedPaths over multipart input with a shared path split at an input vertex

Point-only intersections¶

Crossing at an interior point and touching at an endpoint are both zero-dimensional contacts. Neither is a shared path, so both MultiLineString elements are empty.

ST_SharedPaths ignores interior crossings and endpoint-only touches

SQL examples¶

Wrap the result in ST_AsText to display its two direction buckets as WKT.

Same direction¶

The shared interval is returned in element 0 because both inputs traverse it from left to right.

SELECT ST_AsText(ST_SharedPaths(
    ST_GeometryFromWKT('LINESTRING (0 0, 10 0)'),
    ST_GeometryFromWKT('LINESTRING (5 0, 15 0)')
));

Output:

GEOMETRYCOLLECTION (MULTILINESTRING ((5 0, 10 0)), MULTILINESTRING EMPTY)

Opposite direction¶

The shared interval is returned in element 1 because B traverses it from right to left. Its coordinates still follow the left-to-right direction of A.

SELECT ST_AsText(ST_SharedPaths(
    ST_GeometryFromWKT('LINESTRING (0 0, 10 0)'),
    ST_GeometryFromWKT('LINESTRING (15 0, 5 0)')
));

Output:

GEOMETRYCOLLECTION (MULTILINESTRING EMPTY, MULTILINESTRING ((5 0, 10 0)))

Same and opposite paths in one result¶

One input pair can populate both elements. Here, B first follows A, leaves it, and later returns along A in the opposite direction.

SELECT ST_AsText(ST_SharedPaths(
    ST_GeometryFromWKT('LINESTRING (0 0, 100 0)'),
    ST_GeometryFromWKT('LINESTRING (20 0, 30 0, 30 50, 80 0, 70 0)')
));

Output:

GEOMETRYCOLLECTION (MULTILINESTRING ((20 0, 30 0)), MULTILINESTRING ((70 0, 80 0)))

Multipart input and noded output¶

Shared paths can come from different components of a MultiLineString. The overlay is noded at vertices from either input, so a shared path may be shorter than its containing input segment.

SELECT ST_AsText(ST_SharedPaths(
    ST_GeometryFromWKT(
        'MULTILINESTRING ((1 3, 4 2, 7 2, 7 5), (13 10, 14 7, 11 6, 15 5))'
    ),
    ST_GeometryFromWKT(
        'LINESTRING (2 1, 4 2, 7 2, 8 3, 10 6, 11 6, 14 7, 16 9)'
    )
));

Output:

GEOMETRYCOLLECTION (MULTILINESTRING ((4 2, 7 2)), MULTILINESTRING ((14 7, 11 6)))

No lineal overlap¶

Lines that cross only at a point do not share a path, so both elements are empty.

SELECT ST_AsText(ST_SharedPaths(
    ST_GeometryFromWKT('LINESTRING (0 0, 10 0)'),
    ST_GeometryFromWKT('LINESTRING (5 -5, 5 5)')
));

Output:

GEOMETRYCOLLECTION (MULTILINESTRING EMPTY, MULTILINESTRING EMPTY)

Practical use: comparing linear networks¶

ST_SharedPaths is useful when two road, rail, pipeline, or utility-network datasets contain coincident centerlines and direction matters. For example, it can distinguish a road segment digitized consistently in two datasets from a one-way segment digitized in reverse. It can also measure how much of an inspected utility route follows its reference alignment in either direction.

The following query assumes that a spatial join has already paired candidate segments. Element 0 measures consistently directed overlap, while element 1 measures reversed overlap:

WITH compared AS (
    SELECT
        segment_id,
        ST_SharedPaths(reference_geom, candidate_geom) AS shared
    FROM network_segment_pairs
)
SELECT
    segment_id,
    ST_Length(ST_GeometryN(shared, 0)) AS same_direction_length,
    ST_Length(ST_GeometryN(shared, 1)) AS opposite_direction_length
FROM compared;

Rows with a positive opposite_direction_length are useful candidates for direction or one-way attribute review. Apply ST_SharedPaths after candidate matching rather than to every possible pair in two large networks. The function finds exact lineal overlap; snap or otherwise normalize nearly coincident datasets first when their coordinates differ within an accepted tolerance.