clustering similar linestring with direction in PostgreSQL(PostGIS)

Question

Linestrings with directions are stored in the form of geometry in PostgreSQL, as shown in the figure below.

I would like to cluster similar(location, length, and direction) shapes through functions such as ST_ClusterDBSCAN of PostGIS, but there is no option in ST_ClusterDBSCAN considering the direction of the straight line. (direction of the line is determined by the start and end points of the stored node. )

Is there any good way to solve the problem?

Answer 1

A common way is to collapse multi-dimensional observations into low-dimensional realms that can be handled with simpler algorithms - in this case could try to find a coefficient for your 4 dimensions (x, y, length, azimuth) to project onto the 2 dimensions (x, y) needed for the cluster functionality in PostGIS.

The general idea is to create a surrogate table with Point geoms having their coordinates augmented by a given coefficient derived off the 4 line predicates (x, y, length, azimuth); a simple starting point could be to use the ST_Centroid of your segments as positional indicator and work in weighted factors of the predicates:

SELECT
  line_id,
  ST_MakePoint(
    1 / ( <positional_weight> * x + <length_weight> * len + <directional_weight> * azm ),
    1 / ( <positional_weight> * y + <length_weight> * len + <directional_weight> * azm )
  ) AS geom
FROM
  <lines>,
  LATERAL ST_Length(geom) AS len,
  LATERAL ST_Centroid(geom) AS ctr,
  LATERAL ST_X(ctr) AS x,
  LATERAL ST_Y(ctr) AS y,
  LATERAL ST_StartPoint(geom) AS sp,
  LATERAL ST_EndPoint(geom) AS ep,
  LATERAL DEGREES(ST_Azimuth(sp, ep)) AS azm
;

This is a simplistic weighting: you can and should normalize/enhance the factorization to suit your needs; the idea is to be able to weight all predicates individually based on their importance for the actual clustering.

On a table like the one above you can apply ST_ClusterDBSCAN (or ST_ClusterKMEANS) with the eps parameter referencing the 'observation distance' of the surrogate Point coordinates. You can link back to your original lines by their line_id.