Computes the distance between a trajectory object and its longest common subsequence (LCSS) sub-trajectory.
Syntax
float8 ST_lcsSubDistance(trajectory traj1, trajectory traj2, float8 dist, distanceUnit unit default 'M');
float8 ST_lcsSubDistance(trajectory traj1, trajectory traj2, float8 dist, interval lag, distanceUnit unit default 'M');Parameters
| Parameter | Description |
|---|---|
| traj1 | The first trajectory object. |
| traj2 | The second trajectory object. |
| dist | The distance tolerance between two trajectory points. Unit: meters. |
| lag | The time tolerance between two trajectory points. |
| unit | The unit of the distance. Valid values: 'M' (meters, default), 'KM' (kilometers), 'D' (degrees, valid only when the spatial reference system identifier (SRID) of the trajectory object is WGS84, which defaults to 4326). |
Description
Identifies the sub-trajectory of traj1 whose trajectory points match those in the LCSS sub-trajectory, then returns 1 - (LCSS point count / traj1 sub-trajectory point count).
In the preceding figure, trajectory points 2, 3, and 5 are in the LCSS sub-trajectory, and trajectory points 2, 3, 4, and 5 are in the sub-trajectory of traj1. The result is 1 - 3/4 = 0.25.
A smaller return value indicates higher similarity between the LCSS sub-trajectory and traj1. If no SRID is specified, the default is 4326 (WGS84).
Examples
Example 1: Compute the distance using only a distance tolerance.
With traj AS (
Select ST_makeTrajectory('STPOINT', 'LINESTRINGZ(114.000528 33.588163 54.87, 114.000535 33.588235 54.85, 114.000447 33.588272 54.69, 114.000348 33.588287 54.73, 114.000245 33.588305 55.26, 114.000153 33.588305 55.3)'::geometry,
ARRAY['2010-01-01 11:30'::timestamp, '2010-01-01 11:31', '2010-01-01 11:32', '2010-01-01 11:33','2010-01-01 11:34','2010-01-01 11:35'], NULL) a,
ST_makeTrajectory('STPOINT', 'LINESTRINGZ(114.000529 33.588163 54.87, 114.000535 33.578235 54.85, 114.000447 33.578272 54.69, 114.000348 33.578287 54.73, 114.000245 33.578305 55.26, 114.000163 33.588305 55.3)'::geometry,
ARRAY['2010-01-01 11:29:58'::timestamp, '2010-01-01 11:31:02', '2010-01-01 11:33', '2010-01-01 11:33:09','2010-01-01 11:34','2010-01-01 11:34:30'], NULL) b)
Select st_LCSSubDistance(a, b, 100) from traj;Result:
st_lcssubdistance
------------------------
0.66666666666666666667
(1 row)Example 2: Compute the distance using both a distance tolerance and a time tolerance.
With traj AS (
Select ST_makeTrajectory('STPOINT', 'LINESTRINGZ(114.000528 33.588163 54.87, 114.000535 33.588235 54.85, 114.000447 33.588272 54.69, 114.000348 33.588287 54.73, 114.000245 33.588305 55.26, 114.000153 33.588305 55.3)'::geometry,
ARRAY['2010-01-01 11:30'::timestamp, '2010-01-01 11:31', '2010-01-01 11:32', '2010-01-01 11:33','2010-01-01 11:34','2010-01-01 11:35'], NULL) a,
ST_makeTrajectory('STPOINT', 'LINESTRINGZ(114.000529 33.588163 54.87, 114.000535 33.578235 54.85, 114.000447 33.578272 54.69, 114.000348 33.578287 54.73, 114.000245 33.578305 55.26, 114.000163 33.588305 55.3)'::geometry,
ARRAY['2010-01-01 11:29:58'::timestamp, '2010-01-01 11:31:02', '2010-01-01 11:33', '2010-01-01 11:33:09','2010-01-01 11:34','2010-01-01 11:34:30'], NULL) b)
Select st_LCSSubDistance(a, b, 100, interval '30 seconds') from traj;Result:
st_lcssubdistance
-------------------
0.666666666666667
(1 row)