ST_Compress Trajectory Compression Overview - PolarDB

Compresses a trajectory object by discarding points that fall within specified thresholds. Returns the compressed trajectory object.

ST_Compress is a lossy operation. Points that do not exceed the specified thresholds are permanently discarded. When only dist is specified (Syntax 1), points with large angle changes or velocity changes may also be removed. Use Syntax 2 or Syntax 3 to retain those points.

Syntax

trajectory ST_Compress(trajectory traj, float8 dist);

trajectory ST_Compress(trajectory traj, float8 dist, float8 angle, float8 acceleration);

trajectory ST_Compress(trajectory traj, float8 dist, float8 angle, float8 acceleration, cstring velocity_field);

Parameters

Parameter	Type	Description
`traj`	trajectory	The original trajectory object.
`dist`	float8	The Euclidean distance offset threshold. Trajectory points whose Euclidean distance offset exceeds this value are kept to maintain the spatial trend of the original trajectory.
`angle`	float8	The angle offset threshold. Trajectory points whose angle change exceeds this value are kept. Pass `null` to skip this constraint.
`acceleration`	float8	The acceleration threshold. Trajectory points whose velocity change exceeds this value are kept. Pass `null` to skip this constraint.
`velocity_field`	cstring	The name of the velocity attribute field in the trajectory object. When specified, ST_Compress computes acceleration directly from the values of this field.

Which syntax to use

Syntax	Parameters used	When to use
Syntax 1	`dist`	Spatial-shape-only compression. Fast and simple, but may drop points at sharp turns or speed changes.
Syntax 2	`dist`, `angle`, `acceleration`	Retains points at sharp turns and velocity changes. Pass `null` for any threshold you want to skip.
Syntax 3	`dist`, `angle`, `acceleration`, `velocity_field`	Same as Syntax 2, but uses a named velocity attribute field in the trajectory to compute acceleration directly. Use this when the trajectory contains a pre-recorded speed field (for example, `sog`).

Examples

The following examples use a test trajectory with 89 points (MMSI 477027500, Speed Over Ground (SOG), Course Over Ground (COG), and heading attributes).

Create test data

Create table If not exists traj_test(id integer, mmsi integer, traj trajectory);

INSERT INTO traj_test(mmsi, traj) VALUES(477027500, ST_makeTrajectory('STPOINT'::leaftype, 'LINESTRING(-179.48077 51.72814,-179.47416 51.73714,-179.47187 51.74027,-179.46964 51.74325,-179.46731 51.74634,-179.46502 51.74934,-179.46183 51.75378,-179.45943 51.75736,-179.45560 51.76273,-179.44845 51.77186,-179.43419 51.78977,-179.42595 51.80094,-179.42343 51.80411,-179.42078 51.80719,-179.41821 51.81025,-179.41562 51.81308,-179.41259 51.81643,-179.41001 51.81941,-179.40751 51.82223,-179.40497 51.82505,-179.40242 51.82796,-179.39981 51.83095,-179.39734 51.83398,-179.39499 51.83709,-179.39264 51.84023,-179.39037 51.84333,-179.38699 51.84791,-179.38467 51.85114,-179.38216 51.85439,-179.37997 51.85762,-179.37772 51.86144,-179.37474 51.86568,-179.37219 51.86869,-179.36983 51.87156,-179.36755 51.87467,-179.36001 51.88423,-179.35754 51.88712,-179.34216 51.90644,-179.33935 51.90995,-179.33704 51.91298,-179.18826 52.10105,-179.18096 52.11031,-179.17504 52.11786,-179.16482 52.12996,-179.16233 52.13289,-179.15967 52.13590,-179.14599 52.15132,-177.76666 52.85042,-177.48459 52.89898,-177.47841 52.90001,-177.47319 52.90084,-177.46251 52.90268,-177.38188 52.91595,-177.37102 52.91765,-177.36378 52.91877,-177.34492 52.92173,-177.33217 52.92364,-177.32581 52.92468,-177.31238 52.92697,-177.03751 52.97394,-176.93063 52.99160,-176.92406 52.99265,-176.91471 52.99423,-176.90643 52.99554,-176.89912 52.99674,-176.89246 52.99791,-176.88342 52.99942,-176.87697 53.00060,-176.86594 53.00256,-176.85946 53.00370,-176.85294 53.00481,-176.84640 53.00592,-176.83985 53.00705,-176.83238 53.00830,-176.82589 53.00950,-176.81848 53.01084,-176.80553 53.01310,-176.79879 53.01419,-176.79115 53.01548,-176.78466 53.01668,-176.77901 53.01765,-176.77256 53.01879,-176.76301 53.02039,-176.75649 53.02141,-176.74700 53.02296,-176.73757 53.02450,-176.71683 53.02795,-176.70741 53.02950,-176.68481 53.03327)'::geometry, ARRAY['2017-01-15 09:06:39'::timestamp,'2017-01-15 09:10:08'::timestamp,'2017-01-15 09:11:20'::timestamp,'2017-01-15 09:12:29'::timestamp,'2017-01-15 09:13:39'::timestamp,'2017-01-15 09:14:48'::timestamp,'2017-01-15 09:16:28'::timestamp,'2017-01-15 09:17:48'::timestamp,'2017-01-15 09:19:48'::timestamp,'2017-01-15 09:23:19'::timestamp,'2017-01-15 09:30:28'::timestamp,'2017-01-15 09:34:40'::timestamp,'2017-01-15 09:35:49'::timestamp,'2017-01-15 09:36:59'::timestamp,'2017-01-15 09:38:09'::timestamp,'2017-01-15 09:39:18'::timestamp,'2017-01-15 09:40:40'::timestamp,'2017-01-15 09:41:49'::timestamp,'2017-01-15 09:42:58'::timestamp,'2017-01-15 09:44:08'::timestamp,'2017-01-15 09:45:18'::timestamp,'2017-01-15 09:46:29'::timestamp,'2017-01-15 09:47:38'::timestamp,'2017-01-15 09:48:49'::timestamp,'2017-01-15 09:49:58'::timestamp,'2017-01-15 09:51:08'::timestamp,'2017-01-15 09:52:49'::timestamp,'2017-01-15 09:53:58'::timestamp,'2017-01-15 09:55:09'::timestamp,'2017-01-15 09:56:18'::timestamp,'2017-01-15 09:57:38'::timestamp,'2017-01-15 09:59:09'::timestamp,'2017-01-15 10:00:20'::timestamp,'2017-01-15 10:01:29'::timestamp,'2017-01-15 10:02:39'::timestamp,'2017-01-15 10:06:29'::timestamp,'2017-01-15 10:07:40'::timestamp,'2017-01-15 10:15:00'::timestamp,'2017-01-15 10:16:20'::timestamp,'2017-01-15 10:17:29'::timestamp,'2017-01-15 11:30:09'::timestamp,'2017-01-15 11:33:58'::timestamp,'2017-01-15 11:36:58'::timestamp,'2017-01-15 11:42:00'::timestamp,'2017-01-15 11:43:10'::timestamp,'2017-01-15 11:44:20'::timestamp,'2017-01-15 11:50:28'::timestamp,'2017-01-15 18:01:00'::timestamp,'2017-01-15 18:54:13'::timestamp,'2017-01-15 18:55:21'::timestamp,'2017-01-15 18:56:22'::timestamp,'2017-01-15 18:58:21'::timestamp,'2017-01-15 19:13:21'::timestamp,'2017-01-15 19:15:21'::timestamp,'2017-01-15 19:16:41'::timestamp,'2017-01-15 19:20:11'::timestamp,'2017-01-15 19:22:31'::timestamp,'2017-01-15 19:23:41'::timestamp,'2017-01-15 19:26:10'::timestamp,'2017-01-15 20:15:49'::timestamp,'2017-01-15 20:34:39'::timestamp,'2017-01-15 20:35:49'::timestamp,'2017-01-15 20:37:30'::timestamp,'2017-01-15 20:39:00'::timestamp,'2017-01-15 20:40:19'::timestamp,'2017-01-15 20:41:30'::timestamp,'2017-01-15 20:43:08'::timestamp,'2017-01-15 20:44:19'::timestamp,'2017-01-15 20:46:19'::timestamp,'2017-01-15 20:47:29'::timestamp,'2017-01-15 20:48:40'::timestamp,'2017-01-15 20:49:49'::timestamp,'2017-01-15 20:50:59'::timestamp,'2017-01-15 20:52:21'::timestamp,'2017-01-15 20:53:29'::timestamp,'2017-01-15 20:54:50'::timestamp,'2017-01-15 20:57:09'::timestamp,'2017-01-15 20:58:20'::timestamp,'2017-01-15 20:59:40'::timestamp,'2017-01-15 21:00:49'::timestamp,'2017-01-15 21:01:50'::timestamp,'2017-01-15 21:02:58'::timestamp,'2017-01-15 21:04:40'::timestamp,'2017-01-15 21:05:50'::timestamp,'2017-01-15 21:07:29'::timestamp,'2017-01-15 21:09:11'::timestamp,'2017-01-15 21:12:49'::timestamp,'2017-01-15 21:14:30'::timestamp,'2017-01-15 21:18:30'::timestamp], '{"leafcount": 89,"attributes" : {"sog" : {"type":"float","length":8,"nullable":false,"value":[10.5,10.4,10.5,10.7,10.8,10.3,10.7,10.4,10.5,10.1,10.2,11.0,11.2,10.8,10.3,10.3,10.1,10.7,10.6,10.0,10.3,10.5,10.6,10.3,10.8,10.9,10.8,10.8,10.8,11.0,11.2,11.2,10.3,10.2,10.8,10.0,10.4,10.7,10.2,10.6,9.1,10.2,10.1,9.7,10.4,10.6,9.9,12.3,12.1,12.0,12.0,12.2,12.3,12.2,12.3,12.2,12.3,12.2,12.2,12.8,12.8,12.9,12.5,12.6,12.5,12.6,12.6,12.3,12.6,12.6,12.5,12.7,12.8,12.5,12.7,12.5,12.8,13.0,12.9,12.6,12.9,12.8,12.7,12.8,13.0,12.7,12.8,12.6,12.7]}, "cog" : {"type":"float","length":8,"nullable":false,"value":[23.3,25.7,25.9,23.6,25.3,24.1,23.0,21.6,20.7,24.8,22.4,28.5,23.1,30.3,26.2,28.1,25.1,28.7,31.4,28.2,30.4,29.4,29.2,23.0,25.1,25.1,23.5,22.7,27.1,23.3,19.2,27.1,31.0,28.8,22.0,30.1,24.6,26.2,26.7,24.7,26.8,29.5,19.9,30.1,28.8,28.7,30.0,74.2,69.1,75.2,81.3,81.3,80.1,72.6,82.4,74.2,74.9,67.7,73.4,74.2,72.2,80.5,78.6,77.3,70.9,80.1,85.4,71.9,67.0,77.5,77.5,72.2,70.5,72.6,70.8,77.8,71.2,71.2,73.8,75.4,67.1,77.5,74.3,76.9,80.1,72.8,76.0,75.4,72.9]},"heading" : {"type":"float","length":8,"nullable":false,"value":[22.0,23.0,23.0,23.0,23.0,21.0,21.0,25.0,24.0,26.0,25.0,27.0,28.0,29.0,31.0,30.0,28.0,29.0,29.0,28.0,28.0,27.0,24.0,24.0,25.0,25.0,25.0,26.0,25.0,24.0,25.0,29.0,31.0,28.0,29.0,31.0,28.0,29.0,29.0,29.0,27.0,27.0,26.0,26.0,26.0,27.0,27.0,69.0,71.0,72.0,72.0,71.0,73.0,72.0,72.0,72.0,72.0,71.0,71.0,72.0,72.0,72.0,73.0,72.0,71.0,72.0,72.0,72.0,71.0,72.0,72.0,73.0,71.0,72.0,71.0,72.0,73.0,72.0,72.0,72.0,72.0,73.0,74.0,73.0,73.0,73.0,73.0,73.0,73.0]}}}'));

Syntax 1: compress by spatial distance only

89 points in, 8 points out. Only the spatial trend is preserved.

SELECT st_compress(traj, 0.001) AS traj FROM traj_test;

Output:

{"trajectory":{"version":1,"type":"STPOINT","leafcount":8,"start_time":"2017-01-15 09:06:39","end_time":"2017-01-15 21:18:30","spatial":"LINESTRING(-179.48077 51.72814,-179.42595 51.80094,-179.39734 51.83398,-179.37474 51.86568,-179.17504 52.11786,-179.14599 52.15132,-177.76666 52.85042,-176.68481 53.03327)","timeline":["2017-01-15 09:06:39","2017-01-15 09:34:40","2017-01-15 09:47:38","2017-01-15 09:59:09","2017-01-15 11:36:58","2017-01-15 11:50:28","2017-01-15 18:01:00","2017-01-15 21:18:30"],"attributes":{"leafcount":8,"sog":{"type":"float","length":8,"nullable":false,"value":[10.5,11.0,10.6,11.2,10.1,9.9,12.3,12.7]},"cog":{"type":"float","length":8,"nullable":false,"value":[23.3,28.5,29.2,27.1,19.9,30.0,74.2,72.9]},"heading":{"type":"float","length":8,"nullable":false,"value":[22.0,27.0,24.0,29.0,26.0,27.0,69.0,73.0]}}}}
(1 row)

Syntax 2: compress by spatial distance, angle, and acceleration

89 points in, 13 points out. Additional points at sharp turns and speed changes are retained.

SELECT st_compress(traj, 0.001, 5, 0.3) AS traj FROM traj_test;

Output:

{"trajectory":{"version":1,"type":"STPOINT","leafcount":13,"start_time":"2017-01-15 09:06:39","end_time":"2017-01-15 21:18:30","spatial":"LINESTRING(-179.48077 51.72814,-179.42595 51.80094,-179.39734 51.83398,-179.37474 51.86568,-179.35754 51.88712,-179.18826 52.10105,-179.17504 52.11786,-179.14599 52.15132,-177.76666 52.85042,-177.47841 52.90001,-177.47319 52.90084,-176.83238 53.0083,-176.68481 53.03327)","timeline":["2017-01-15 09:06:39","2017-01-15 09:34:40","2017-01-15 09:47:38","2017-01-15 09:59:09","2017-01-15 10:07:40","2017-01-15 11:30:09","2017-01-15 11:36:58","2017-01-15 11:50:28","2017-01-15 18:01:00","2017-01-15 18:55:21","2017-01-15 18:56:22","2017-01-15 20:52:21","2017-01-15 21:18:30"],"attributes":{"leafcount":13,"sog":{"type":"float","length":8,"nullable":false,"value":[10.5,11.0,10.6,11.2,10.4,9.1,10.1,9.9,12.3,12.0,12.0,12.5,12.7]},"cog":{"type":"float","length":8,"nullable":false,"value":[23.3,28.5,29.2,27.1,24.6,26.8,19.9,30.0,74.2,75.2,81.3,72.6,72.9]},"heading":{"type":"float","length":8,"nullable":false,"value":[22.0,27.0,24.0,29.0,28.0,27.0,26.0,27.0,69.0,72.0,72.0,72.0,73.0]}}}}
(1 row)

To apply only dist and angle without an acceleration constraint, pass null for acceleration:

SELECT st_compress(traj, 0.001, null, null) AS traj FROM traj_test WHERE traj_id = 5;

Output:

{"trajectory":{"type":"STPOINT","leafsize":8,"starttime":"2017-01-15 09:06:39","endtime":"2017-01-15 21:18:30","spatial":"LINESTRING(-179.48077 51.72814,-179.42595 51.80094,-179.39734 51.83398,-179.37474 51.86568,-179.17504 52.11786,-179.14599 52.15132,-177.76666 52.85042,-176.68481 53.03327)","timeline":["2017-01-15 09:06:39","2017-01-15 09:34:40","2017-01-15 09:47:38","2017-01-15 09:59:09","2017-01-15 11:36:58","2017-01-15 11:50:28","2017-01-15 18:01:00","2017-01-15 21:18:30"],"themeline":{"leafs":8,"sog":[10.5,11.0,10.6,11.2,10.1,9.9,12.3,12.7],"cog":[23.3,28.5,29.2,27.1,19.9,30.0,74.2,72.9],"heading":[22.0,27.0,24.0,29.0,26.0,27.0,69.0,73.0]}}}
(1 row)

Syntax 3: compress using a velocity attribute field

89 points in, 10 points out. Acceleration is computed directly from the sog field.

SELECT st_compress(traj, 0.001, 5, 1.1, 'sog') AS traj FROM traj_test;

Output:

{"trajectory":{"version":1,"type":"STPOINT","leafcount":10,"start_time":"2017-01-15 09:06:39","end_time":"2017-01-15 21:18:30","spatial":"LINESTRING(-179.48077 51.72814,-179.42595 51.80094,-179.39734 51.83398,-179.37474 51.86568,-179.33704 51.91298,-179.18826 52.10105,-179.17504 52.11786,-179.14599 52.15132,-177.76666 52.85042,-176.68481 53.03327)","timeline":["2017-01-15 09:06:39","2017-01-15 09:34:40","2017-01-15 09:47:38","2017-01-15 09:59:09","2017-01-15 10:17:29","2017-01-15 11:30:09","2017-01-15 11:36:58","2017-01-15 11:50:28","2017-01-15 18:01:00","2017-01-15 21:18:30"],"attributes":{"leafcount":10,"sog":{"type":"float","length":8,"nullable":false,"value":[10.5,11.0,10.6,11.2,10.6,9.1,10.1,9.9,12.3,12.7]},"cog":{"type":"float","length":8,"nullable":false,"value":[23.3,28.5,29.2,27.1,24.7,26.8,19.9,30.0,74.2,72.9]},"heading":{"type":"float","length":8,"nullable":false,"value":[22.0,27.0,24.0,29.0,29.0,27.0,26.0,27.0,69.0,73.0]}}}}
(1 row)