ST_Compress Function for Trajectory Object Compression - AnalyticDB for PostgreSQL

Compresses a trajectory object using lossy compression based on specified thresholds, and returns the compressed trajectory.

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 preserve the spatial trend of the original trajectory.
`angle`	float8	The angle offset threshold. Trajectory points whose angle changes exceed this value are kept.
`acceleration`	float8	The acceleration threshold. Trajectory points whose velocity changes exceed this value are kept.
`velocity_field`	cstring	The name of the velocity attribute field in the trajectory object. The values of this field are used to compute acceleration.

Description

ST_Compress discards trajectory points based on specified thresholds to produce a compressed trajectory.

Syntax 1 compresses the trajectory based only on the spatial distance offset threshold. This preserves the spatial trend of the original trajectory but may discard points where significant angle or velocity changes occur.

Syntax 2 compresses the trajectory based on the spatial distance offset threshold, angle offset threshold, and acceleration threshold. This preserves both the spatial trend and important inflection points where angle or velocity changes are large. Any one or two of the three thresholds can be specified; pass null for thresholds you do not want to apply.

Syntax 3 achieves the same effect as Syntax 2 and is designed for trajectory objects that contain a velocity attribute field. When velocity_field is specified, the function computes acceleration directly from the values in that field.

Note ST_Compress supports GPU-accelerated computing for trajectory objects with large angle or velocity changes. GPU acceleration is enabled automatically when a GPU is available.

Examples

Example 1: Compress using distance threshold only (Syntax 1)

-- Create the table and insert sample 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]}}}}'
  )
);

-- Compress using the distance threshold only.
SELECT st_compress(traj, 0.001) AS traj FROM traj_test;

The result retains 8 trajectory points (reduced from 89):

{"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)

Example 2: Compress with distance threshold only, null angle and acceleration (Syntax 2)

Passing null for angle and acceleration produces the same spatial result as Syntax 1:

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

Output (8 points retained):

{"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)

Example 3: Compress with distance, angle, and acceleration thresholds (Syntax 2)

Adding angle and acceleration thresholds retains more points at significant direction and speed changes:

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

Output (13 points retained):

{"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)

Example 4: Compress using a velocity attribute field for acceleration (Syntax 3)

When the trajectory contains a velocity attribute field (here sog), pass its name to let the function derive acceleration from it:

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

Output (10 points retained):

{"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)