This function compresses a trajectory object based on the Euclidean distance offset threshold.

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 Description
traj The original trajectory object.
dist The Euclidean distance offset threshold. After this value is specified, trajectory points whose Euclidean distance offset is greater than this value are kept, so as to maintain the spatial trend of the original trajectory object.
angle The angle offset threshold. After this value is specified, trajectory points whose angle changes are greater than this value are kept.
acceleration The acceleration threshold. After this value is specified, trajectory points whose velocity changes are greater than this value are kept.
velocity_field The name of the velocity attribute field in the trajectory object. The values of this attribute field are used to compute the acceleration.

Description

This function discards trajectory points based on specified thresholds to compress the trajectory object in lossy mode, and returns the compressed trajectory object.

Syntax 1: Compresses the trajectory object based on the specified spatial distance offset threshold. This function maintains the spatial trend of the original trajectory object, but may delete important trajectory points with large angle or velocity changes.

Syntax 2: Compresses the trajectory object based on the specified spatial distance offset threshold, angle offset threshold, and acceleration threshold. This function not only maintains the spatial trend of the original trajectory object, but also keeps important trajectory points with large angle or velocity changes. You can specify any one or two of the thresholds for compression.

Syntax 3: This function achieves the same effect as syntax 2. It applies when the trajectory object contains the velocity attribute field. After the name of the velocity attribute field is specified, this method directly computes the acceleration of trajectory points based on the values of the velocity attribute field.

Examples

-- Create 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 the trajectory object.
select st_compress(traj,0.001) as traj from traj_test;
           traj
-------------------------------------------------------------
 {"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)

select st_compress(traj,0.001,null,null) as traj from traj_test where traj_id=5;
           traj
-------------------------------------------------------------
{"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)

select st_compress(traj,0.001,5,0.3) as traj from traj_test;
           traj
-------------------------------------------------------------
{"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)

select st_compress(traj,0.001,5,1.1,'sog') as traj from traj_test;
          
            traj
-------------------------------------------------------------
{"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]}}}}