[postgis-users] Creating trajectory/lines from millions ofpoints[PostGIS]

Wed Nov 26 00:05:04 PST 2014

Hi,

I may be wrong, but with this method, I'm afraid you will have the same start and end time for each vehicle and status (the start time of the first line and le end time of the last one by vehicle and status) instead of a unique start / end time for each line.

To have the start and end for each line, I think you will have to retrieve them in a second time comparing start and end point of each line with the original gps points. Another solution would be to create a plpgsql function to build the linestring from a loop.

Regards,

Hugues.

De : postgis-users-bounces at lists.osgeo.org [mailto:postgis-users-bounces at lists.osgeo.org] De la part de Roxanne Reid-Bennett
Envoyé : mardi 25 novembre 2014 19:17
À : postgis-users at lists.osgeo.org
Objet : Re: [postgis-users] Creating trajectory/lines from millions ofpoints[PostGIS]

On 11/25/2014 11:48 AM, Oliver Burgfeld wrote:

	I also tried that and it works but it does not give me those two columns in my new table. There are only id and status inside.

>From the quer below change
SELECT id, status, (ST_Dump(mylines)).geom
to 

SELECT id, status, (ST_Dump(mylines)).geom, time_start, time_end

Something to keep an eye out for... Depending upon your version of PostGIS and the underlying libraries, because we were working with an older version of the underlying libraries, I don't know if this is still a potential issue.  We ran into issues with "stacked" points (2 GPS points with the same coordinates), and GPS drift causing issues with the linestrings being pretty goofy (jagged points in the linestring that were clearly NOT what the vehicle did).    We wrote cleanup scripts to take care of those issues (that are still in place).  I've not taken time to revisit the library routines to see if they now handle those conditions cleanly.

Roxanne

Am Dienstag, 25. November 2014 17:39:21 UTC+1 schrieb Brent Wood: 

as in my previous reply, I figured that would be useful...

WITH multis AS (

                SELECT id, status, min(timestamp) as time_start, max(timestamp) as time_end, ST_MakeLine( point_geom ORDER BY timestamp) AS mylines

                FROM your_table

GROUP BY id, status

)

SELECT id, status, (ST_Dump(mylines)).geom

FROM multisBrent Wood

________________________________

From: Oliver Burgfeld <oliver.... at gmail.com <javascript:> >
To: postgi... at googlegroups.com <javascript:>  
Cc: pcr... at pcreso.com <javascript:> ; postgi... at lists.osgeo.org <javascript:> ; postgi... at lists.osgeo.org <javascript:> ; remi... at gmail.com <javascript:>  
Sent: Wednesday, November 26, 2014 5:10 AM
Subject: Re: [postgis-users] Creating trajectory/lines from millions of points[PostGIS]

Thank you and all the others who were answering :)

I tried that and it seems that its working. Nevertheless I only tried it with a small part of my data (round about 1 million rows out of ~500 million) but if it's working now, it should also work with the whole dataset.

Is there a way to also include the time_field into the result? I created a new table with this statement given but there are only two columns (vehicleid and status) included. 
I know thats logical because I only included those two into my select clause but it would be great to not only order by time but also have a time column in my table.

For example:

vehicleid | status | time_start | time_end

I hope its understandable and not to mixed up...

Thanks!

Am Dienstag, 25. November 2014 16:06:33 UTC+1 schrieb Rémi Cura: 

Hey, a small correction :

ST_MakeLine is already an aggregate, and you may want to enforce the order inside the aggregate (see at the end).

Another interesting point is the possiblity to pu somehting in the M value of each point of the line, for instance the time.

This comes very handy when you want to extrat parts of the lines.

So for instance for the first proposition :

WITH multis AS (

                SELECT id, status, ST_MakeLine( point_geom ORDER BY time_field) AS mylines

                FROM your_table

GROUP BY id, status

)

SELECT id, status, (ST_Dump(mylines)).geom

FROM multis

Cheers,
Rémi-c

2014-11-25 9:53 GMT+01:00 Brent Wood <pcr... at pcreso.com>:

or automatically get the start & end times for each trackline in the record like this:

WITH multis AS (

                SELECT id, min(time_field) AS time_start, max(time_field) as time_end, status, ST_MakeLine(array_agg(point_ geom )) AS mylines

                FROM your_table

GROUP BY id, status

ORDER BY time_field

)

SELECT id, status, (ST_Dump(mylines)).geom

FROM multis;

Cheers,

   Brent Wood

________________________________

From: Hugues François <hugues.... at irstea.fr>
To: PostGIS Users Discussion <postgi... at lists.osgeo.org <mailto:postgi... at lists.osgeo.org%20> > 
Sent: Tuesday, November 25, 2014 8:13 PM
Subject: Re: [postgis-users] Creating trajectory/lines from millions of points[PostGIS]

Hello,

In your case I would have try to make multilines for each taxi and each status (i.e. two multi by taxi) and then dump them into  simple linestrings. All in a query that may look like this assuming you have a taxi id field:

WITH multis AS (

                SELECT id, status, ST_MakeLine(array_agg(point_ geom )) AS mylines

                FROM your_table

GROUP BY id, status

ORDER BY time_field

)

SELECT id, status, (ST_Dump(mylines)).geom

FROM multis

You may want to add a time reference to your lines. To do this, you can add an extraction from your timestamp field (e.g. day or month) and add it into the WITH and to the group by clause.

Hugues.

De : postgis-us... at lists. osgeo.org [mailto:postgis-us...@ lists.osgeo.org] De la part de Oliver Burgfeld
Envoyé : mardi 25 novembre 2014 07:09
À : postgi... at lists.osgeo.org
Objet : [postgis-users] Creating trajectory/lines from millions of points[PostGIS]

Hi,

I have millions of points in a PostGIS database containing taxi gps tracks. Now I want to create lines from these points by vehicleid and ordered by timestamp. But, and that's my problem right now, at first I want to include every column of my point table into the "line table" and I also need to intersect those lines at specific points.

I have one column representing the "taxi_is_occupied" status with 0 or 1. 

What I want now is to create lines which are divided every time this status changes. In the end I need lines which show the path of every taxi over time, divided every time the status of the car changes so that I can query all lines where the taxi is occupied, for example.

What do I have to use therefore? I know that there is the ST_MakeLines tool existing in PostGIS, but as I am a new PostGIS user... I do not know exactly how to use it to get the results I need. 

Thanks a lot

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