[postgis-users] Generating new random points throughout an update

[Brent Wood]

 Hi Regina,
The seed was an int generated from random(), so I'd expected to generate a different result every time. This didn't happen.
Do I understand that if I omit the seed, I'll get a different point each time by default?

Thanks,
   Brent 

    On Saturday, November 18, 2023 at 06:01:37 PM GMT+13, Regina Obe <lr at pcorp.us> wrote:  
 
 
If you want the answer different each time, you don’t want to feed a seed to ST_GeneratePoints.  

The seed argument was added because some people wanted to generate the same answer for each run.

  

https://postgis.net/docs/ST_GeneratePoints.html  (note the sentence: The optional seed is used to regenerate a deterministic sequence of points, and must be greater than zero.)

  

  

From: postgis-users <postgis-users-bounces at lists.osgeo.org> On Behalf Of Brent Wood via postgis-users
Sent: Friday, November 17, 2023 11:53 PM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Cc: Brent Wood <pcreso at yahoo.com>
Subject: [postgis-users] Generating new random points throughout an update

  

Hopefully someone can help with a problem I'm having.

  

I have a table with simple linestrings that I need to create a randomly modified version of.

  

The linestrings represent vessel tracks. I can identify a set of "similar" tracks & create a single "average" linestring that is somewhat representative.

  

Many of the records don't have a linestring, but for statistical purposes I need to assign a linestring to each - by creating a jittered version of the average linestring so they are not all identical.

  

The simplest approach I have tried is to use an update with ST_Project() given a random() distance & random() direction applied to each vertex in the average line.

  

I use the first two vertices with ST_Makeline(), then append a vertex for the third point, as in the SQL below. 

  

My problem is that every new line is identical. From some Googled hints, I figure the optimiser has decided to run random() once & re-use the value instead of running the function for every iteration (but I could be wrong!).

  

Any suggestions as to how I can force a different random result for each record that is updated?

I also tried using ST_GeneratePoints() in a buffer around each point, but need to use something like (random()::int as the seed, and this seems to do exactly the same - valid linestrings are generated, but they are identical, so I'm assuming the seed is not being recalculated for each record.

  

  

update events
set jittered = ST_MakeLine(
                           (select ST_Project(
                                         ST_POINTN(std_track,1),
                                         (random()*5000),
                                         radians(random()*360))::geometry
                           from std_tow),
                  (select ST_Project(
                                         ST_PointN(std_track,2),
                                         (random()*5000),
                                           radians(random()*360))::geometry
                           from std_tow)
                  );

  

  

Thanks,

  

  Brent Wood

  
  
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