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
<[email protected]> 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 <[email protected]> On Behalf Of Brent
Wood via postgis-users
Sent: Friday, November 17, 2023 11:53 PM
To: PostGIS Users Discussion <[email protected]>
Cc: Brent Wood <[email protected]>
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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