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List:       postgis-users
Subject:    Re: [postgis-users] strange behavior in ST_SnaptoGrid
From:       Paul Ramsey <pramsey () cleverelephant ! ca>
Date:       2023-09-03 23:29:39
Message-ID: 4A91ECA6-FF4F-4148-B740-F2D23487B0FA () cleverelephant ! ca
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One thing to always ask is "why exactly am I doing this snapping operation?"
If the answer is "so my numbers look nicer in my UI" then using the second parameter \
of the ST_AsText() function will give you nice, perfectly rounded-looking outputs \
just like you want them.

ATB,
P

> On Sep 3, 2023, at 3:07 PM, Mike Taves <mwtoews@gmail.com> wrote:
> 
> On Mon, 4 Sept 2023 at 00:06, Sajjad Hassany Pazoky
> <ijustloveit13@gmail.com> wrote:
> > 
> > I observed a strange behavior when using ST_SnaptoGrid.
> > I don't know whether I am making a mistake or if there is a bug.
> > The following code:
> > SELECT ST_AsText(ST_SnapToGrid(
> > ST_GeomFromText('LINESTRING(334729.13 4103548.88,334729.12 4103548.53)'),
> > 0.001)
> > );
> > 
> > produces:
> > LINESTRING(334729.13 4103548.88,334729.12 4103548.5300000003)
> 
> This is an annoying quirk of floating point numbers. What is happening
> behind the scenes is simple:
> 
> value = 4103548.53
> grid = 0.001
> snapped = round(value / grid) * grid
> 
> which results with 4103548.5300000003 which is one floating point unit
> too many! There is a nextafter C function that could be used to bring
> the result down a notch. Using Python/NumPy the example would be:
> 
> np.nextafter(np.rint(value / grid) * grid, -np.inf)
> 
> But it's difficult to know when to increment/decrement the float by a
> small amount, and which direction. There is a better solution
> somewhere, but I haven't figured it out yet.
> 
> Note that the actual source code for PostGIS' implementation is:
> https://github.com/postgis/postgis/blob/11876ebb5e70dfb3c1b40e48bfc343820e6000d9/liblwgeom/ptarray.c#L2082-L2155
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