# [postgis-users] Are there script examples for determine where vertices run clockwise or anti-clockwise?

Paul Ramsey pramsey at cleverelephant.ca
Thu Feb 17 13:39:56 PST 2022

Without closure it is not possible to say what orientation a linestring has.

P

> On Feb 16, 2022, at 11:31 PM, Shaozhong SHI <shishaozhong at gmail.com> wrote:
>
> How about each line gets split into segments.  .  Each segment is constructed with two points (current and next points.
> Then, calculate bearing/azimuth
> Then, determine whether it is clockwise or counter-clockwise.
>
> Can this be done?
>
> Regards,
>
> David
>
> On Wed, 16 Feb 2022 at 20:39, Marcin Mionskowski <mionskowskimarcin at gmail.com> wrote:
> It doesn't make sense for lines.
> Imagine LINESTRING(1 0,1 1) - which "part of a square" is this i.e. where is "the center of the square": (1) 0.5,0.5 or (2) 1.5,0.5?
> If (1) it's CCW, if (2) CW.
>
>
> śr., 16 lut 2022 o 21:29 Shaozhong SHI <shishaozhong at gmail.com> napisał(a):
> The problem now is that I can not find anything for lines.
>
> Anything like ST_IsPolygonCW|ST_IsPolygonCCW for lines for instance, something like ST_IsLineCW|ST_IsLineCCW.
>
> Regards,
>
> David
>
> On Wed, 16 Feb 2022 at 19:43, Marcin Mionskowski <mionskowskimarcin at gmail.com> wrote:
> Assuming simple polygons, composed from exterior ring only (i.e. no multipolygons), I think that you are looking for a function {ST_IsPolygonCW|ST_IsPolygonCCW}.
>
> Greetings,
> Marcin
>
> śr., 16 lut 2022 o 19:55 Imre Samu <pella.samu at gmail.com> napisał(a):
> > how vertices run?  clockwise or anti-clockwise?
>
> maybe the:
> https://postgis.net/docs/ST_Angle.html  "Computes the clockwise angle between two vectors."
> or https://postgis.net/docs/ST_Azimuth.html
>
> regards,
>  Imre
>
>
> Shaozhong SHI <shishaozhong at gmail.com> ezt írta (időpont: 2022. febr. 16., Sze, 17:06):
> Can we use PostGIS to find out how vertices run?  clockwise or anti-clockwise?
>
> Regards,
>
> David
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