[postgis-users] ST_Simplify

[Kevin Neufeld]

That's what I thought ... my morning coffee hadn't kicked in yet :)

It still looks like PostGIS might benefit from a 
ShiftCoordinates(linearring geometry) function to solve this particular 
problem.
-- Kevin

On 10/22/2010 10:45 AM, David William Bitner wrote:
> D-P simplification is not going to modify your start/end points. For a 
> bit of reference on the Douglas-Peuker simplification algorithm, see 
> http://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm
>
> On Fri, Oct 22, 2010 at 11:13 AM, Kevin Neufeld 
> <kneufeld at refractions.net <mailto:kneufeld at refractions.net>> wrote:
>
>     I don't see any pdf posted, but that's ok, I get the idea.
>
>     Isn't that interesting.  To me, this looks like a robustness issue
>     with ST_Simplify.  Here, the starting point of the polygon's
>     exterior ring is not on the corner.
>
>     select
>      st_astext(
>        st_simplify(
>          'POLYGON ((
>            25 300,
>            275 300,
>            300 300,It
>            300 275,
>            300 25,
>            300 0,
>            275 0,
>            25 0,
>            0 0,
>            0 25,
>            0 275,
>            0 300,
>            25 300
>           ))', 1
>        )
>      );
>                        st_astext
>     --------------------------------------------------
>      POLYGON((25 300,300 300,300 0,0 0,0 300,25 300))
>     (1 row)
>
>     I don't know why, but I too was expecting to see something like:
>     POLYGON (( 0 300, 300 300, 300 0, 0 0, 0 300 ))
>
>     You almost could use a function that shifts the ordinates in the
>     exterior linear ring ... you could probably put something together
>     fairly quickly in plpgsql (extract the exterior ring, use
>     ST_GetPointN and ST_SetPointN, and rebuild the polygon).
>     Once shifted, the starting ordinance would be on the corner and
>     the (25 300) vertex would disappear with a rerun of ST_Simplify.
>
>     JTS has the same issue.
>
>     -- Kevin
>
>
>
>     On 10/22/2010 2:33 AM, lplateandy wrote:
>
>         Hi Kevin,
>
>         Hopefully the attached (hopefully) PDF will help to show what
>         i mean.
>
>         In producing the PDF i think i can see why the issue happens.
>         It seems that
>         the start and end point of the polygon geometery are the node
>         i was hoping
>         might be removed during the simplify process.
>
>         Not sure if there is a way around this?
>
>         I do realise there are ways for a simple rectangle etc to fix
>         this but the
>         example is mainly for illustration as the issue is a problem
>         in more complex
>         geometries.
>
>         Thanks
>
>         Andy
>
>
>         Kevin Neufeld wrote:
>
>             Hi Andy,
>
>             Can you post a small example?
>             -- Kevin
>
>
>             On 10/21/2010 10:02 AM, lplateandy wrote:
>
>                 Hi,
>
>                 I have, in the simplest example, a square polygon of 1
>                 by 1 kilometer
>                 size
>                 with nodes every 25 meters.
>
>                 I can use ST_Simplify (the_geom,5) to remove most of
>                 the points.
>
>                 However, for some reason i end up with the four corner
>                 nodes and a fifth
>                 node 25m clockwise from the top left corner.
>
>                 In itself, this would be fine - 5 nodes is better than
>                 160.
>
>                 However, on some of the polygons, there are more
>                 complex boundaries which
>                 i
>                 wish to smooth out using simplify. I would do this
>                 with  ST_Simplify
>                 (the_geom,26) example - going over the minimum 25
>                 metres between  any
>                 nodes.
>
>                 In the example of the square with 5 nodes, the problem
>                 i find then is
>                 that
>                 the top left node is removed and the one 25m clockwise
>                 remains. The
>                 result
>                 is that i no longer have a 1km square but sloping
>                 Western boundary
>                 correctly
>                 going 1km North South but incorrectly veering across
>                 25m to the East.
>
>                 Is there a way to remove the stray node so i am more
>                 likely to maintain
>                 my
>                 overall polygon shape. I did try
>                 st_simplifiypreservetopology but saw no
>                 noticeable difference.
>
>                 Hope that makes sense, any help much appreciated
>
>                 Andy
>
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>
>         http://old.nabble.com/file/p30027046/ST_simplify_issue.pdf
>         ST_simplify_issue.pdf
>
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>
> -- 
> ************************************
> David William Bitner
>
>
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