D-P simplification is not going to modify your start/end points. For a bit of reference on the Douglas-Peuker simplification algorithm, see <a href="http://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm">http://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm</a><br>
<br><div class="gmail_quote">On Fri, Oct 22, 2010 at 11:13 AM, Kevin Neufeld <span dir="ltr"><<a href="mailto:kneufeld@refractions.net">kneufeld@refractions.net</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
I don't see any pdf posted, but that's ok, I get the idea.<br>
<br>
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.<br>
<br>
select<br>
st_astext(<br>
st_simplify(<br>
'POLYGON ((<br>
25 300,<br>
275 300,<br>
300 300,<br>
300 275,<br>
300 25,<br>
300 0,<br>
275 0,<br>
25 0,<br>
0 0,<br>
0 25,<br>
0 275,<br>
0 300,<br>
25 300<br>
))', 1<br>
)<br>
);<br>
st_astext<br>
--------------------------------------------------<br>
POLYGON((25 300,300 300,300 0,0 0,0 300,25 300))<br>
(1 row)<br>
<br>
I don't know why, but I too was expecting to see something like:<br>
POLYGON (( 0 300, 300 300, 300 0, 0 0, 0 300 ))<br>
<br>
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).<br>
Once shifted, the starting ordinance would be on the corner and the (25 300) vertex would disappear with a rerun of ST_Simplify.<br>
<br>
JTS has the same issue.<br><font color="#888888">
<br>
-- Kevin</font><div><div></div><div class="h5"><br>
<br>
<br>
On 10/22/2010 2:33 AM, lplateandy wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi Kevin,<br>
<br>
Hopefully the attached (hopefully) PDF will help to show what i mean.<br>
<br>
In producing the PDF i think i can see why the issue happens. It seems that<br>
the start and end point of the polygon geometery are the node i was hoping<br>
might be removed during the simplify process.<br>
<br>
Not sure if there is a way around this?<br>
<br>
I do realise there are ways for a simple rectangle etc to fix this but the<br>
example is mainly for illustration as the issue is a problem in more complex<br>
geometries.<br>
<br>
Thanks<br>
<br>
Andy<br>
<br>
<br>
Kevin Neufeld wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi Andy,<br>
<br>
Can you post a small example?<br>
-- Kevin<br>
<br>
<br>
On 10/21/2010 10:02 AM, lplateandy wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi,<br>
<br>
I have, in the simplest example, a square polygon of 1 by 1 kilometer<br>
size<br>
with nodes every 25 meters.<br>
<br>
I can use ST_Simplify (the_geom,5) to remove most of the points.<br>
<br>
However, for some reason i end up with the four corner nodes and a fifth<br>
node 25m clockwise from the top left corner.<br>
<br>
In itself, this would be fine - 5 nodes is better than 160.<br>
<br>
However, on some of the polygons, there are more complex boundaries which<br>
i<br>
wish to smooth out using simplify. I would do this with ST_Simplify<br>
(the_geom,26) example - going over the minimum 25 metres between any<br>
nodes.<br>
<br>
In the example of the square with 5 nodes, the problem i find then is<br>
that<br>
the top left node is removed and the one 25m clockwise remains. The<br>
result<br>
is that i no longer have a 1km square but sloping Western boundary<br>
correctly<br>
going 1km North South but incorrectly veering across 25m to the East.<br>
<br>
Is there a way to remove the stray node so i am more likely to maintain<br>
my<br>
overall polygon shape. I did try st_simplifiypreservetopology but saw no<br>
noticeable difference.<br>
<br>
Hope that makes sense, any help much appreciated<br>
<br>
Andy<br>
</blockquote>
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</blockquote>
<a href="http://old.nabble.com/file/p30027046/ST_simplify_issue.pdf" target="_blank">http://old.nabble.com/file/p30027046/ST_simplify_issue.pdf</a><br>
ST_simplify_issue.pdf<br>
</blockquote>
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