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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Thanks for your reply Remi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">for now we are going with simply a union of a buffer of the points (as you suggested) as it makes sense in the context of the origin of the data
anyway.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">cheers<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">-i<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">De :</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> postgis-users-bounces@lists.osgeo.org [mailto:postgis-users-bounces@lists.osgeo.org]
<b>De la part de</b> Rémi Cura<br>
<b>Envoyé :</b> Tuesday, 14 January 2014 11:39<br>
<b>À :</b> PostGIS Users Discussion<br>
<b>Objet :</b> Re: [postgis-users] ST_ConcaveHull giving up too soon, aka too convex<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Hey,<o:p></o:p></p>
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<p class="MsoNormal">sadly I can't answer your questions, <o:p></o:p></p>
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<p class="MsoNormal">but it looks like you don't want concave hull but instead Alpha Shape (see SFCGAL , maybe they have integrated it).<o:p></o:p></p>
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<p class="MsoNormal"><br>
You have still a lot of workaround :<br>
if you don't mind expending the bouyndary, a union of buffers of points<br>
if you don't mind quantization (anyway your input looks quantized) : convert to raster, then any classical image processing easily available (watershed for example) (you can use out of the box QGIS 2.0 raster processing tools)<br>
if you want a sharp boudary : and assuming your points are regularly spaced : maybe something liek this (untested)
<br>
create a segment for each point to his K nearest neighbours (K would be 2 or 3, and you would put a max distance before drawing a line)<o:p></o:p></p>
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<p class="MsoNormal">Then pick the segments that have no intersection with other segments, and either use ST_Polygonyze, or cut the bounding box polygon with the segments and remove the part outside<br>
.<o:p></o:p></p>
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<p class="MsoNormal">Maybe you can try to reduce your point precision for processing (snap to grid) and/or put the points in an appropriate srid (not too much digits)<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers,<br>
<br>
Rémi-C<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><o:p> </o:p></p>
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<p class="MsoNormal">2014/1/14 Ivan Price <<a href="mailto:Ivan.Price@noveltis.fr" target="_blank">Ivan.Price@noveltis.fr</a>><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">Hi there,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">I am experimenting with ST_ConcaveHull, am using Postgres 9.2.2 on windows with postgis 2.1.1 r12113</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">i have a table of 218 points forming roughly an arc (something like the nike swoosh)</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">when i ask for the concave hull i'm expecting a poly shrink-wrapped to them, however this is not the case, even when i set a very low percentage area parameter.
for example:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">SELECT ST_ConcaveHull(ST_Collect(geom), 0.000001) as geom FROM points;</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">i attach a screenshot of the points and the convext hull, as well as an sql dump of the points themselves.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">also as a side note, in researching the problem i upgraded from postgis 2.01 to 2.1.1 and noticed a change in the processing time for the above statement from
1 minute to ~ 5 minutes.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">are there any pointers for making ConcaveHull more 'aggressive' ?</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU">thanks and regards</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU" style="color:#888888"> </span><span style="color:#888888"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU" style="color:#888888">-i</span><span style="color:#888888"><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-AU" style="color:#888888"> </span><span style="color:#888888"><o:p></o:p></span></p>
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