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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">So you have an uncategorized bunch of points and you wish to have an algorithm that recognizes outer and inner rings automatically? I fear that is not as easy as it may appear. Have
a look at the annexed image with two polygons having the same vertices. How could an algorithm know which interpretation is correct? In this simple case you could make a convex hull, delete the vertices which were consumed for the outer ring, and create a
new ring from the remaining points, but generally it will not work if there are two holes, for example.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">If the points come from a GPX device, make the application to record indexes for both the points and the rings “ring1-point1, ring1-point2---ring2-point1,ring2-point2” and construct
the polygon ring by ring.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">-Jukka Rahkonen-<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b>Lähettäjä:</b> gdal-dev <gdal-dev-bounces@lists.osgeo.org>
<b>Puolesta </b>Paul Meems<br>
<b>Lähetetty:</b> perjantai 3. maaliskuuta 2023 14.52<br>
<b>Vastaanottaja:</b> gdal-dev@lists.osgeo.org<br>
<b>Aihe:</b> [gdal-dev] Convex hull with holes<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">How can I algorithmically create a parcel border that accounts for islands or holes within the parcel based on a list of coordinates? <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">While GDAL+GEOS has a Convex hull method, it only identifies the outer border. <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Has GDAL+GEOS a method that I can use to detect inner holes as well, such as in a donut-shaped field?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The output can be json or shapefile, that doesn't matter.<br clear="all">
<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<br>
<br>
Paul Meems<o:p></o:p></p>
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