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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US"><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;mso-fareast-language:EN-US">Hi,<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">I do not know of any automatic method that prevents the duplicate drawing. With simple symbology of solid, opaque lines
 there is no need to do anything because the result looks the same anyway. For advanced symbology a simple rendering trick with a solid, opaque background may give good result for some kind of symbology but sometimes nothing else helps but to remove the overlapping
 segments permanently from the data that is used for rendering the borders.<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;mso-fareast-language:EN-US"><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;mso-fareast-language:EN-US">I believe that there are some examples with images about the “line with white background and symbols above that trick”
 in gis.stackexchange but this answer is just literal<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;mso-fareast-language:EN-US"><a href="https://gis.stackexchange.com/questions/171182/how-to-make-dashed-lines-between-adjacent-polygons-render-correctly-in-qgis">https://gis.stackexchange.com/questions/171182/how-to-make-dashed-lines-between-adjacent-polygons-render-correctly-in-qgis</a><o:p></o:p></span></p>
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The “convert polygons into polylines and clean overlapping segments workflow”  explained with images in an ESRI blog<br>
<a href="https://blogs.esri.com/esri/arcgis/2007/09/26/excerpt-for-dash-dot-line-entry/">https://blogs.esri.com/esri/arcgis/2007/09/26/excerpt-for-dash-dot-line-entry/</a><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;mso-fareast-language:EN-US"><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;mso-fareast-language:EN-US">The operation “polygons into polylines without overlapping segments” can be performed by creating a planar graph or the
 GIS software may have a special tool for that purpose like my favorite OpenJUMP. However, it is rather common that polygon shapefiles are not topologically clean and borders between adjacent polygons do not match exactly. In that case the source data must
 be made to match first. Usually I do that with OpenJUMP and the Adjust Polygon Boundaries tool. Fine tuning can be tedious and next update from the data provider will have the same errors so I tend to use simple styles
</span><span lang="EN-US" style="font-size:11.0pt;font-family:Wingdings;color:#1F497D;mso-fareast-language:EN-US">J</span><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">.<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;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">-Jukka Rahkonen-<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">Lähettäjä:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Björn Danielsson [<a href="mailto:bjorn.danielsson@falubo.se">mailto:bjorn.danielsson@falubo.se</a>]
<br>
<b>Lähetetty:</b> 12. lokakuuta 2017 13:03<br>
<b>Vastaanottaja:</b> Rahkonen Jukka (MML) <<a href="mailto:jukka.rahkonen@maanmittauslaitos.fi">jukka.rahkonen@maanmittauslaitos.fi</a>>; Anton Bakker <<a href="mailto:anton.bakker@geocat.net">anton.bakker@geocat.net</a>>;
<a href="mailto:lars.schylberg@blixtmail.se">lars.schylberg@blixtmail.se</a><br>
<b>Kopio:</b> <a href="mailto:mapserver-users@lists.osgeo.org">mapserver-users@lists.osgeo.org</a><br>
<b>Aihe:</b> Re: [mapserver-users] Polygon border with marker line symbol<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black">Hi,<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black">Jukka R,<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black">I would love to learn how to prevent polygon borders to be drawn twice.<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black">Björn D<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black"> <o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma",sans-serif;color:black"> 
<o:p></o:p></span></p>
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