<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
Tobias,<br>
<br>
Indeed Buffer 0 won't do any good on self intersections. Thats just
not the way to repair them. <br>
<br>
Hence I wrote "Buffer 0 on INVALID geometries IMHO can lead to good
results depending on the <i>nature of the invalidity</i>. To my
experience <i>repeating points,</i><i>wrong coordinate ordering and
holes touching outer shells in one point</i><i> </i>are being
repaired well without data loss.".<br>
<br>
Of course self intersections can be easily overlooked. For that we
have validity checkers.<br>
<br>
Marco<br>
<br>
<br>
<div class="moz-cite-prefix">On 29-10-19 19:35, Tobias Wendorff
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:e454edd6-2cb9-c9bf-613d-b9ba1f06cde7@tu-dortmund.de">
<pre class="moz-quote-pre" wrap="">Hi Marco,
Am 29.10.2019 um 10:49 schrieb Marco:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Buffer 0 on INVALID geometries IMHO can lead to good results depending
on the nature of the invalidity. To my experience repeating points,
wrong coordinate ordering and holes touching outer shells in one point
are being repaired well without data loss.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
I'm sorry to say that the side effects are worse. Maybe the
implementation of QGIS is wrong, but try the example below.
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">Please enlighten me with a few links to the tons of tests on the web
showing that buffer 0 is destroying data. My Google skills are not up to it.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
I've just updated the example I'm normally showing students with an
actual thematic, I'm calling in "wronxit": [1]
Try to "buffer(geom, 0)" it in QGIS...
This problem isn't constructed (well, this geojson is of course), but
I'm got lots of self-intersecting polygons from different sources all
the time. "Buffer 0" is destructive here and since many
self-intersections are damn small, most users simply don't see them.
"Fix Geometries" gives the correct result.
Best regards,
Tobias
references:
[1] <a class="moz-txt-link-freetext" href="https://gist.github.com/tobwen/39bc3562d1cb957cbaf4305ef53f77df">https://gist.github.com/tobwen/39bc3562d1cb957cbaf4305ef53f77df</a>
</pre>
</blockquote>
<br>
</body>
</html>