<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;"><div dir="auto" style="overflow-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;">It would be slow to compute, but one thought is if you had an additional characterization from filters.covariancefeatures <a href="https://pdal.io/en/stable/stages/filters.covariancefeatures.html">https://pdal.io/en/stable/stages/filters.covariancefeatures.html</a> you could craft a filters.expression that that uses those attributes to help you identify the "linear tunnels".<br id="lineBreakAtBeginningOfMessage"><div><br><blockquote type="cite"><div>On Nov 10, 2025, at 7:54 AM, Roland Spielhofer via pdal <pdal@lists.osgeo.org> wrote:</div><br class="Apple-interchange-newline"><div><div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">Hi,</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">I tried cell = 0.02 - 0.04 and thresholds of 0.001 to 0.01.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">In the second image it is quite good to see that the sinusoidal line of points is quite dense in z-direction and I guess that is the problem that the threshold does not work well. It must be quite small, but larger than the variance in the surface.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;"> </span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">In the meantime I also tried this:</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;"> </span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">{<br> "type": "filters.gridDecimation",<br> "output_type":"max",<br> "resolution": "0.03",<br> "value":"Classification=9"<br> },</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;"> </span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">Keep only the highest of a grid cell - with resolution of 0.03 the low points go away, with 0.025 or 0.02 they stay - 0.02 is roughly the distance between the transversal scan lines. Hoewever, it decimates the concrete surface as well by a mangitude, which I would try to avoid.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;"> </span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">Regards,</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="background-color: #ffffff;">Roland</span></div>
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<div><strong>Gesendet: </strong>Montag, 10. November 2025 um 14:37</div>
<div><strong>Von: </strong>"Bradley Chambers" <brad.chambers@gmail.com></div>
<div><strong>An: </strong>"Roland Spielhofer" <rspiel@gmx.net></div>
<div><strong>CC: </strong>pdal@lists.osgeo.org</div>
<div><strong>Betreff: </strong>Re: [pdal] Delete low points below surface</div>
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<div>I’d be interested to know what ELM parameters you used. It is configurable, though perhaps still difficult to fine tune. </div>
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<div class="gmail_attr">On Mon, Nov 10, 2025 at 06:38 Roland Spielhofer via pdal <<a href="mailto:pdal@lists.osgeo.org" target="_blank" rel="noopener noreferrer">pdal@lists.osgeo.org</a>> wrote:</div>
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<div style="font-family: verdana; font-size: 12px;"><span style="font-family: verdana;">Hi,</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="font-family: verdana;">I am loking for a PDAL pipline to get rid of low lying points.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="font-family: verdana;">The point cloud I am working with is very dense and represents the surface of a concrete pavement.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="font-family: verdana;">The slabs of the pavement are divided by joints - and the points lying in these joints just below the surface I want to get rid of.</span></div>
<div style="font-family: verdana; font-size: 12px;"><span style="font-family: verdana;"> </span></div>
<div style="font-family: verdana; font-size: 12px;">I created some images showing details of one joint:</div>
<div style="font-family: verdana; font-size: 12px;"><a style="font-family: verdana;" href="https://imgur.com/a/oX7dZcE" target="_blank" rel="noopener noreferrer">https://imgur.com/a/oX7dZcE</a></div>
<div style="font-family: verdana; font-size: 12px;"> </div>
<div style="font-family: verdana; font-size: 12px;">The surface is very flat, and the point density is ~ 5 mm in transversal and 20 mm in longitudinal direction (=driving direction).<br>The joints have a depth of ~40 mm (see images).</div>
<div style="font-family: verdana; font-size: 12px;"> </div>
<div style="font-family: verdana; font-size: 12px;">I tried out the elm filter, but I guess that the relative z-difference between the points is too little to mark them as noise.</div>
<div style="font-family: verdana; font-size: 12px;"> </div>
<div style="font-family: verdana; font-size: 12px;">Any idea to filter the points in the joint with PDAL would be appreciated (I can also provide the LAZ pointcloud itself for experiments).</div>
<div style="font-family: verdana; font-size: 12px;"> </div>
<div style="font-family: verdana; font-size: 12px;">Thanks in advance and best regards,</div>
<div style="font-family: verdana; font-size: 12px;">Roland</div>
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