<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>Chris:</p>
<p>Super. In retrospect, the negative observer height should come
back with a null result as you cannot see much if you are standing
in a well.</p>
<p>You could run a buffer of 2 m around every observation point and
recalculate your raster DSM values to a set elevation (ie 10m) and
calculate your observation elevation by subtracting 10 m from
your actual observation elevation. That way, your observation
height will be modeled correctly. <br>
</p>
<p>Have fun</p>
<p>Kirk Schmidt<br>
</p>
<p><br>
</p>
<div class="moz-cite-prefix">On 5/14/2020 6:48 PM,
<a class="moz-txt-link-abbreviated" href="mailto:cgray@graytechsoftware.com">cgray@graytechsoftware.com</a> wrote:<br>
</div>
<blockquote type="cite"
cite="mid:003801d62a39$732fe730$598fb590$@graytechsoftware.com">
<meta name="Generator" content="Microsoft Word 15 (filtered
medium)">
<div class="WordSection1">
<p class="MsoNormal">I’ve done what you described – select a
point away from the edge and manually adjust the observer
height. This does work. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I also tested negative observer height. If
it is surrounded, the viewshed simply shows nothing
(neighboring pixels only).</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">However, I’m going to be doing 100+ points
over multiple locations. Ideally, when I setup a viewshed
point (including elevation), I can just run an analysis from
that location and ignore the first 2m or so. I think
manipulating every point will be cumbersome.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thank you, Chris</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"> </p>
<div>
<div>
<p class="MsoNormal"><b>From:</b> kirk
<a class="moz-txt-link-rfc2396E" href="mailto:kirk@nortekresources.com"><kirk@nortekresources.com></a> <br>
<b>Sent:</b> Thursday, May 14, 2020 5:36 PM<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:cgray@graytechsoftware.com">cgray@graytechsoftware.com</a>;
<a class="moz-txt-link-abbreviated" href="mailto:qgis-user@lists.osgeo.org">qgis-user@lists.osgeo.org</a><br>
<b>Subject:</b> Re: [Qgis-user] Viewshed >From Specific
Elevation?</p>
</div>
</div>
<p class="MsoNormal"> </p>
<div>
<p class="MsoNormal">You could try converting the first lidar
returns to a raster layer and using a negative elevation
observer value or select point that is one or 3 pixels away
from the edge of the roof. I am not sure if the negative
observer height will work but it may. If you need to remove
obstructions, you can create a mask and recalculate the
elevations if the obsructions below the observer height and
rerun the viewshed. </p>
</div>
<div>
<p class="MsoNormal"> </p>
</div>
<div>
<p class="MsoNormal"> </p>
</div>
<div>
<p class="MsoNormal"> </p>
</div>
<div id="composer_signature">
<div>
<p class="MsoNormal"><span>Sent from my Samsung Galaxy
smartphone.</span></p>
</div>
</div>
<div>
<p class="MsoNormal"> </p>
</div>
<div>
<div>
<p class="MsoNormal"><span>-------- Original message
--------</span></p>
</div>
<div>
<p class="MsoNormal"><span>From: <a
href="mailto:cgray@graytechsoftware.com"
moz-do-not-send="true">cgray@graytechsoftware.com</a>
</span></p>
</div>
<div>
<p class="MsoNormal"><span>Date: 2020-05-14 5:42 PM
(GMT-04:00) </span></p>
</div>
<div>
<p class="MsoNormal"><span>To: <a
href="mailto:qgis-user@lists.osgeo.org"
moz-do-not-send="true">qgis-user@lists.osgeo.org</a> </span></p>
</div>
<div>
<p class="MsoNormal"><span>Subject: [Qgis-user] Viewshed
From Specific Elevation? </span></p>
</div>
<div>
<p class="MsoNormal"><span> </span></p>
</div>
</div>
<div>
<p class="MsoNormal">I am attempting to test viewsheds from
multiple heights below the roofline of particular buildings.
I have been identifying viewing positions from directly from
LiDAR data, but I cannot find a way to use absolute values
for elevation. The build-in visibility analysis seems to
always use the raster elevation value plus the relative
observer height. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I would like to use an absolute Z value,
and have some immediate zone that is ignored (i.e., ignore
the first 2m of obstructions) so that a viewpoint that might
be lower than the raster elevation value can get outside of
a building. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Is there a plugin or alternative
visibility analysis tool that can do this? I was able to get
it to work in Manifold Viewer, but I would like to keep the
workflow within QGIS if possible. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thank you, Chris</p>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
Qgis-user mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Qgis-user@lists.osgeo.org">Qgis-user@lists.osgeo.org</a>
List info: <a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/qgis-user">https://lists.osgeo.org/mailman/listinfo/qgis-user</a>
Unsubscribe: <a class="moz-txt-link-freetext" href="https://lists.osgeo.org/mailman/listinfo/qgis-user">https://lists.osgeo.org/mailman/listinfo/qgis-user</a></pre>
</blockquote>
<pre class="moz-signature" cols="72">--
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
RR # 1
Thorburn, NS
B0K 1W0
Tel (902) 922.3607
Email: <a class="moz-txt-link-abbreviated" href="mailto:kirk@nortekresources.com">kirk@nortekresources.com</a>
Web: <a class="moz-txt-link-abbreviated" href="http://www.nortekresources.com">www.nortekresources.com</a></pre>
</body>
</html>