<html><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"></head><body><div>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. </div><div><br></div><div><br></div><div><br></div><div id="composer_signature"><div style="font-size:85%;color:#575757" dir="auto">Sent from my Samsung Galaxy smartphone.</div></div><div><br></div><div style="font-size:100%;color:#000000"><!-- originalMessage --><div>-------- Original message --------</div><div>From: cgray@graytechsoftware.com </div><div>Date: 2020-05-14 5:42 PM (GMT-04:00) </div><div>To: qgis-user@lists.osgeo.org </div><div>Subject: [Qgis-user] Viewshed From Specific Elevation? </div><div><br></div></div><div class="WordSection1"><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. <o:p></o:p></p><p class="MsoNormal"><o:p> </o:p></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. <o:p></o:p></p><p class="MsoNormal"><o:p> </o:p></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. <o:p></o:p></p><p class="MsoNormal"><o:p> </o:p></p><p class="MsoNormal">Thank you, Chris<o:p></o:p></p></div></body></html>