Chris:
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.
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.
Have fun
Kirk Schmidt
On 5/14/2020 6:48 PM, cg...@graytechsoftware.com wrote:
I’ve done what you described – select a point away from the edge and
manually adjust the observer height. This does work.
I also tested negative observer height. If it is surrounded, the
viewshed simply shows nothing (neighboring pixels only).
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.
Thank you, Chris
*From:* kirk <k...@nortekresources.com>
*Sent:* Thursday, May 14, 2020 5:36 PM
*To:* cg...@graytechsoftware.com; qgis-user@lists.osgeo.org
*Subject:* Re: [Qgis-user] Viewshed >From Specific Elevation?
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.
Sent from my Samsung Galaxy smartphone.
-------- Original message --------
From: cg...@graytechsoftware.com <mailto:cg...@graytechsoftware.com>
Date: 2020-05-14 5:42 PM (GMT-04:00)
To: qgis-user@lists.osgeo.org <mailto:qgis-user@lists.osgeo.org>
Subject: [Qgis-user] Viewshed From Specific Elevation?
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.
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.
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.
Thank you, Chris
_______________________________________________
Qgis-user mailing list
Qgis-user@lists.osgeo.org
List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
--
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
RR # 1
Thorburn, NS
B0K 1W0
Tel (902) 922.3607
Email: k...@nortekresources.com
Web: www.nortekresources.com
_______________________________________________
Qgis-user mailing list
Qgis-user@lists.osgeo.org
List info: https://lists.osgeo.org/mailman/listinfo/qgis-user
Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-user
