[Qgis-user] Viewshed From Specific Elevation?

[Kirk Schmidt]

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, cgray at 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 <kirk at nortekresources.com>
> *Sent:* Thursday, May 14, 2020 5:36 PM
> *To:* cgray at graytechsoftware.com; qgis-user at 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: cgray at graytechsoftware.com <mailto:cgray at graytechsoftware.com>
>
> Date: 2020-05-14 5:42 PM (GMT-04:00)
>
> To: qgis-user at lists.osgeo.org <mailto:qgis-user at 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
>
>
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-- 
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
RR # 1
Thorburn, NS
B0K 1W0
Tel (902) 922.3607
Email: kirk at nortekresources.com
Web: www.nortekresources.com

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