[GRASS-dev] Re: Line of Sight Update

[Michael Barton]

On Jul 9, 2008, at 11:11 AM, <grass-dev-request at lists.osgeo.org> <grass-dev-request at lists.osgeo.org 
 > wrote:

This is a very nice update, especially the extraordinary speed  
increase. I hope that this can eventually be included into a  
cumulative viewshed module. Thanks very much. Some comments in the  
text below.


> Date: Wed, 9 Jul 2008 17:11:26 +0100 (BST)
> From: Paul Kelly <paul-grass at stjohnspoint.co.uk>
> Subject: [GRASS-dev] Re: Line of Sight Update
> To: Will <willster3021 at gmail.com>
> Cc: Laura Toma <ltoma at bowdoin.edu>, grass-dev at lists.osgeo.org
> Message-ID: <Pine.LNX.4.62.0807091657120.15193 at vortex.ukshells.co.uk>
> Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
>
> Hello Will,
>
> On Wed, 9 Jul 2008, Will wrote:
>
>>
>> Firstly, r.los has a lot of options, suchas observer elevation,  
>> curviture of
>> the earth, and max distance to look at.  Do you still want all or  
>> some of
>> those in r.viewshed?
>
> Max distance is IMHO only a requirement because of the extreme
> inefficiency of r.los, where limiting the calculation to a circular
> sub-region  can reduce the running time significantly. I think if
> r.viewshed performs well enough, it should be fine to leave it  
> calculating
> the viewshed over the whole of the current region.
>
> Observer elevation is a useful shortcut to have and especially  
> relevant
> for radio masts etc. Do you have a default observer elevation in
> r.viewshed?
>

I agree with Paul. Observer elevation is handy.


> Earth curvature calculation would seem to be important when covering a
> very large area, but I imagine it is not the simplest thing to add  
> so it
> may not be necessary immediately. I must confess I have no idea how  
> much
> of a difference it makes to the calculation, nor what is the threshold
> when it starts to become an important consideration. Perhaps someone  
> else
> on the list can comment.

Following up on later posts to this thread, I suppose that earth's  
curvature might be of importance in viewsheds from the sea or from  
high mountains.

>
>
>> Secondly, r.los outputs a map that sets the value of each visible  
>> point to
>> the vertical angle (in degrees) required to see those cells.  Do  
>> you want
>> this for r.viewshed, or something else?  Right now, I just have it  
>> output
>> the elevation of the visible points, but that can always change.
>
> Perhaps there could be multiple output options, e.g. (a) elevation of
> visible cell, (b) difference in elevation between observer and visible
> cell, (c) angle between observer and visible cell... I'm not sure on  
> this
> though and again perhaps someone else has an opinion.

I've run into a few cases where the angle of view is useful. The  
distance between the observer and each cell could be very useful to  
permit assessing resolving ability and creating fuzzy viewsheds. I'm  
not sure about the difference in elevation between the observer and  
cell. The elevation of the cell would be a nice shortcut.

One thing that would be really helpful would be to have a flag that  
would create a binary viewshed: 1=visible and 0=not visible. In this  
way, multiple viewsheds could easily be combined to create cumulative  
viewsheds. As it is now, non-visible cells are always coded as NULL,  
making this more complicated.

Michael