On Jul 9, 2008, at 11:11 AM, <[EMAIL PROTECTED]> <[EMAIL PROTECTED]
> 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 <[EMAIL PROTECTED]>
Subject: [GRASS-dev] Re: Line of Sight Update
To: Will <[EMAIL PROTECTED]>
Cc: Laura Toma <[EMAIL PROTECTED]>, grass-dev@lists.osgeo.org
Message-ID: <[EMAIL PROTECTED]>
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
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