[GRASS-dev] Re: Line of Sight Update

[Paul Kelly]

Hello Will,

On Wed, 9 Jul 2008, Will wrote:

> Hi Paul,
>
> That all sounds good.  I'll move r.terraflow and r.viewshed (I decided to
> take up the r.viewshed name) into that iostream directory that you
> mentioned.  Otherwise though, the code is working and ready to use.

That's great! I'm sure I can speak for the other developers in saying that 
we're very grateful that you continued to work on this even without the 
Google funding.

> I just have a couple of questions about details of the output.
>
> 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?

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.

> 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.

> As soon as those issues are sorted out, I think its all done.
>
> -Will
>
> P.S. I can post this all to the mailing list if you want me to.  Laura Toma
> has told me that I need to be approved or something like that to post, and I
> was wondering what I need to do to get approved.

You only need to be a subscribed, and I just checked and you are already 
subscribed so all you should need to do is send a mail to 
grass-dev at lists.osgeo.org. I've copied this mail to the grass-dev list 
already, so you can just reply to it to follow-up.

Paul