[GRASS-dev] Re: Line of Sight Update

[Markus Neteler]

Hi Will,

On Wed, Jul 9, 2008 at 8:11 PM, Will <willster3021 at gmail.com> wrote:
> On Wed, Jul 9, 2008 at 12:11 PM, Paul Kelly <paul-grass at stjohnspoint.co.uk> wrote:
>> On Wed, 9 Jul 2008, Will wrote:
...
> I am having a great time doing this work;   I am funded from an NSF  grant
> that Laura has.  Hopefully I'll be able to get funding from Google next
> summer and contine working with GRASS.

this is great and much appreciated!

...
>> 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?
>
> Observer elevation will not be too hard to implement, so I can put it in.
> As of right now, the default observer elevation for r.viewshed is ground
> level.  For r.los, the default value is 1.75- does anyone on the list have
> an opinion on if I should make 1.75 the default for r.viewshed, or if it
> should be a different value?

Since it is rather arbitrary, please keep it for backward compatibility.

>> 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.
>
> Looking at the r.los code, it doesn't seem to be too hard to put in
> curvature, though like you I have no idea what effect it has or at what
> threshold it becomes important.

It is important - please, if not too hard (I ported it from r.cva to
r.los, wasn't
so hard) add it. It makes a difference... I have an ongoing "batlle" with
an ex colleague if we can see the sea from our mountain here or not,
r.los and his algorithm do not agree. :)

> It is done exactly the way you mentioned in the other email, finding the
> radius of the elipsoid and reducing the elevatin of the cell that you are
> looking at depending on the distance it is away from the viewpoint.  The way
> r.los does it is they have a flag that you set if you want the curvature to
> be considered, though I can impliment it for all calculations without a flag
> if we think that will be better- any input from the list on this?

I would keep the flag if there aren't too many.

Benjamin provided a useful link in his mail.

Markus