[GRASS-dev] Re: Line of Sight Update

[Benjamin Ducke]

Sorry for the late reply to this -- too caught up in other things
at the moment.

Earth curvature correction and other generic issues involved in LOS
application design are detailed nicely here:

http://www.scs.org/PUBs/jdms/vol1num1/article04.pdf

The article talks about using an approximation instead of a precise
correction, which should be good enough in most cases and kick in
beyond a range of ca. 3 kilometers.

A redesign of the GRASS LOS tool is certainly a great thing. Would be
good to  try and keep the actual LOS core nice and simply and cleanly 
separated from the other program logics.

That way, we can later easily build up on this and add things like
cumulative visibility, probabilistic viewsheds and visiblity fall-off
curves to account for things like atmospheric effects.

Cheers,

Ben

Paul Kelly wrote:
> 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
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> 


-- 
Benjamin Ducke
Senior Applications Support and Development Officer

Oxford Archaeological Unit Limited
Janus House
Osney Mead
OX2 0ES
Oxford, U.K.

Tel: +44 (0)1865 263 800 (switchboard)
Tel: +44 (0)1865 980 758 (direct)
Fax :+44 (0)1865 793 496
benjamin.ducke at oxfordarch.co.uk




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