[Radiance-general] Building on a hillside

[Greg Ward]

Hi Randolph,

It's a bit tricky to modify skybright.cal to account for an elevation causing the horizon to be below a 0-degree altitude, but it's possible.  First, you should check that it's necessary.  The formula you want for the angle below the horizon is:

	theta = acos(R/(h+R))

where R is the radius of the Earth (about 6360 km) and h is the distance above the horizon (sea level, presumably).  Even from the top of Mt. Everest, the change in the horizon is only about 3 degrees, assuming you could see something at sea level from there, which I'm not sure you could.  (Anyone who's been to the top of Everest, please pipe in.)

Cheers,
-Greg

> From: "Randolph M. Fritz" <RFritz at lbl.gov>
> Date: November 15, 2010 1:45:55 PM PST
> 
> I'm simulating a test structure on a hillside site; the ground drops away dramatically.  So a lot of sky is visible below what would be the horizon on a level site.  So far as I can tell, the only way to simulate this correctly (if the ground reflectance and sky light is significant) is to give the model an actual elevation, z = 700 feet or whatever.  Do I have that right?
> 
> -- 
> Randolph M. Fritz • RFritz at lbl.gov