[GRASS-dev] Re: Line of Sight Update

[Paul Kelly]

Hi Will/Laura
(Cc grass-dev)
Some comments in-line below.

On Wed, 9 Jul 2008, Will wrote:

> Hi again everyone,
>
> I agree, it will be easy, but I wanted to make sure it was wanted before I
> did it.  I'll put it in now.
>
> I have the code written for curvature when it runs in memory, but I have not
> tested it to see if it works.
>
> I'll do whatever we all think is best.
>
> -Will
>
> On Wed, Jul 9, 2008 at 12:05 PM, Laura Toma <ltoma at bowdoin.edu> wrote:
>
>> Will,
>> It will be very easy to add an option for the user to specify the elevation
>> of the observer.  I think you should do it.
>>
>> From what you told me, you are already considering the curvature of the
>> earth when you compute elevations? right?

How does the earth curvature calculation work? Does it just involve 
reducing the height of elevation cells based on their distance from the 
observer and the earth's ellipsoid before considering them in the 
calculation? GRASS has a function G_get_ellipsoid_parameters() to get the 
current radius and eccentricity squared of the earth's ellipse - is that 
relevant? Using that would make sure the calculation would still work 
correctly if someone used it on the Moon or Mars I suppose. It would also 
need to function correctly (i.e. be disabled) in an XY (unprojected) 
location. But maybe all this is something we can look at later.

(There are also some functions G_meridional_radius_of_curvature(), 
G_transverse_radius_of_curvature() and 
G_radius_of_conformal_tangent_sphere() which may or may not be relevant.)

>> Regarding the output:   my suggestion was to output, for every cell that is
>> visible, its  height  (or slope) *above* the line of sight;  this will give
>> an indication of how visible the point is.  Similarly, for the cells that
>> are *not* visible,  you could compute their height (or slope) below the line
>> of sight.  High positive values will mean the point is very visible;  high
>> negative values will mean the point is very un-visible.

I don't quite understand what "the line of sight" is here. What I think of 
as the line of sight is a straight line from the observer to the visible 
cell, passing straight through both. Some kind of output relating to how 
visible the cell is definitely sounds like a very good idea though.

Paul