Re: [Jsbsim-devel] Field elevation, altitude ASL and altitude AGL

[Ron J. <wi...@je...>]

On Wed, 2009-08-05 at 16:32 +0200, Fabian Grodek wrote:
> Hello,
> It seems there's still something broken in the field elevation and
> flight altitude issue.
>  
> If in the init file I set :
>   <elevation unit="FT">   5,000  </elevation>
>   <altitude unit="FT">    15,000  </altitude>
>  
> In the output I get:
>    Altitude ASL = 20,000 ft, while  Altitude AGL = 15,000 ft.
>  
> I would expect:
>    Altitude ASL = 15000 ft, and Altitude AGL = 15,000 - 5,000 =
> 10,000 ft.
>  
> The property "position/terrain-elevation-asl-ft" correctly reports
> 5,000 ft.
>  
> Fabian


  It looks to me like <altitude unit="FT"> is AGL not ASL.

Altitude ASL = 15,000 + 5,000.

  Whether that is the desired input is open to debate.

(pro current behavior side)
  It prevents rewriting both properties in an init file when changing
the ground elevation, which is a good thing since the elevation
parameter was put in to test take-off/landings. By that I mean the AGL
behavior allows <altitude> to be a constant in an init file that starts
on the ground.  The AGL behavior also allows  <altitude> to be a
constant if the init file sets up an approach position. If you want
<altitude> to be from sea level either leave the <elevation> tag out or
explicitly set it to 0.

(con current behavior side)
  Having <altitude> in AGL is somewhat confusing.

Thanks,

Ron