fixed points in survex

[John Halleck]

On Thu, 6 Feb 2003, John Halleck wrote:

> [...]

>   Don't forget refraction.  Because the air closer to the ground 
>   is denser than the air farther up, in isothermal conditions the the
>   line of sight drops.  (Thereby making up somewhat for the curviture
>   of the earth.
> 
>   If I remember aright, the earth curvature drops 1foot/mile, and
>   the refraction drops line of sight 1/2 ft/mile, for a net APPARENT
>   curviture of about 0.6 feet/mile.

  And, to address some thermal effects...
  If the air blowing down the passage is warmer than the walls, there
  is a thermal (and therefore density) gradient that bends line of
  sight towards the walls.  And if the air blowing down the passage
  is cooler than the walls then there is a gradient bending line of
  sight towards the center of the passage.  (These conditions can, 
  of course, occur in any periodicly breathing cave)  This effect
  can EASYLY exceed 1 ft/mile of line of sight bend.

> [...]