fixed points in survex

[Lev Bishop]

On Thu, 6 Feb 2003, John Halleck wrote:

> On Thu, 6 Feb 2003, John Halleck wrote:
> 
>   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.

I believe your numbers and I would not be surprised if errors 10 times as
big weren't possible in extreme cases (where different bodies of air come
into contact and mix with each other, for example near to the entrance of
the cave (heat-shimmer can be visible to the naked eye there), or even
when the bodies of the surveyors set up thermal gradients in the cave (or
they're using carbide lights)). However, the curvature is proportional to
the square of the length of the leg, so 1ft/mile-squared, for a typical
leg of 40' length and even taking 10 ft/mile-squared, the effect is still
under a 100th of an inch.

Lev