fixed points in survex

[John Halleck]

On Thu, 6 Feb 2003, John Halleck wrote:

> > > > 1) Tidal effects on the earth. The two sources of earth tides are the sun,
> > > > causing effects of 17cm amplitude, and the moon, with 36cm (the next most
> > > > influential body is venus, whose effects are less than 0.05% of the
> > > > above). However, as long as our measurements cover an area of limited
> > > > dimensions then the effects will be small. For example, over a 100km
> > > > distance, the tides should come to less than a cm.
> > > 
> > >    Where does this figure come from?  It is higher than what I've been told.
> > >    (Unless you mean "MUCH less than a cm".)
> > 
> > Rough calculation as follows: take combined tidal effect to be sinusoidal
> > and of 50cm amplitude (eg a spring tide when sun & moon line up
> > 36cm+17cm=53cm). assume we're on the equator and feel the full +-53cm
> 
>   But where are THOSE figures come from?  The claim for the NGS HARN
>   point around the corner from me is that the earth tide here is in mm's.
>   It is possible I've been told incorrectly, but I don't know where
>   your figures come from.

  I'm wrong.  The figure I remember appears to be for the displacment 
  of the monument with respect to other local (order of 100 miles) monuments
  in the network.  Not the absolute compared to monuments on the other side
  of the earth.

  Lev is correct.