fixed points in survex

[Lev Bishop]

On Thu, 6 Feb 2003, John Halleck wrote:

> On Thu, 6 Feb 2003, Lev Bishop wrote:

> > 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.

It's possible they're talking about the earth tide relative to the Geoid, 
rather than relative to the ellipsoid. The figures I got came from 
http://einstein.gge.unb.ca/tutorial/tutorial.pdf

>   It doesn't.  What you argue is a matter of measuring north, not truely
>   a matter of what direction north is.  It is in the direction of that
>   pole one meter away from you.
> 
>   There seems to be some confusion with actual angles vs. posible to measure
>   angles in your discussion.

Possibly. You can think of it either way: you can take "down" to mean the
perpendicular to the geoid or the perpendicular to the ellipsoid, either
is a valid definition. Equally, you can take "north" to be parallel to the
ellipsoid or parallel to the geoid. Its just a matter of definitions. If
your software assumes one but you give it the other then bad things will
happen. I think most coordinate systems take down perpendicular to the
geoid and north parallel to the ellipsoid. With this set of conventions
north and down aren't even perpendicular and you can't even build a
locally valid coordinate system with them. Since the ellipsoid is an
artificial construction, no instruments will give readings directly
related to the ellipsoid (ie, your spirit level or your automatic
compensating prism on your theodolite are only interested in the geoid, as
is the gimbal on your compass) and if you want to relate your measurements
back to the ellipsoid you need to make the a correction. I believe
"laplace correction" to the azimuth is the term used in land surveying.

Lev