Curvature of earth.

**electrosys** · 29-02-12, 22:07

Ok - so a fair few years ago the Earth was nowt but a ball of molten metal and other minerals, which slowly began to cool and form a crust, which is more-or-less exactly where we are today.

So - where did all the water come from ?

**mjcoon** · 29-02-12, 22:12

Originally Posted by electrosys

Ok - so a fair few years ago the Earth was nowt but a ball of molten metal and other minerals, which slowly began to cool and form a crust, which is more-or-less exactly where we are today.

So - where did all the water come from ?

Comets, amazingly! (Now you are going to ask where comets got their water; it's like having a curious child, I imagine...)

Mike.

**Roberto** · 29-02-12, 22:24

Originally Posted by armchairsailor

In best Bogie voice:

Here's lookin' at Euclid.

always wanted to use that one.

May I borrow it one or two times
dalan dalan dalan, dalan dalan dalan, dalan dalan dalan dan dan

r

**BAtoo** · 29-02-12, 22:29

Originally Posted by lustyd

And also

Tut tut...........

**BAtoo** · 29-02-12, 22:36

Originally Posted by SHUG

So the answer is...140m,559m,1800ft,1700ft,558m or 196m
Great!!!!!!

1800ft is 548.64m - or 300 fathoms

1700ft is 16.666667fathoms less than 548.46m

and they are all somewhat more than 140 or 196m

**AntarcticPilot** · 29-02-12, 22:57

Originally Posted by geoid96

You are correct that the geoid is not exactly the same as MSL. The geoid is a gravity equipotential surface whereas MSL is also influenced by the other local effects that you mention. That’s why I said the geoid is an attempt to model the MSL surface and also extend it underneath the land masses. The point I was making is that globally the geoid is far closer to MSL than it is to the ellipsoid and that MSL is the most common vertical datum for heights on maps.

I think my avtar was based on an even older global geoid model, OSU91a. EGM96 and EGM2008 are considerably refined, but at the scale and resolution, I think it would be difficult to see any difference. I’m currently working on a project using AUSGeoid09. This is a regional refinement of EGM2008 that also incorporates a several thousand extra gravity measurements and precise levelling. The trouble is that nothing stands still. The earth is slowly getting fatter and the tectonic plates are slowly shifting.

I think the OSU Geoid was the first one I used in anger!

**alan_d** · 01-03-12, 00:18

Originally Posted by Kintail

To confuse the issue further. The curvature is so gradual that when we are sailing we are by and large on a flat surface. However the world is round so are we ever sailing " uphill or downhill" however gradual?

No, because wherever you are on the earth's surface "level" is at right angles to a line running to the centre of the earth at that point.

**geoid96** · 01-03-12, 10:34

Originally Posted by alan_d

No, because wherever you are on the earth's surface "level" is at right angles to a line running to the centre of the earth at that point.

Level is a surface that is perpendicular to the local vertical as defined by the direction of a precise plumb bob. There are two reasons that this does not point exactly to the centre of mass of the earth. Firstly, the earth not being spherical means that it tends to point to a point perpendicular to the surface of the ellipse. Secondly the earths crust is very irregular in terms of terrain and density. This means there are significant local deviations of the vertical.

As AntarcticPilot has described earlier, other than gravity, there are other factors that can influence sea level Mean Sea Level. On that basis, and forgetting tidal variations, it could be argued that we are sometimes slightly sailing uphill or downhill. However I don't think it's worth trying to allow for it in your passage planning.