bottomlesspit
Well-Known Member
Interesting read.... prompts more googling...
. The Cornish hut that gave rise to sea level benchmark
. The Cornish hut that gave rise to sea level benchmark
Ordnance Datum Newlyn (ODN
- Thread starter bottomlesspit
- Start date
Tomahawk
Well-Known Member
Many years ago I was working as a land surveyor and setting out engineer. Everythng in my work was about AOD Newlyn.
I went on a rugby tour to Newlyn so whilst in the town I made a pilgrimage to the datum. Very unassuming for such a thing..
I went on a rugby tour to Newlyn so whilst in the town I made a pilgrimage to the datum. Very unassuming for such a thing..
johnalison
Well-Known Member
Very interesting. On the other hand, the Egyptians got there first in a way thousands of years ago with their nilometers to monitor the depth of the Nile flood.
AntarcticPilot
Well-Known Member
The Newlyn datum is the datum of OS maps - but not of charts, which use LAT as the datum. Indeed, there was a very interesting discussion on these fora of chart datums, and there are charts where the datum changes from place to place on the chart - charts of rivers like the Thames were given as an example.
Datums are a fascinating study; because of the vagaries of Antarctic mapping, I had to be very aware of the influence of datums - mainly horizontal ones, rather than vertical, as very few places in Antarctica are mapped sufficiently accurately for the vertical datum to matter much! MY favourite was an analysis of the depths in Port Foster, Deception Island, where I used data from about 5 or 6 charts - all on different vertical datums! The results showed that if you enter Port Foster, you'd better have a cast-iron exit plan just in case things go bang!
Finally, the datum we all know and love, WGS84, has substantial variation from sea-level datums - over + and - 100m in places (one being in the Ross Sea embayment in Antarctica, the other near Hawaii). In the UK it is usually about 10-15m different, so if you measure elevation using GPS, you have to account for the difference between WGS84 and the local gravitational equipotential surface (sea-level is such a surface).
Datums are a fascinating study; because of the vagaries of Antarctic mapping, I had to be very aware of the influence of datums - mainly horizontal ones, rather than vertical, as very few places in Antarctica are mapped sufficiently accurately for the vertical datum to matter much! MY favourite was an analysis of the depths in Port Foster, Deception Island, where I used data from about 5 or 6 charts - all on different vertical datums! The results showed that if you enter Port Foster, you'd better have a cast-iron exit plan just in case things go bang!
Finally, the datum we all know and love, WGS84, has substantial variation from sea-level datums - over + and - 100m in places (one being in the Ross Sea embayment in Antarctica, the other near Hawaii). In the UK it is usually about 10-15m different, so if you measure elevation using GPS, you have to account for the difference between WGS84 and the local gravitational equipotential surface (sea-level is such a surface).
tillergirl
Well-Known Member
"local gravitational equipotential surface "! I am not sure you should be using that sort of language like that on the ECF! 
PeterWright
Well-Known Member
I'm left wondering whether that surface is flat or curved and if curved whether its curvature varies with the proximity of large masses or is fairy constant and equal to the curvature of the earth's surface."local gravitational equipotential surface "! I am not sure you should be using that sort of language like that on the ECF!
Peter.
AntarcticPilot
Well-Known Member
Would you prefer it if I said Geoid? That's the local gravitational equipotential surface at mean sea-level. Some quite nice pictures of EGM96 (I think that's the most recent) here: Geoid - Wikipedia"local gravitational equipotential surface "! I am not sure you should be using that sort of language like that on the ECF!
PS, I'm assuming that people on here aren't NFN!
AntarcticPilot
Well-Known Member
I should have said that SMALL variations are caused by things like mountains (see the Schiehallion Experiment), and somewhat larger variations by deeper geology. But the major influences are much deeper in the Earth. Basically, the greater the extent of an anomaly, the deeper the cause. Deep ocean bathymetry can be derived from very precise measurement of the sea-level (sorry, no simple explanation, but see bathymetry by inversion of satellite altimetry - Google Search and take your pick!). I am proud to say that I was part of this (Greenland Gravitational Experiment), and my part was to derive the topography of the rock beneath the ice to account for its effect on the gravitational force being measured.
tillergirl
Well-Known Member
Oh, if you had said 'Geoid'. What on earth means 'NFN'?
penfold
Well-Known Member
Normal for Norfolk?
penberth3
Well-Known Member
Does NFN include putting the Newlyn Datum in the East Coast Forum?
PeterWright
Well-Known Member
i AP,
Thank you for that explanation, Was the influence of these geological anomalies on gravity best explained by Newtonian, Einsteinian or quantum mechanics?
Peter.
penfold
Well-Known Member
That would be an ecumenical matter.
AntarcticPilot
Well-Known Member
Newtonian. Because a) the effects are on timescales and velocities too low for relativistic effects, b) too large a scale for quantum mechanical effects and c) the Schiehallion Experiment took place to verify predictions of Newtonian theory.
Tomahawk
Well-Known Member
We are an eclectic bunch.