The meridian - how does it work

[saaby93]

I tried posting this in science but no response. Maybe more navigators here

BBC article tries to explain why the Greenwich Meridian is in the wrong place - something to do with contours.
http://www.bbc.co.uk/news/magazine-33919429

I thought the Meridian through Greenwich was chosen in the 1800s by some international agreement.
GPS WGS84 chooses a different Zero line going through somewhere near Paris so is 100metres off Greenwich Line
Surely it's just a different line with a different arbitrary starting point rather than Greenwich being in the wrong place?

Someone explain about contours

[karma mechanic]

http://www.thegreenwichmeridian.org/tgm/articles.p...

That purports to be an explanation, although it is perhaps a little dense...

The short version is that the shape of the Earth isn't an exact sphere, so more exact recent measurements use a different calculation of exactly what is where in relation to the surface features going round the centre of the Earth.

Edited by karma mechanic on Tuesday 18th August 10:32

[saaby93]

Thanks for the link but all I got from it was there was an arbitary decision for some satellites to use NAD27
In 1971 they suggested realigning it with Greenwich OSGB36 but it was forgotten about
Later WGS84 followed NAD27
Unless I'm missing something I cant see anything that suggests the choice of datum is anything other than arbitrary
Why Greenwich and not Bristol or Liverpool?

Which means Greenwich isnt in the wrong place, it's just different systems with different zero points

[Simpo Two]

Stick with Greenwich, anything foreign is beastly and wrong.

I'm surprised the EU hasn't moved it to Brussels; they want to control everything else.

[karma mechanic]

The concept of having a fixed place (Greenwich) as the base for all the measurements is indeed arbitrary, it just happens that Greenwich is it. The difficulties that have arisen since then are simply because with the arrival of GPS it has become clear that to make all of our current system fit together the actual baseline isn't quite where we originally thought it should be - the requirement is to make it work accurately for the whole globe.

By adopting the later system with much more accurate knowledge of where the centre of mass of the Earth is the result is that the Greenwich line is actually about 100 metres out. This is due to the shape of the Earth being more complex than was known about back then. It is based on the centre of mass of the Earth because that's what the GPS satellites orbit around.

It isn't that Greenwich is in the wrong place, it is that refinements to the model undreamed of by the original inventors of it have revealed that the placement of the 'line' is slightly out. Still in Greenwich though...

[maffski]

saaby93 said:

Why Greenwich and not Bristol or Liverpool?

Which means Greenwich isnt in the wrong place, it's just different systems with different zero points

People were working out their position based mainly on the port they had sailed from. But the calculations needed were quite long winded and the Royal Observatory produced a set of tables to make the job easier, these were all calculated with the meridian going through Greenwich as that's where they were based.

Because the tables made life easier lots of ships started using them, including those from other nations so when it was decided to make a single 'prime' meridian it was just the choice most people were familiar with.

Re the second point, I think it's that once you take more accurate account of the earths surface the line marked as the Greenwich merdian isn't actually where the Greenwich calculations put it.

As an aside this tendency to produce tables of things is probably the reason the industrial revolution happened here.

[Simpo Two]

karma mechanic said:

By adopting the later system with much more accurate knowledge of where the centre of mass of the Earth is the result is that the Greenwich line is actually about 100 metres out. This is due to the shape of the Earth being more complex than was known about back then. It is based on the centre of mass of the Earth because that's what the GPS satellites orbit around.

Could they not have moved the GPS satellites 100 metres along so they fitted the existing structure?

Mass is really nothing to do with navigation; only the surface matters. But thanks for the good explanation!

[saaby93]

Simpo Two said:

karma mechanic said:

By adopting the later system with much more accurate knowledge of where the centre of mass of the Earth is the result is that the Greenwich line is actually about 100 metres out. This is due to the shape of the Earth being more complex than was known about back then. It is based on the centre of mass of the Earth because that's what the GPS satellites orbit around.

Could they not have moved the GPS satellites 100 metres along so they fitted the existing structure?

Mass is really nothing to do with navigation; only the surface matters. But thanks for the good explanation!

I'm not sure it's right
If you take the centre of mass you can draw a line elipsoid thingy with its centre at the centre of mass and have it going through Greenwich and call that zero.

I think its more likely what heppend was that when the first satellites were designed they arranged the US system so it was near enough zero point at Greenwich for compatibility with previous.
Otherwise why didnt the zero end up at Houston?
When they brought in the more accurate WGS84 they found it was actually out by 100 metres. They had the opportunity in the 70s to shift it back, but there is now so much reliant on it they just live with Greenwich being the one thats allegedly out.

Its like setting out the footings for a greenhouse
You measure 50 metres from the front gate set it all out, lay the concrete, build the walls, measure back to the gatepost and its 50.5 metres. How did that happen?

[eharding]

Simpo Two said:

Could they not have moved the GPS satellites 100 metres along so they fitted the existing structure?

Mass is really nothing to do with navigation; only the surface matters. But thanks for the good explanation!

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

[Simpo Two]

eharding said:

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

Oh do stop taking silly opportunistic pops Ed, it's getting tedious and noticeable. Saaby is right.

[Tom_C76]

Simpo Two said:

Could they not have moved the GPS satellites 100 metres along so they fitted the existing structure?

Mass is really nothing to do with navigation; only the surface matters. But thanks for the good explanation!

I understood from somewhere (BBC website maybe?) That the issue was that the meridian is relative to astronomical measurements as used for navigation, and that more accurate measurement of the star positions moved the meridian line.

The reference to OSGB is way off, as OSGB is based on a flat sheet projection and meridian lines are on a sphere. To get an accurate translation from the GPS spheroid system to OSGB is a horrible 7 way transformation. When I was a student we had to run a 286 PC for several minutes to convert from GPS to survey coordinates.

[Disastrous]

Also, is Greenwich still in exactly the same place as it was when the meridian was established?

I don't know the answer at all, but I'm just wondering if in the same way that nautical charts account for magnetic deviation each year, would continental drift have an effect on the exact location of it? Or do they move too slowly for it to be a thing?

[Bluetoo]

can we not just move Greenwich 100m to the left or right? :-)

[saaby93]

Tom_C76 said:

I understood from somewhere (BBC website maybe?) That the issue was that the meridian is relative to astronomical measurements as used for navigation, and that more accurate measurement of the star positions moved the meridian line.

I have a sneaky feeling that stars move too. Unless the meridian is determined by the time 0:00 on day 01/02/03
However isnt the time and date something where we've set arbitrary zero points?

Tom_C76 said:

The reference to OSGB is way off, as OSGB is based on a flat sheet projection and meridian lines are on a sphere. To get an accurate translation from the GPS spheroid system to OSGB is a horrible 7 way transformation. When I was a student we had to run a 286 PC for several minutes to convert from GPS to survey coordinates.

Agreed about OSGB and conversion
However OSGB is one mapping system WGS84 another
Each serves its purpose
There's only one point we're interested how to relate the two- Greenwich and the offset 100metres or so

100metres east of Greenwich - can I claim that as a book title?

[Bluetoo]

should your book not be '102m East Of Greenwich'........ or perhaps even '5.3seconds East of Greenwich', equiv to at least 100km/h in any reasonable PH car :-)

[eharding]

Simpo Two said:

eharding said:

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

Oh do stop taking silly opportunistic pops Ed, it's getting tedious and noticeable. Saaby is right.

When you stop talking crap, we'll stop telling you you're talking crap.

[Simpo Two]

eharding said:

Simpo Two said:

eharding said:

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

Oh do stop taking silly opportunistic pops Ed, it's getting tedious and noticeable. Saaby is right.

When you stop talking crap, we'll stop telling you you're talking crap.

What is 'crap' about suggesting that the GPS structure might have been calibrated to fit the existing one? And there's no 'we'; it's just you and your tedious little snipes.

[saaby93]

Simpo Two said:

eharding said:

Simpo Two said:

eharding said:

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

Oh do stop taking silly opportunistic pops Ed, it's getting tedious and noticeable. Saaby is right.

When you stop talking crap, we'll stop telling you you're talking crap.

What is 'crap' about suggesting that the GPS structure might have been calibrated to fit the existing one? And there's no 'we'; it's just you and your tedious little snipes.

Can you two explain whats going on
I cant see the conection with druids stones the A303 and the sun
Is it anything to do with Spinal Tap?

[eharding]

Simpo Two said:

eharding said:

Simpo Two said:

eharding said:

You're a bit like a Druid complaining that the Sun isn't rising in the right place at the Summer Solstice, according to some pile of old stones you've found next to the A303.

Oh do stop taking silly opportunistic pops Ed, it's getting tedious and noticeable. Saaby is right.

When you stop talking crap, we'll stop telling you you're talking crap.

What is 'crap' about suggesting that the GPS structure might have been calibrated to fit the existing one? And there's no 'we'; it's just you and your tedious little snipes.

Because it's *precisely* the same as any other deluded unfortunate assuming that some physical structure placed in the past, subject to the vagaries of time and the error margins inherent in earlier technology, should somehow still be referred to in the face of superior techniques (or simple current reality).

If you've decided to have a gumpy-arse moment about having that pointed out, then that's entirely your problem.

[eharding]

saaby93 said:

Is it anything to do with Spinal Tap?

Possibly - I suspect Simpo has been asked to remove the foil-wrapped courgette from his trousers again, and has got a general snit on as a result.