Greenwich Meridian Laser - Can it Demonstrate Curvature?

Trailblazer

Moderator
Staff member
All of this laser talk makes me think of the laser beam that shines along the Greenwich Meridian in London. It is very powerful and visible from over 30 miles away under good conditions. I would have thought some measurements of the altitude of this beam would be useful.







I'm not sure if or how the beam is levelled, but it should be possible to find out.
 
Last edited:

Trailblazer

Moderator
Staff member
I have been doing a bit of research and it seems the Greenwich beam now has a 0.4-degree tilt, after it had to be adjusted upwards in 2014 because of a new building in its path.

upload_2016-8-23_12-28-46.png
 

Trailblazer

Moderator
Staff member
Just some rough data to work with: the ground elevation of Greenwich Observatory is 46 metres above sea level (AOD = above Ordnance datum).

From these photos I would estimate the laser emerges about 6 metres above ground level.




So the laser emerges at about 52 metres above sea level (AOD).

Stratford Plaza (the building that the laser struck) has a roof height of 98.25 metres AOD:

upload_2016-8-23_15-42-20.png

And is 4.29 miles from the observatory:

upload_2016-8-23_15-44-27.png
 

Mick West

Administrator
Staff member
The problem is you need to measure it against some level surface. It does go over the Thames for about 1.3 miles, but I don't see how you could measure it.

The drop over the water would be about 28 inches. i.e. the beam would be 28" higher on the north side than on the south side, if it were actually level.

You would need to take multiple measurements. There are enough spots where it crosses the shore to do this. But how to measure the height.

Also, the river is tidal.
 

Trailblazer

Moderator
Staff member
So, from the measurements above, the roof of Stratford Plaza is about 46.25 metres higher than the start of the laser.

Which means that on a flat Earth the required angle to clear the building would be arctan (46.25/6900), which is 0.384 degrees.

And they raised it to 0.4 degrees.

Oh well, maybe the Earth is flat after all ;)

Seriously, though, I was thinking of whether it would be possible to measure the height from a longer distance. Over the distance from Greenwich to Stratford, the drop due to curvature is only 16 feet.

For instance, the beam is visible from Pole Hill, which is 11 miles away from Greenwich. Over that distance the drop would be over 80 feet. In other words the beam should be 80 feet higher than it would be on a flat Earth. How could you measure the height, though? Could a drone be rigged to do it?
 
Last edited:

Rory

Senior Member
On a flat Earth the required angle to clear the building would be arctan (46.25/6900), which is 0.384 degrees.

And they raised it to 0.4 degrees.

Oh well, maybe the Earth is flat after all ;)
What angle would be required on a spherical earth?
 

Trailblazer

Moderator
Staff member
But how was the ground measured?
The ground is measured above sea level. In the UK, that is specifically above the mean sea level at Newlyn, in Cornwall, and levelled across the country by manual survey. Sea level should be sea level, regardless of the shape of the Earth.

What angle would be required on a spherical earth?
As I noted above, the drop over the 6.9km from Greenwich to Stratford is only about 16 feet (5 metres), so the difference is pretty negligible compared to the 46.25 metre height difference.

You could take the 5 metres off the building height, and arrive at an angle of arctan (41.25/6900) = 0.343 degrees, so only about 0.04 degrees less.
 

Rory

Senior Member
Info on the beam:
Photo of the beam 'hitting' the Elektron tower in Blackwall:



What's interesting for me about this picture, in relation to the Balaton experiment, is the amount of divergence of the laser. As the article above says, "the centre of the beam is a few inches from the building."

That's over a distance, from source to tower, of approximately 3542 metres.

(GPS coordinates for origin of laser: 51.477859, -0.001483; and that corner of the tower: 51.509734, -0.001481)

Also, the tower is 76.3 metres to the roof, with a floor-floor height of 2.9m (source).

Don't know if that adds...
 

Mick West

Administrator
Staff member
The ground is measured above sea level. In the UK, that is specifically above the mean sea level at Newlyn, in Cornwall, and levelled across the country by manual survey. Sea level should be sea level, regardless of the shape of the Earth.
Sure, I was just thinking of it from the perspective of a Flat Earther who thinks the surveys have been faked to pretend the world is round. Of course that makes no sense at all, however, a key point of these experiments is for the suspicious person to be able to verify for themselves that the earth is round - without relying on government paid surveyors. That's essentially what Sandor is doing (rather pointlessly, as there's many other ways of verify the spherical nature of Earth)

That said, it would be an interesting experiment either way.
 

Whitebeard

Senior Member
What's interesting for me about this picture, in relation to the Balaton experiment, is the amount of divergence of the laser. As the article above says, "the centre of the beam is a few inches from the building."
Laser bloom, caused by the beam deflecting off particles in the atmosphere, is an issue if you require real pin point accuracy, especially over a distance of more than a couple of hundred yards. You need particles to see the beam clearly (hence why bands etc use haze or smoke machines when the use lasers), but the same particles scatter the beam and increase its 'size' when it hits an object.
 

Rory

Senior Member
PS When searching for the elevation of the observatory using the GPS coordinates I actually most commonly found about 40m, rather than 46m, even though that seems to be the 'official' figure.

I don't know if this explains it:
 

Trailblazer

Moderator
Staff member
PS When searching for the elevation of the observatory using the GPS coordinates I actually most commonly found about 40m, rather than 46m, even though that seems to be the 'official' figure.
The large-scale Ordnance Survey maps suggest 46m is accurate. The observatory is above the 45m contour line.

upload_2016-8-23_18-6-9.png


There's also a plaque on the wall with a height of 154.7ft (47.15m) above Ordnance Datum marked:



The site is on quite a steep hill, so "ground level" is not really a fixed quantity. This picture shows the plaque in context: note the courtyard behind the wall, in front of the observatory itself, appears to be more or less level with it.



So in fact the ground level would appear to be more like 47 metres.


Edit: actually, Street View shows it more clearly:

upload_2016-8-23_18-14-21.png

From photos, the laser appears to be roughly 73 brick courses above ground level. A standard UK brick plus mortar joint is 75mm (four courses to the foot), so that's about 5.5 metres above the ground. The arrow on the plaque is 5 or 6 courses above the ground, or about 40cm, giving a ground height of about 46.75m and a laser height of roughly 52.25m above sea level.
 
Last edited:

Rory

Senior Member
Yes, rather than question the Greenwich figures it made me doubt the accuracy of using GPS coordinates on google maps to determine elevations.

Though I guess the plaques and such at Greenwich have been there a pretty long time - perhaps before more accurate ways of measuring these things were available.

(The height of Everest, for example, has been fine-tuned by 4-8 metres since it was first 'measured' in 1856.)

In any case, I guess both of them can't be right. Maybe the key is in using the same system for all measurements in a given set of calculations.
 

Trailblazer

Moderator
Staff member
Yes, rather than question the Greenwich figures it made me doubt the accuracy of using GPS coordinates on google maps to determine elevations.
It's not so much a question of accuracy as of definition. (Can you guess I'm a bit of a map geek? ;))

Actually this is related to the shape of the Earth more than you might imagine. GPS uses a model of the Earth, which is an ellipsoid. There is then a correction to this basic ellipsoid, which takes into account local variations of sea level - this is called the geoid, and it varies from the ellipsoid by a few tens of metres up and down around the world. GPS altitudes are given relative to this geoid, but the geoid is still only an approximate model:



And even then, the height above the GPS geoid is not necessarily the same as the height above sea level as defined by the Ordnance Survey, which makes the maps, and the difference varies across the UK. Many modern GPS units have a built-in look-up table that applies a correction, based on your current position, so that the height reading agrees with the map (to within GPS accuracy limits).

On top of that, the altitudes used by mapping programs are often the free, fairly low-resolution data set which was done by radar from the Space Shuttle. I think Google Earth uses better data than that, but it can still miss local variations on small, steep hills.
 
Last edited:

Rory

Senior Member
Interestingly, clicking around in Google Maps for elevation data around Newlyn itself reports places that are in the sea as being up to 6 feet above sea level!

Screen Shot 2016-08-24 at 3.01.04 AM.png

We always knew the OS were the world's greatest mapmakers. ;)
 

Z.W. Wolf

Senior Member
Laser bloom, caused by the beam deflecting off particles in the atmosphere, is an issue if you require real pin point accuracy, especially over a distance of more than a couple of hundred yards. You need particles to see the beam clearly (hence why bands etc use haze or smoke machines when the use lasers), but the same particles scatter the beam and increase its 'size' when it hits an object.
A Google search of laser bloom always redirects to thermal blooming, which is not at all what you're describing. Is there another name for it?

What you are describing would seem to be a diffraction effect; different from and addition to the refraction caused by atmospheric turbulence I was warning Sandor about.
 
Last edited:

Z.W. Wolf

Senior Member
Info on the beam:
Photo of the beam 'hitting' the Elektron tower in Blackwall:



What's interesting for me about this picture, in relation to the Balaton experiment, is the amount of divergence of the laser. As the article above says, "the centre of the beam is a few inches from the building."

That's over a distance, from source to tower, of approximately 3542 metres.

(GPS coordinates for origin of laser: 51.477859, -0.001483; and that corner of the tower: 51.509734, -0.001481)

Also, the tower is 76.3 metres to the roof, with a floor-floor height of 2.9m (source).

Don't know if that adds...

You're right that this can teach us something right away about the Balaton experiment. At 2.2 miles, not only is there considerable beam divergence, it is also irregular, (and, I'm assuming, ever changing).






Turn the photo on its side and we can imagine Sandor's target.







We can already see that the building or target is not big enough to intercept all of the laser light.

If we chop off even more...






... we are still getting a hit on the target. And a potential "Direct Hit on the Camera!"




Sandor must increase the size of his target.
 
Last edited:

Whitebeard

Senior Member
A Google search of laser bloom always redirects to thermal blooming, which is not at all what you're describing. Is there another name for it?

What you are describing would seem to be a diffraction effect; different from and addition to the refraction caused by atmospheric turbulence I was warning Sandor about.
dunno, my only real experience with lasers is firing them around music venues. But I do know that the longer you shoot a standard laser beam the thicker and less intense a beam gets. If a beam is say 2mm across when leaves the projector, 100 yards away it projects a spot nearly 1cm across. I always understood this due to the beam being 'scattered' as it passes through the atmosphere and 'deflects' off water droplets, dust and other particles (like smoke). The longer the distance the wider and weaker the beam becomes. I dunno the exact terms, my thing is history and politics, I'm not that up on the science stuff.
 

Related Articles

Top