Reference Instruments - DIY Laser Level.

[mcbrien410]

I contributed the U - Tube to the DIY Laser Level thread as this is the only rudimentary reference that I can think of for getting something level.

In the vertical axis we have the "Plumb Line". This is what I call a reference for getting something to stand or hang vertically.

I have asked other people what they consider is a horizontal refrence instrument and they all say the "Spirit Level". This I disagree with as it can be adjusted or altered. This is why I say the "U - Tube is the reference for horizontals as there is no adjustment, you just wait for the liquid to settle.

What did the Egyptians, Romans et al use for horizontals. They could have constructed a form of U-Tube but not a moveable/flexible one.

OK Pundits, what do you rekon?

Chris

[Malcolm Stewart]

You really should have asked this in a surveying group. The beauty of a spirit level, at least a decent one as fitted to a theodolite, is that it's possible to make the base of the theodolite (very close to true) level by turning it through 180degrees, and taking out about

1/2 the error. Repeat after turning through 180 degrees. Repeat at 90degrees etc. In a way, surveying as practised up to about 15-20 years ago was all about "managing" the inevitable errors in the instruments as a result of manufacturing and centring erros. So most readings would be repeated 180 degrees from the first, and another complete set taken with the telescope turned over. Careful work reduced erros to around 1" of arc, or better for map making. Modern instruments use electronics, and some of the "problems" are hidden.

One of my theodolites, a Wild T2, has simple, but sensitive vials, for all axes. The other, a Kern DKM2-A, uses reflection from the surface of liquid for one of its references, so you're effectively back to the plumbline. With care, both can measure to 1" repeatedly, and placed side by side, agree to a remarkable degree as to where the horizon is. (Whilst lurking on the previous thread, I compared my surveyor's level against my cheap laser level. No contest.)

[Andy Dingley]

Go read L Sprague du Camp's "The Ancient Engineers"., or a few other similar books.

Generally a long stone beam with an open "U tube" carved into the surface, carried around and set up with water where needed.This level was then magnified optically by sighting onto wooden staffs (boning rods)

[Peter Taylor]

Putting on my anorak for a second, given that we live on the surface of a sphere, there is no definite answer to this. Over 1km distance, the water level of a lake will have curved 10mm lower than a straight sighting looking out into space. In a similar way, because a plumb bob always points to the centre of the earth, two readings any distance apart will not be truly parallel.

[Andy Dingley]

Gravitational isopotential surface, as a rough approximation (ignoring local geological variations).

[mcbrien410]

Those of you with 1km. window sills should watch out!!

Seriously though thanks for the answers. I usually do do that with a spirit level, turn it through 180 deg and see if the difference is the same.

The towers of the Forth Road Bridge are different distances apart from base to tip, can't remember what it is but it can be calculated. Also driving across the Nullabor plain can be enlightning.

Chris.

[Rob Morley]

You just turn it the other way and check if it's still level.

[Andy Dingley]

I used to have the world's most complex tide gauge sharing my office windowsill (back when I was laser student in Hull, with a desk on the rooftop). An interferometer measured the distance to a retroreflector mounted on the top of the Humber Bridge Northern tower. As the tide moved, you could measure the bridge shifting.

[Peter Taylor]

Yes, you have a valid point there. But the local geological variations do play an important part. Don't forget here in GB we're on the other side of the earth from the bulk of its ferrous mass. Maybe in the middle of the Gobi desert your point would be more relevant. Not forgetting also that the isopotentiality is largely cancelled out by the refraction of gravitational pull caused by the Doppler effect of the Moon in its third phase, I feel maybe your concern takes on rather insignificant proportions. However, put up a theory and there'll usually be someone who wants drive a hole through it, so, as always, I stand to be corrected.

I am now closing this topic and returning to my main occupation of caring for my antique raingauge collection.

Peter ;)

[Chris Bacon]

What?? The earth is by composition about 1/3 iron. Do you say that minor (in the great scheme of things) concentrations of this element in extractable form have a serious influence on the composition of one hemisphere? If no, what do you say?

[Dave]

Can you please expand on this doppler effect? I would think that it occurred on all aspects of gravitational pull, regardless of the moon's phases.

Dave

[Peter Taylor]

Yes by all means. For certain people, and under specific conditions during the third phase of the moon, the Doppler effect causes them to turn their backs and wait while someone uses a key to increase the torsion of their life-force.

[Dave]

Very nicly put :-)

Thanks Dave