Laser levels - why not just use a ball of string?

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On Fri, 25 Aug 2006 15:43:37 +0100 Mike Barnes wrote :

Ah yes. On a tour we went to see the longest pier in Queensland, used for loading sugar. Constant height above the water and, we were told, the middle is six foot above a straight line joining the ends.
--
Tony Bryer SDA UK \'Software to build on\' http://www.sda.co.uk


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Tony Bryer wrote:

The pier is six miles long? If it's six inches rather than six feet, that would make the pier about one and three-quarter miles long. I found Urangan Pier, which is supposedly 1.4 km, a little under a mile. Is that the one?
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On Fri, 25 Aug 2006 12:21:57 -0400 Harlan Messinger wrote :

No, Lucinda, further north:
"Situated about 100 kilometers north of Townsville, on the northeast coast of Australia, the Port of Lucinda is dedicated to the export of raw sugar from the Ingham sugar-growing district. It comprises on-shore sugar handling and storage facilities, with a single trestle jetty and conveyor running out to an offshore berth and ship loader. The jetty is one of the longest of its type in the world, extending for 5.6 kilometers and dipping 1.2 meters over its length as it follows the curvature of the earth. Sugar takes 22 minutes to travel along the conveyor from the on-shore storage sheds to the ship loader."
So four foot in old money
http://66.102.9.104/search?q Κche:o7ytnJu5jDUJ:www.diveweb.com/uw/archives/arch/042.03.shtml+pier+conveyor+Ingham&hl=en&ct=clnk&cd=1&lr=lang_en&client=opera
--
Tony Bryer SDA UK \'Software to build on\' http://www.sda.co.uk


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Hmmm. I woulda thunk half that, ie r-sqrt(r^2-2.8^2), where r = 12,756.3 km.
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Having spent a frustrating afternoon putting a coat of white paint on an already white ceiling, it occurs to me that any line you could follow would be a bonus. Is that something a laser level could do? The tinted paint that's supposed to show you where you've been is totally useless IMO
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Yes, even one of the cheapies you get at the petrol station as long as it has a prism to throw a line instead of a dot.
--
Skipweasel
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snipped-for-privacy@ece.villanova.edu wrote:

That's the earth's diameter, 2r.
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Oops. I meant:
I woulda thunk half that, ie r-sqrt(r^2-2.8^2), where r = 12,756.3/2 km.
Try it.
Nick
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I'm no math whiz, and I probably have the terms screwed up, but wouldn't you have a hump instead of a dip if it followed the earth?
STeve
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Sure. I guess they mean "The middle is 0.6 m higher than the ends."
Nick
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'nautical'. Sailors refer to a 'dipping' distance. They assume a sight line , perfectly straight, horizontal and _tangential to the earths surface_ at the observation point. The Earth's surface is falling away from this observation point, the jetty, a constant height above the Earth's surface is 'seen' to be falling away ( _dipping_ ), If one held one of those red/white surveyors poles upright (and plumb to the Earth's surface) at the far end, the sight line will intercept the pole 1.2 metres (allegedly) _higher Up_ the pole. The pier is said to have _dipped_ 1.2 metres.
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Brian




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In uk.d-i-y, Brian Sharrock wrote:

Steve's envisaging a pier that's horizontal in the middle. You're envisaging a pier that's horizontal at the observer's end.
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Mike Barnes

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Hmmm .... I'm of the opinion that we are _both_ thinking of a pier that's _horizontal_ ( = tangential to the Earth's Surface and at a constant height above mean-seal-level) at all points on the pier's surface, both at the middle and the end. {Give or take the tolerances available to the builders). Would you care to rethink your 'interesting' theory? would you care to try to determine it's 'horizontal-ness' with a ball of string?
--

Brian




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Seems to me it would be 0.6 meters up...
| - 2.8 km - |
p - o---------------- l d = r - y --------------- e . . . . . . . . . . . . . . . .----------- | = 0.61459 meters / | / | / | / | / | / | | / | | / r | / | y = sqrt(r^2-2.8^2) / / | | / | | / r = 6378.15 km | / / | / | / | / | / | / | / | / | / - |/
Nick
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Note the 'weasel word' (allegedly): I re-used the figure given in the original post. You may be, or not be, correct ... as I'm unlikely to stand on this Antipodean Pier, nor likely to be bamboozled by a Strine Guide (somewhat similar to a Strain Gauge but less accurate) I don't really care how fast it conveys nor how undulating its surface is. I'll await the development of the "during the day the tremendous heat causes the end to expand and touch the Sky'; next episode .... :)
--

Brian



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So the middle's 2' above each end.
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Skipweasel
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It can be locally horizontal - but as you imply it'll be a tangent to the horizon.
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Skipweasel
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== snip

Good lord - take care of that book! A quick scan on Google showed it at £250!!!
Roy
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from snipped-for-privacy@hotmail.com contains these words:

Because string does not self-level, does not reach past obstructions like doorframes, doesn't easily go round inside corners, is fiddly to adjust if you're not sure exactly where you want the level in the first place and droops.
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