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

[Guy King]

The message from Stuart Noble contains these words:

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.

[Ian White]

Position the laser in the open doorway and project a horizontal line, inside and out, at the height where you want to the holes to be. If you position the laser just right, you'll be able to see the line of light running along the drill bit. Use that as a guide to drill the first hole dead level, and then repeat from the other side. The shadow of the door-frame may get in the way, so you may need to re-position the laser to do the other side (make a mark on the door-frame so you can return to the same height).

[RzB]

Good lord - take care of that book! A quick scan on Google showed it at £250!!!

Roy

[John Rumm]

Yup, I have a laser pointer called "Kitty Remote" ;-)

[HLAH]

We have two cats a tiny, bright (for a cat) black one and a rather dim but very loveable fat grey one. The grey cat will run the entire length of the house, sliding to a crashing halt each end chasing that little red dot, until exhaustion, . The black one on the other hand shows absolutely no interest at all. It took me months to figure out that she could actually see the dot but just isn't interested in chasing it unlike all the local wildlife or indeed string.

H

[HLAH]

"Steve B" wrote

Anyone can throw up a

Anyone can screw up with a dodgy spirit level as well.

H

[Italian Mason]

[nicksanspam]

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

[Steve B]

[Steve B]

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

[Al Bundy]

"Mathew Newton" wrote in news: [e-mail]:

Yet Hover Dam, the Golden Gate, Empire State Bldg, etc. still stand.

They must have had laser pencils and GPS's in their lunch boxes though :-)

[Ian White]

I hope your American TV doesn't show 'Grand Designs' - your blood pressure probably couldn't stand it. In last night's repeat, the architects had tried to "help" the self-builders to lay out a large arc of concrete block piers for steelwork... and were out by 2 feet.

[nicksanspam]

Sure. I guess they mean "The middle is 0.6 m higher than the ends."

Nick

[Brian Sharrock]

This is probably a confusion in terminology between 'tourist' and '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.

[Mike Barnes]

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

[nicksanspam]

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

[Brian Sharrock]

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 Sharrock]

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 .... :)