Rotating laser for foundation layout - how does it work?

[shannyla]

More for my own general knowledge than anything else, I've seen rotating lasers on site and could never work out how you used them.

I assume they provide a horizontal datum, and perhaps some distance measuring capability, but how do you set them up in the first place? Do you just choose a random spot as your reference location and measure from there, replacing the laser in the same spot everyday? How about distance? How do you see the beam and measure it?

Guess I should have taken the surveying module at college...

 

[kevindgas]

they work on a tripod with adjusters once you have the level set correctly it just shines a beam then the whole body rotates.

 

[jeds]

Laser provides a horizontal datum. You measure off the datum with a reciever attached to a height rod. This gives you level above or below. Most good ones are self levelling. Set the laser on a tripod, switch on and start levelling. The further you are from the laser head the thicker the beam becomes and accuracy starts to tail off. Better machines work over hundreds of metres but cost a grand or more. The beams are quite visible when used indoors but outside the beams are invisible - which is why you need the receiver.

Rotating lasers don't measure distance. For that you need an EDM. (electronic distance measuring)

 

[shannyla]

So it's the receiver and height rod that does the hard work, giving you how much off you are from the set datum.

Last question, how do you ensure repeatability from day to day if the laser isn't left in situ? I get that the tripod gives you the height adjustment, is it just a case of ensuring that you put the tripod in exactly the same place and at the same height each day? Or is there something in the system that negates any variation in positioning?

I tried a web search for a guide to this and found nothing.

 

[^woody^]

The principle of setting out levels is that you always start with a reference level, or point, and then all subsequent readings are relative to the reference point.

This allows the instrument to be moved, and then the first reading becomes the new reference.

The first point is the backsight, the last is the foresight, and those in between are intermediate readings.

So when setting up the level, a backsight reading is the first to be noted, and then subsequent readings added or deducted from this

 

[shannyla]

That makes perfect sense.

 

[JohnD]

if doing extensions, patios etc, the DPC of the main house is a good reference point.

 

[noseall]

I agree with JohnD, dpc is a useful datum.

As well as dpc level relative to the overall site ground levels, you should also observe dpc level relative to the road level, should this be relevant.

Falls to drains in the road is also important.

 

[ColJack]

you'll often see little metal pins hammered into paths and such, sometimes with a white spray paint ring round them..
these are used for reference points when doing road surfaces etc..

 

[^woody^]

When I was an apprentice on site, one of the mundane jobs given to apprentices would be the building up of manholes and road gullies to a set level.

The site engineer would knock a peg or bar in the ground and put a mark on it. The metal cover then had to be built up to this level.

But, if it was a bit difficult to build up the brickwork (ie it did not work bricks or brick on edge) then we would simply knock down or lift up the peg to suit the cover.

So if, when driving around, the manhole or drain is too low or too high and you feel a bump in the car or stub your toe, then now you know why

 

[stuart45]

When I was an apprentice on site, one of the mundane jobs given to apprentices would be the building up of manholes and road gullies to a set level.

The site engineer would knock a peg or bar in the ground and put a mark on it. The metal cover then had to be built up to this level.

But, if it was a bit difficult to build up the brickwork (ie it did not work bricks or brick on edge) then we would simply knock down or lift up the peg to suit the cover.

So if, when driving around, the manhole or drain is too low or too high and you feel a bump in the car or stub your toe, then now you know why

I've known a few bricklayers do that with the DPC datum pegs when building the footings because they couldn't be bothered to gauge up.

 

[jeds]

The principle of setting out levels is that you always start with a reference level, or point, and then all subsequent readings are relative to the reference point.

This allows the instrument to be moved, and then the first reading becomes the new reference.

The first point is the backsight, the last is the foresight, and those in between are intermediate readings.

So when setting up the level, a backsight reading is the first to be noted, and then subsequent readings added or deducted from this

And to complete a survey with the highest confidence you should close the survey by ending up at the starting point. So, over a large site, you would start at a reference point, foresight and backsight along a number of features and finally end up with the last one at your start point. When you then add up the sum of the level changes noted in your notebook they should come to zero. Unless, that is, you have a certain bricklayer from the Midlands on your site.

 

[^woody^]

Don't forget collimation error though, if doing it with an old skool instrument