Checking laser accuracy

[Alan Ambrose]

I'm trying to measure height on various bits of my slab accurately to figure out whether I need to adjust some structural post heights which assume a flat slab. I went round the 8 points needed a couple of times and measured the height with the receiver (new Spectra HL760). I tried four circuits with the rotating laser (a 6-month old Imex) in various positions on the slab. I thought I got the rod accurately vertical at each point. I went round 4 times and the results were a bit nonsense:

 

 

That is, the height of a post base seems sometimes 20mm different when measured again. The slab is ~12mx6m i.e. not huge.

 

I thought I would check the accuracy of the set-up today by leaving the receiver and laser in the same positions and just rotated the laser on the tripod carefully through 45 degree intervals. I got rained off before finishing, but I was getting readings that varied only a couple of mm. I did notice that the wind made a difference and the tripod (the Imex one) is a bit lightweight.

 

Any ideas or tips on how to ensure laser accuracy?

[nod]

Put it on the floor 3 m from a wall Strike a line on the wall

Turn the laser round and see if the lines line up 

[Alan Ambrose]

Ah thanks, I should have said that this is an outside site laser with only a horizontal sweep.

[markc]

Quick check of auto level accuracy.

set in middle of room, mark laser position on opposite walls.

Turn laser around 180 degrees and points (or difference in points) should be the same.

laser receivers are not that accurate, when checking floors it’s easier to set laser low and use a rule or tape to check height

Edited Wednesday at 12:46 by markc

[MikeSharp01]

Take the laser off the tripod and put it on the slab, use some thin packers to get it level - that stops any wavering from the tripod and minimises the rod verticality issue. You can then use a very short rod on the receiver or maybe the laser will scan roughly the middle when its standing on the ground.

[nod]

1 hour ago, Alan Ambrose said:

Ah thanks, I should have said that this is an outside site laser with only a horizontal sweep.

Exactly same 

we check ours this way each week 

[Russell griffiths]

Do you need to be accurate with your posts

you normally pack them up with steel packers and then grout underneath them. 

[Alan Ambrose]

OK understand, let me try those ideas and report back.

 

>>> Do you need to be accurate with your posts you normally pack them up with steel packers and then grout underneath them.

 

Yeah. that's exactly what I'm at - but either way I need to get them all level - which means measuring that they're level.

Edited Wednesday at 15:17 by Alan Ambrose

[SteamyTea]

Hose pipe, couple of clear pipes on each end, coloured water and some small stands/clamps.

 

When I did my surveyors course, the 'levellers' where flipped over 3 times, then the measurements were corrected.

Do a search on Bowditch Correction.

Explains it better than I can.

[saveasteading]

Instrument check.

It is basically a spirit level with a torch on the end.

if the laser is not projecting level then it describes a circle below or above the correct line.

The error increases with distance.

To check it, set it up and use it to measure the height difference between 2 points at equal distance from the machine, a distance of say 20m.

then move.

Move the machine close to one of those points and still distant from the other. Measure the height difference between those same points again.

 

The close one has very little distance over which to project its error. The far one has the same potential error as before,

 

If the machine is accurate then the number will be much the same. if it is  out of adjustment then the number will be rather different.

 

[Alan Ambrose]

OK I repeated my rotating the laser on the tripod thing again with the laser and receiver fixed at the same points:

 

 

 

So, the laser seems fairly consistent and close to spec. I don't think the E60 has any adjustment possible. One thing I learnt is that '3mm @ 30m' really means '+-3mm (i.e. 6mm total) at 30m'.

 

Maybe this argues for figuring out the most consistent laser direction and then positioning the laser to keep most of the measurements in that arc.

 

Edited Wednesday at 18:14 by Alan Ambrose

[Spinny]

Laser levels have error factors, have you compared the tolerance of your chosen laser(s) on the product specification with what you are achieving in practice ?

Do you have anything to calibrate them against ?

 

My window/door company came to site twice - once to survey, and once to fit. Insisted that I must have changed my floor levels by 12+mm over 9m, despite me saying with certainty there was no such change. People get fixated with the idea that laser levels are 100% accurate, and that they never go out of calibration, even if rattling around in the back of vans. They made me get the builder in to cut down a timber but for other reasons couldn't fit. So I bought myself a water level for £20 and carefully struck true levels onto some walls - inside and outside. When they came back my levels were shown to be good within 1mm.

 

Over 12m, I'd say use a water level. As good today as it was in ancient times. Along with the plumb bob.

https://youtu.be/29lzibkKeAI?feature=shared

[Gus Potter]

57 minutes ago, saveasteading said:

nstrument check.

It is basically a spirit level with a torch on the end.

if the laser is not projecting level then it describes a circle below or above the correct line.

The error increases with distance.

To check it, set it up and use it to measure the height difference between 2 points at equal distance from the machine, a distance of say 20m.

then move.

Move the machine close to one of those points and still distant from the other. Measure the height difference between those same points again.

 

The close one has very little distance over which to project its error. The far one has the same potential error as before,

 

If the machine is accurate then the number will be much the same. if it is  out of adjustment then the number will be rather different.

Ah this is a variant of the traditional two peg test we use when we get the level out. Normally I set the level up in the middle of two pegs 25m apart and mark the level. Then move the level 25m outside off one of the pegs and remark. This makes the difference easy to see. But for a lazer level it's often not that bright so you need to do it late or early in the day and work over a shorter distance. 

 

Do this a few times each time setting the tripod up again to check if the self leveliing on your lazer level is working ok and not sticky. 

 

For abosulte confidence:

1 hour ago, SteamyTea said:

Hose pipe, couple of clear pipes on each end, coloured water and some small stands/clamps.

The water level is fool proof provided you purge all air from the pipe. Sometimes in the past if I turned up and the found was flooded I marked the water levels! Every cloud has a silver lining. 

 

 

 

[G and J]

You wouldn’t believe how many peeps laughed at my home made water level.  But it literally sees round corners and despite telling peeps our site was sloping by yay much none of them believe me till we were blocking up. 

[G and J]

Anyway on a more practical level always set the laser up facing the same way, e.g. north.  
 

That way when you mop the floor it’ll all run down one end and be easier to clean up.   😉

[SteamyTea]

30 minutes ago, Gus Potter said:

Sometimes in the past if I turned up and the found was flooded I marked the water levels

@Pocster

[Gus Potter]

31 minutes ago, G and J said:

You wouldn’t believe how many peeps laughed at my home made water level.  But it literally sees round corners and despite telling peeps our site was sloping by yay much none of them believe me till we were blocking up. 

Lot's do but I bet your house is level... and you have had the last laugh!

 

You can check a water level the same way as the two peg test.

 

Set it up by pinning it to your datum where you want to start.. go round the corners (laser levels need a line of sight unless you are good at transferring the datum, encourages error) and mark the bottom of the miniscus always. 

 

Then un pin and start from where you ended. Mark your starting miniscus, could be 200mm below / above does not matter and go back the other way. If you have no air in the pipe you will get a 200 mm difference as you retrace your steps. 

 

The great thing about this is that you can plug the ends and use it as often as you like. 

[SteamyTea]

The curvature of the Earth should be taken into account.

https://jerrymahun.com/index.php/home/open-access/31-i-basic/364-chapter-e-systematic-errors-2

(sorry about the silly old units)

But at 500' (152.4 m) the difference is 0.006' (0.00183 m).  So a couple of millimetres.

 

Now at 304.8 m, the error is 0.00732 m as it is not a linear error.  Nearly 10 mm.

 

Light refraction needs to be taken into account as well.

 

 

 

 

Edited Wednesday at 20:13 by SteamyTea

[Gus Potter]

1 hour ago, SteamyTea said:

Light refraction needs to be taken into account as well

I was talking to a Client about this the other day about the difference between a geodetic survey and a plan survey that we might do on a small site. 

 

But @steamy love your post as for high accuracy we need to decide if this is material. Case in point could be if we want to monitor settlement in a sensitive location.

 

For all if you get a professional surveyor in with modern equipment there is a button you press that does all this complicated maths in the software with a bit of manual input on the curent weather conditions. 

 

 

[Spinny]

When it comes to measurement, 'there ain't 'alf been some clever bastards'...

 

Part 1...

https://youtu.be/YdOXS_9_P4U?feature=shared

 

Part 2...

https://youtu.be/hFMaT9oRbs4?feature=shared

[saveasteading]

3 hours ago, SteamyTea said:

The curvature of the Earth should be taken into account

One of my early jobs was setting the line of the A9 using theodolites for line, and optical levels for gradient.  Over 14 miles the curvature is significant, but not as much as the hills, woodland and midges.

[saveasteading]

5 hours ago, G and J said:

literally sees round corners

Yes, the only way I could survey beneath a working pharmaceutical factory with 4ft undercroft an pillars regularly. Yeuch.... all sorts of smelly stuff dripping through.

 

I have a high tech equivalent, a long tube connected to a pressure meter, and giving height difference digitally. Great for working alone when nobody will trip or drive on it, and subtly checking the day's work.

Edited Thursday at 00:12 by saveasteading

[saveasteading]

The main error I have observed is a groundworker holds the staff until the receiver beeps, then as he bends to mark the base level, let's it all move. 

And they get fling about hence very approximate.

They should need compulsory training before buying one.

 

 

[Pocster]

A load of guys trying to do accurate measurements. None of you know what 6 inches is !

[Tom]

Stop dicking around with lasers and get a Ziplevel! https://www.groundscare-products.co.uk/ziplevel.html