Methods for setting up & using site datums?

[Alan Ambrose]

Has anyone got some good methods for setting up and using site datums? Both FFL planes and also, if possible positions.

 

Sure you want your site marked out by some experienced guys with a total station. But what will you ask them for? What kind of physical datums do you use? How do you protect the datums from being dislodged? How do you cope with general ground movement or do you ignore it? Given your plot won't be perfectly flat, how do you decide where your official ground level is relative to some absolute level ('ordnance datum'). How accurate in practice are total stations anyway?

 

When your total station guys are not around how do check that something is in the right position? Do many people use their own station for convenience & accuracy anyway?

 

etc etc etc. All tips gratefully received.

[markc]

1) you don’t need a total station unless you are setting out / checking specific positions on complex geometry - which can still be done with distance and plumb etc. but takes far too long.

2) for a new build house etc. FFL will be dictated (usually) by the surrounding ground level, or a specific point that the roof level cannot go over etc.

For a datum, a solid fixed point that will not move, be dug up etc. a point in a road, pavement or concrete slab etc. is usual and you then give this a figure (can be Above Sea Level but not necessary unless there are multiple sites and locations working together, roads, bridges, shopping centres etc.)

Then use a dumpy level or water level from this point.

3) plan setting out is straightforward using tapes / large square / diagonal checking etc. - loads of YouTube stuff showing how to do it.

[Russell griffiths]

Get all critical measurements added to your drawings. 
so get a foundation drawing, not a drawing of the house with the foundations on it, but a dedicated layout drawing just for foundations. 
then a set out drawing for brickwork on top of that foundation, you don’t want the location of the kitchen sink on it, that’s not relevant, you want all the different aspects that are on a set of plans pulled off into individual elements. 
all these drawings go to the surveyor and they will plot all the points into their total station. 
 

when they come to site for the first time to do an initial site set out get them to put finished floor height, damp cours height and anything else relevant on a timber post that you have fixed somewhere, preferably out of the way of the digger, probably screwed to a fence or something. 
 

after the site scrape you can measure down from those marks very roughly to get the bottom of your trenches or whatever foundation you are doing. 
remember this stage heights are very rough, accuracy is not needed in just preparing trenches. 
from the ffl mark you can work out how many courses of brick or block you have below this and this will give you top of concrete height. 
 

it’s all fairly simple measurements depending on what you are building. 
 

I had the surveyor out 4 times I believe 

1. Rough house layout 

then I scraped it down to depth. 
 

2 piling locations 

then the piling company came in

then I installed the ringbeam. 

3 wall position on top of ringbeam. 
from here up it’s all set in stone, your corners are marked, that’s where it’s going, it’s just a case of adding the layers you have on your drawing untill your up to ffl. 
4 drainage points. Where added as we brought the blockwork up to work out position of cut outs in the block walls. 
 

a slab is slightly different, but the same information is needed. 

[saveasteading]

It is perfectly possible with a  long measuring tape , or preferably two. You will need them anyway. 

I use branded ones, eg Stanley, and if you get them on an open reel then the mud washes off and they are easily untangled.

Non-branded  ( or made up names ) can be very approximate. I had two from tool station that were 100mm out in 30m...I think they were called silverline. I happened to check with a different tape. Could have been expensive.

 

And for levels i recommend an old fashioned site level on a tripod.  BUT groundworkers and brickies will use  a laser why? because it doesn't involve so many sums. They still sometimes get it wrong for other reasons.

 

But that is me.  I've done it all my life and believe that working with these tools gives  an understanding of the process and more control.

 

PLUS I had one job where a professional came in with a total station and got it very badly wrong.

 

BUT I've seen that many highly intelligent people can't handle these tools, as it needs some maths (mostly arithmetic)  skills which are not universal, and working in the weather.

 

Other points to consider.  If someone knocks over a marker peg, do you call the pros out specially?

Where are they taking their datums from?

 

It has to be up to you. who would teach you the setting out techniques?

 

[Oz07]

Datum heights are easily done off surrounding fixed points or a timber stake. 

 

When i set out myself i like to bang 2 3x2 stakes in with a horizontal 2x1 screwed in across the 2 at ffl. Put these behind the corners 4 or 5m out the way. Better still if can be fixed to a boundary marker like fence or wall. Set out on these. You have to work methodically to get in the right place start with your longest most important wall. Come 90 of this for next wall then all starts to get pretty easy. I used to use 345 or trig to get the 90 but my laser now has 90 function. Still like to check diagonals. Bits of tape on the string lines help as a temporary marker for checking sizes

[Canski]

For levels I had a 100 mm board fixed to the fence. I had the 2 FFL plus 1 m marked on there for the 2 houses. I had an engineer come out and mark out the building. While he was there I got him to extend the long line of the brickwork and marked it on the rear fence with a nail at the front I marked it next to the pavement. I then had the rear wall points extended and marked on the fence. I also took a couple of measurements from a fixed point to the corners of the house in a few places just in case. I had a 2 stakes with a nail in it for this and used 2 tapes to get the corner points. again to set out the brickwork corners. Levels were taken with a laser from the FFL points from the fence. Once up to DPC and everything is checked you don't need them again. I was glad I did it as the digger lost a couple of corner points whilst manouvering. 

[saveasteading]

So @Alan Ambrose you are a "general purpose engineer". The numbers kind?

 

Then you can do it.

if you are following the suggestions above then great. If not, get someone to do it.

 

The projected lines to safe positions are vital. The corners will get knocked out or dug out, then have to go back in.

 

Most new houses are built out of square and level. This is becaise the managers and operatves can't use tapes and levels properly.

If you can control that, then all the workmanship is better.

 

I've got a total station. A waste of £10k. But I use tapes and a site level. And string line.

This is because the building is usually a rectangle and i know where it is / goes.

 

Actually I have a sophisticated sort of water level, using oil in the tube, so I can do levelling without someone holding a staff.

Checking stuff discretely after the workers have left avoids pressure and awkwardness.

[G and J]

I like my water level.  It’s cheap and reliable.  
 

@Alan Ambrose is this a general question or are you soon to be setting out some footings?

[saveasteading]

I haven't ever done it but here is my design for efficient use, avoiding bending.

 

 A 2 x2 timber, about 2m long. with a scale on it from an old tape.

Put a ply base plate on one end so it stands up.

Another the same then fit a clear tube linking them and fill with water.

 

Then one post can stand on the datum point while the other is moved around. All points are then are plus or minus from the datum.

No helper needed.

For sophistication, colour the water and have plugs for the end.

 

Is oil better? I don't know.

[G and J]

I imagine oil stabilises more quickly, and is maybe more visible in the plastic tube.

 

Me being a cheapskate, I bought a 3m length of clear plastic tube the right diameter for hozelock fittings and I use it on the end of a long garden hose.  Works a treat.

[saveasteading]

48 minutes ago, G and J said:

 

Me being a cheapskate

Ermmm. You would get flung out of  BH if you wasted money.

Efficient  is the word. Or a skilled  designer. 

[joe90]

I used an indoor laser level at night with the glasses on. Stood the laser on a 1m stand and a pole with a tape measure stuck to it I carried around . At night the laser was strong enough to be seen at quite a distance or with the glasses on looked into it. 

[JohnMo]

A man came round, with a thing connected to satellites, marked it all out. He also said how how above datum we were. Same day trenches were dug. Once tranches filled, he came back and hammered pins into concrete at every corner of the building outline. Easy quick accurate.

[G and J]

Hmmmm, the American military deliberately introduce positional errors into GPS, and clever stuff can go wrong in clever ways that are way beyond me, so I think I’ll stick to me hosepipe, but I’m glad it worked for you.

 

But what were the pins for?

[JohnMo]

15 minutes ago, G and J said:

But what were the pins for

Hard point that wouldn't wash off could be found if soil covered it I suppose.

 

Accuracy is relative. Who cares if your a few millimetres off position, as long all relative measurements are true to each other. You can always throw a tape measure over to check, like I did.

[Conor]

Highly recommend that when you have an initial site survey done, that you have a couple (ideally three) bench marks setout around your site. A nail driven in to a hard surface is best. Allows you to easily check vertical levels in the future with just a laser level and tape. Ironically, me, as a surveyor didn't do this and when setting out made a 250mm error  

[JohnMo]

6 minutes ago, Conor said:

as a surveyor didn't do this and when setting out made a 250mm error

We once bought a house where the builder messed up and the garage was 1m longer than it should have been👍

[saveasteading]

2 hours ago, JohnMo said:

with a thing connected to satellites

I was marking out a site boundary recently. The drawing provided was done by satellite references. I asked how they had decided where the red line on the drawing was in the first place.

Estate agent's sketch on  Google earth superimposed on a site survey.

Where is the datum for the site survey? It's done by satellites. 

But the very real gate is in the wrong place acc to the drawing.

 It's done by satellites.

 

It's like " computer says no". 

Anyway I got the line shifted where it mattered.

 

[Alan Ambrose]

@saveasteading you are a "general purpose engineer". The numbers kind?

 

Is there another kind? I studied first 'Engineering Science' - everything from semiconductor physics to metallurgy to soil mechanics. I still have a hand drawing of our old-fashioned theodolite work in the parks -  not very accurate work if I remember. Enough knowledge about each subject to know which was was up but not enough to do any real work.

 

@G and J are you soon to be setting out some footings?

 

I'm still playing whack-a-mole with the planners, but there's half a chance that we will get permission before xmas. Or, they could string it out for another year. You have to be optimistic, no?

 

>>> Checking stuff discretely after the workers have left avoids pressure and awkwardness.

 

Yes, that's exactly what I was thinking of - spotting things going wrong before they actually go wrong.

 

>>> But the very real gate is in the wrong place acc to the drawing.

 

Nice ☺️.

 

 

I knew the hive BH mind will have figured this out. Thanks.

 

 

 

[saveasteading]

6 hours ago, Alan Ambrose said:

The numbers kind?

 

Is there another kind

The word is not protected. So it annoys me when a van says 'civil engineer' when it is a groundworker.

Plus there are are expert mechanics with real flair who might be called engineers, because they work on engines. 

 

From the excellent advice above you will have seen that the theory is important, but so are the practical aspects.

At the right time we can run through some more.

[SteamyTea]

When I did my surveyors course, most of the work was checking, then double checking, then triple checking.

When measuring levels we had to take 3 readings, which is actually 6 as we had to turn the leveller 'upside down', while someone else moved the measuring staff back and forth.

(I was told that in the olden days, the OS used to take 7 readings, which is 14 really, but where allowed to drop 1, leaving 13).

Once all the readings were taken, we then had to do loads of mathematical corrections to make sure that the levels made sense.

Then we had to go out and do it all again.

 

I actually quite enjoyed it, but we did have 2 weeks of good weather.  If it had been raining, and considering it was at Exeter University's Tremough campus, it would have been miserable.

I did not enjoy my time at Exeter.

[saveasteading]

44 minutes ago, SteamyTea said:

44 minutes ago, SteamyTea said:

When measuring levels we had to take 3 readings, which is actually 6 as we had to turn the leveller 'upside down', while someone else moved the measuring staff back and forth.

 

Yes I did this for real for a couple of years. In my training we were sent round a housing estate loop and were expected to be within a few mm when back to the original datum.

We were taught by mining surveyors.

The same applied to theodolite use, with the added precision of only rotating it one direction to keep the innards perfectly aligned. Motorways were aligned to a few mm or seconds accuracy.

 

Then at the end a man lays a concrete kerb and taps it til it looks right.

[G and J]

26 minutes ago, saveasteading said:

Then at the end a man lays a concrete kerb and taps it til it looks right.

A source of much head shaking over the years.  I plan, I measure, I calculate, I re-check obsessively, then J walks in and shoves it a little to the left so it looks right.  Sigh. 

[SteamyTea]

51 minutes ago, saveasteading said:

We were taught by mining surveyors

Exeter University took over Camborne School of Mines (just seems to be a cupboard of minerals now), but the guy running the course was important on the Channel Tunnel surveying team (think his name was Andy).

51 minutes ago, saveasteading said:

within a few mm

Was 3mm in our case.  Actually quite achievable and repeatable.

51 minutes ago, saveasteading said:

Then at the end a man lays a concrete kerb and taps it til it looks right

After adding extra water with a dash of Fairy.

 

For housebuilding, I would think that some decent matched tape measures, matching staffs, a large protractor, a long hose with clear ends filled with coloured water, a recap of SohCahToa, a laser level and some decent handwriting is all you really need.

AND a fixed know point as a datum.

Edited November 29 by SteamyTea

[G and J]

2 minutes ago, SteamyTea said:

some decent handwriting

Oh dear.  We’re screwed.