Dumpy/site levels

[ToughButterCup]

On 6/26/2016 at 17:42, JSHarris said:

Sorry, missed this.

[...]

For site setting out then I suggest that hiring kit to set a few datum points, as already suggested, then using a reasonable (Class 2 accuracy) laser level with staff and receiver, together with a decent surveying tape, will be fine.

Thanks very much indeed. Deep into it all now: it's an extremely interesting subject. I am particularly grateful for the reading on Error Correction stuff (Least Squares method) Bit too high-brow for me, but no less interesting for all that. 

I have decided to use an optical level, and I found this brilliant set of videos that takes beginners like me from how to read a staff to being able to check the readings

I think I'm going to begin by doing all the readings and noting the results by 'steam'; maybe then see if I can find a simple Excel Spreadsheet that systematises - and checks for errors.

Never a dull moment this house-building lark. 

[SteamyTea]

Sum of Least Squares is used all the time in mathematical modelling.  It works well with Solver in Excel.  You get Solver to change 'something' in your equation, then it checks the Sum of Least Squares result, if it is heading in the right direction, usually to a value of zero, or close to it, then it repeats the change until it can go no further.  That is the point where the differences between all the results are the minimum, or least.

The reason that the difference is squared is to make all the numbers positive, as any negative number squared is a positive (unless you want to explore imaginary numbers that is).

So in effect you end up with two sets of numbers for your model, one the observed, and one the predicted.  You take one from the other, it does not matter which way around, and then square the answer.

(Observed - Predicted)²

You do this for all the the groups of numbers, then add up (sum) the answers.  The closer that number is to zero the better.

[ToughButterCup]

Ah! Excellent. I was hoping to get a reply from you Steamy. Thanks. Ian

[TerryE]

I had a look through those videos.  Ignoring the wonderful US centifeet units, it was useful 101 stuff if you haven't done this before.  The Excel spreadsheet is fairly straight forward as the most complex bit is adding / subtracting one column from another. 

The thing that I would re-emphasise is: don't assume that your Dumpy is properly calibrated: check!  So:

1. Having exactly centred the levelling bubble at one orientation, it should remain exactly centred as you turn your level through 4×90° allowing the bubble to settle each time.
2. Set up two reference points at a minimum of 40m apart.  Setup your level next to one and use the precision side of your staff to get the relative heights as close to nearest mm as you can.  Now move your dumpy to the other end and repeat.  If the two measurements are more than a few mm out then your optic level is out from the swivel plane.

If either of these are off, then your manual will explain how to recalibrate the level.  Any surveying that you do can only be as accurate as the calibration of your Dumpy.

[SteamyTea]

If you have a look at the road near your house, you may well find a surveying stud, worth using that as your reference point.

The leveller that we used during our surveying course was not self calibrating, so we had to 'turn it over', which meant we had to do 6 measurements, then we applied the Bowditch correction to the results.  All this does is assume that the precision is equal but the accuracy is fine, or it may be the other way around.  It is the same as using anomalies rather than absolute values really.  So it gets rid of any systematic errors.

 

https://books.google.co.uk/books?id=5d1B0Vy52dQC&pg=PA132&lpg=PA132&dq=Bowditch+levelling+correction+surveying&source=bl&ots=ULFO8hFti5&sig=g12JvxLNcaJiAgCbbSXkfc3iu9s&hl=en&sa=X&redir_esc=y#v=onepage&q=Bowditch levelling correction surveying&f=false

[Jeremy Harris]

1 hour ago, SteamyTea said:

If you have a look at the road near your house, you may well find a surveying stud, worth using that as your reference point.

 

Indeed it is.  We were fortunate to find a stud in the lane right alongside where our drive entrance was to be.  This was marked as a spot height on the OS vector map I purchased, so became the datum for everything we laid out.  Because it was low down we couldn't use it for day-to-day use, but we created a new datum from it in the top corner of the plot, where it couldn't easily be disturbed.

The big advantage of the OS datum pin was that we could always cross check any level on site in a minute or two from it, after we'd excavated down nearly 3 metres.  This came in handy when we had the planners around to check the ridge height (someone complained) as we'd already checked and knew we were 90mm lower than the limit and I was able to just show the council chap the pin and the OS spot height and he checked off the ridge height in a few minutes, with no arguments.