Levelling Staff Expansion
Hi,
If a staff expands due to temperature, then the reading will be smaller, therefore the expansion value should be added to the initial reading. Is this correct?
Thanks in advance.
ss
- 25 Replies
> Hi,
>
> If a staff expands due to temperature, then the reading will be smaller, therefore the expansion value should be added to the initial reading. Is this correct?
>
> Thanks in advance.
>
> ssIMHO, Generally the answer is no. Level rods are made of low expansion materials and expansion should not be a factor. Also, since for all level work there is a plus reading and a minus reading any expansion would be cancelled out. One could come up with a scenario of extremes in temperature where the surveyor is using a steel tape for a rod, but that scenario is very improvable.
Thanks for your thoughts Merlin.
I’ve recently done a levelling survey using an aluminum staff. This material has a thermal expansion of 0.23mm per metre per degree in centigrade. It is true what you say about the back and fore sights (approximately) cancelling one another, as this is reflected in my results. I guess staff expansions/contractions due to sudden temperature changes would also be cancelled out.
In any case, am I right when I say that the adjusted values should be “added” to the observed values? I ask this because I sometimes find levelling tricky to visualise.
ss
If colder that normal the rod will have shrunk, thus causing a larger reading so…
subtract the value to get the reading.
What are you building? My guess would be a Swiss watch if you are adjusting 🙂
Now if you have huge differences in elevations and it’s mission critical
that you be as close as possible then by all means adjust.
What are you using for turning points?
Are they more accurate that what the adjustment is?
How plumb was the rod?
Did you rock it or use a bubble?
When was the last time you pegged your level?
Were your turns balanced?Lots of things to consider besides adjusting the rod readings for temp.
> Thanks for your thoughts Merlin.
>
> I’ve recently done a levelling survey using an aluminum staff. This material has a thermal expansion of 0.23mm per metre per degree in centigrade. It is true what you say about the back and fore sights (approximately) cancelling one another, as this is reflected in my results. I guess staff expansions/contractions due to sudden temperature changes would also be cancelled out.
>
> In any case, am I right when I say that the adjusted values should be “added” to the observed values? I ask this because I sometimes find levelling tricky to visualise.
>
> ssAdd the correction taking into account the sign of the correction. To clarify what I mean, sketch out the two possible scenarios. One where the temperature is higher than the calibrated temperature for the “staff” and one where the temperature is lower than the calibrated temperature of the “staff”. Come up with an acronym for the process, or, just sketch it out every time until it becomes part of your everyday thinking.
The only process I can possibly think of where temperature would be a consideration would be in Industrial Machine Leveling and in that environment usually the building temperature is set to a standard temperature for this very reason.
I know there are Industrial Surveyors here on the board who know much more than me on this subject and will hopefully contribute.
Going one step forward, if the temperature is above the standard temperature that the rod was calibrated at and at the calibrated temperature you would be reading 6m, you would be now reading something less than 6M because of expansion and would have to add the correction to your reading. If the temperature was below the standard calibration temperature the rod would contract and your reading would be higher than 6M or a negative correction which you would add to your reading. If you sketch out the two scenarios it will be clear.
Well yes, but keep in mind there is more ‘play’ in the connections of each rod section than there is in the expansion of a rod itself
You might be off a bit, here. 0.23 mm per meter per degree sounded high, so I checked, and I find Aluminum listed as 0.0222 mm per meter per degree. That’s 10 times less than you originally posted. Over a couple of meters (the length of a rod), that’s probably negligible.
>0.23mm per metre per degree in centigrade
Check your decimal place. Isn’t it 23 parts per million per degree C? If I’m awake, that means 0.023 mm per meter per degree.
If you do make the correction, you can wait until your are all done doing the calculation in “staff units” and then apply the correction to the elevation difference between end points to get corrected meters. That should be less confusing than trying to correct each reading.
As an extreme case, if you are working 20 degrees below the standard temperature over an elevation difference of a km, that’s a million mm and so the correction is 23 mm times 20 degrees or 460 mm. But how often do you run levels up a mountain in the cold?
For most applications, you are probably better off studying gravity and refraction than temperature effects.
.> You might be off a bit, here. 0.23 mm per meter per degree sounded high, so I checked, and I find Aluminum listed as 0.0222 mm per meter per degree. That’s 10 times less than you originally posted. Over a couple of meters (the length of a rod), that’s probably negligible.
I agree, the more important issue would be plumbness of the Rod.
Ralph
Levelling Error Distribution
Thanks guys for your input. You’ve put me on the right track now.
I have another question:
When calculating misclose adjustments between stations in a two way level run by taking the ratio of the misclose with the distance, is it the “direct” distance between each station or the “indirect” total distance over each back and fore sight between those stations?
In other words, the direct distance between A and B might be 100m, but the total traversed distance along say 3 back and fore sights between A and B might be 140m.
As an example, if the total direct distance A-B-C=250m, the misclose is 0.015m, the direct distance from A-B=100m, but the indirect distance along my levelling traverse is 120m, do I go 100/250*0.015=0.006m adjustment, or 140/250*0.015=0.008m?
ss
Levelling Error Distribution
In leveling, only the sighting paths count. Existence of a shortcut between stations does not affect the result.
.“I ask this because I sometimes find levelling tricky to visualise.”
Run to the nearest used book store and find any of dozens of surveying books from the inception of the press to mid last century. Even the back page of most field level books has the graphic. If it isn’t in your study materials already, your prof is pretty lazy. Mayby go back to the Survey 101 materials and find it.
A pair of triangles, pencil and paper is all that is required. Cadd can replicate these simple geo diagrams that help you visualize the errors and adjustments. Then add your notes and conclusions. Then, proceed with adjustments as required by the project tolerances.
Levelling Error Distribution
Precise leveling is A LOT MORE than results.
You can find all you will need to know (well, almost all) at your Friendly Neighborhood NGS Web Site.
Radar
I hope everyone has a great day; I know I will!I have found thermal expansion a problem when using level rods 50 feet or longer, it not only changes length over the entire day as temperatures vary, but passing clouds raise havoc with them also.
judFor geodetic applications, the scale on a one-piece aluminum rod is made of invar and used to be calibrated (every half-centimeter graduation) by the National Bureau of Standards (now NIST). Now, it’s no longer done in the U.S. – only in Europe and maybe in Canada.
Couple of thoughts.
1. The expansion is nearly negligible for 99% of all levels. Unless you’re Ralph or Scott or John on here, don’t sweat it.
2. Rock the rod and read the lowest number. It takes the plumb out of the equation.
3. Balance your shots. Then curvature is irrelevant and you can apply a proration to the legs for the balancing of the level loop.
4. If you close a 1000′ of levels more than 0.015 FEET, then re-run it.
The trick here is to know that you don’t know, and know when to scrap it and do it again.
Levels are easy. Peg the level before each run, tap the side or hit the compensator button to make sure it swings freely, and you’ll be fine.
Oh.
5. Read Foote and Davis or some other OLD book on the subject and you will have ALL questions answered.
Can I add a couple of thoughts from my side of the pond.
I’ve monitored a large shopping complex for some 25 years now. It was built on an old power station ash lagoon site and has suffered substantial movements over the years – partly remedied by several massive ground stabilisations costing upwards of $20M.
When the job first started (and was considered a minor problem) I used Wild sectional staves made of aluminium. Two staves were formed (ht. 2metres) by locking two sections together to make them rigid. Levelling used a precise level and parallel plate.
To find stable ground meant coming down from higher ground some distance away and we realised that temperature changes during the day would affect the readings. To overcome this we used clamp on dial thermometers and recorded the temperature at each set up.
Without the temperature corrections misclosures of quite a few millimetres would have occurred. With the corrections the closures were down to 1-2mm. max.
Once the extent of the problem with the site was understood we invested in invar staves. (The overall network involves some 15 kilometres of levelling, spread over several days).
If you have to carry out precise levelling:
1) use invar, rather than temperature correct a normal stave – makes things a lot easier. (unless you are dealing with short runs AND at the same general height.)
2) place permanent change points which are slightly raised – we use stainless steel machine screws (6mm) drilled into convenient concrete surfaces. Rocking the staff may be OK for normal levelling, but the pivot point changes from the front edge where the graduations are to the back edge as you rock and can then introduce an error of its own.
3) keep foresights and backsights equal, use two staffmen AND always read the same staffman first. This eliminates any error due to minor settling of the instrument while you are reading the staves. It’s surprising how quickly a level will settle into hot tarmac.
I trust this helps.
> Can I add a couple of thoughts from my side of the pond.
>
> I’ve monitored a large shopping complex for some 25 years now. It was built on an old power station ash lagoon site and has suffered substantial movements over the years – partly remedied by several massive ground stabilisations costing upwards of $20M.
>
> When the job first started (and was considered a minor problem) I used Wild sectional staves made of aluminium. Two staves were formed (ht. 2metres) by locking two sections together to make them rigid. Levelling used a precise level and parallel plate.
>
> To find stable ground meant coming down from higher ground some distance away and we realised that temperature changes during the day would affect the readings. To overcome this we used clamp on dial thermometers and recorded the temperature at each set up.
>
> Without the temperature corrections misclosures of quite a few millimetres would have occurred. With the corrections the closures were down to 1-2mm. max.
>
> Once the extent of the problem with the site was understood we invested in invar staves. (The overall network involves some 15 kilometres of levelling, spread over several days).
>
> If you have to carry out precise levelling:
>
> 1) use invar, rather than temperature correct a normal stave – makes things a lot easier. (unless you are dealing with short runs AND at the same general height.)
>
> 2) place permanent change points which are slightly raised – we use stainless steel machine screws (6mm) drilled into convenient concrete surfaces. Rocking the staff may be OK for normal levelling, but the pivot point changes from the front edge where the graduations are to the back edge as you rock and can then introduce an error of its own.
>
> 3) keep foresights and backsights equal, use two staffmen AND always read the same staffman first. This eliminates any error due to minor settling of the instrument while you are reading the staves. It’s surprising how quickly a level will settle into hot tarmac.
>
> I trust this helps.:good:
Expansion/contraction is best seen using an elastic band with marks on it so you ‘see’ what is happening when the elastic is stretched (due to heat) and brought back to ‘rest’ (cooled) when loosened to its natural length.
Cheers
Derek
> Hi,
>
> If a staff expands due to temperature, then the reading will be smaller, therefore the expansion value should be added to the initial reading. Is this correct?
>
> Thanks in advance.
>
> ssPretty much, all the posts are right. However, for the student surveyor, the answer to your question is yes….that’s correct. You are envisioning exactly right.
A lot of people get it backwards. (ie: It’s too long therefore you need to subtract…)
You need to picture it correctly (like you did). You will have the same dilemma when thinking about measuring with a tape/chain. If you are measuring between existing points and the chain is too long, you will read a smaller number than “true” and need to correct by adding your factor. If you are setting out a point, you are going to far, because the chain is to long, so you would need to subtract. Think it through before answering. you will get that kind of question on tests, including licensing tests, etc.
Log in to reply.