Accurate Linear Measurements

How accurate can your linear measurements be?

Why do we need accurate measurement?

Whenever we take a measurement, we want it to be as accurate as possible. We need the measurement to be right. But we know that any measurement that we take will have an error associated with it. If we do not have accurate measurements, then whatever we are building or constructing will not fit together. Inaccurate measurement means that the piece that connects two items together maybe either too big or too small.

Hence why we have tolerances for our measurements. Tolerances ensure that whatever we are building or constructing fits together just right. The finer the tolerance, the more we have to eradicate errors from our measurements. The more accurate the measurement needs to be, the longer it is going to take to eradicate the errors from our measurements.

For information on measurement errors in construction setting out, have a look at the article on this website on how to avoid the 3 measurement errors in construction setting out.

How do we know what accuracy we need for our measurements?

The desired accuracy of the measurement is dependent on what we are building or constructing and the size of the building or construction. No matter what it is that you are building or constructing, the accuracy of the work will be determined. The accuracy required in the completed work, or stages of the work, will be determined by the specification. Any work that is carried out should be specified by the client.

That doesn’t mean that we don’t have to pay attention to our measurement accuracy until the end of the works. Quite often greater accuracy may be necessary in parts of the work in order to achieve this. This is often the case when we are working with different types of materials and manufacturing processes. This is often true when items are made or cast off site.

What is accuracy and precision in surveying?

Accuracy refers to how close the measurement is to the true value. In this case how close is the Easting, Northing and Elevation to the true value.

Precision refers to how close repeated measurements are to each other. In this case how close together are the measured Eastings, Northings and Elevations.

There is no doubt that measurement mistakes in construction can be very costly. But we also need to bear in mind that the actual act of doing these measurements can also be costly, so therefore we do need a balance. This is the balance between getting the measurement perfect and good enough.

For this reason, we have in every industry a standard of accuracy that competent operators can achieve with certain types of equipment. An indication of the accuracies obtainable by trained operators following good practice is given below.

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For a more information on the needs for accuracy, methods of achieving it, error avoidance and correction you need to refer to the ICE guide to the management of setting out in construction. The management of setting out in construction publication is also available here.

What accuracy can you achieve with steel tapes?

Getting accurate results from taping can be more difficult than most people realise. The accuracy that can be achieved is so dependent on how the tape is used and the corrections that need to be applied. The corrections due to tape sag, ambient temperature, tension, and the slope need to be applied. This is all dependent on the distances being measured.

With a properly tensioned steel tape, and temperature factored in, then at a distance of 30m you should be within 6mm of the true distance. If measuring along a slope and correcting for the slope then you should only expect to be within 25mm of the true distance.

What accuracy can you achieve with a laser tape measure?

You can achieve a much greater accuracy with the laser tape measure than you ever will with a steel tape measure. With a laser tape measure, a 50m measurement should be accurate to within 3mm. The main advantage of using a laser tape measure is that you do not have to account for tape sag or tape tension in your measurement. Therefore, the measurement has a greater accuracy.

When using either a steel tape or laser tape measure the longer the linear measurement, the less accurate the measurement will be. The accuracy you can achieve is inversely proportional to the distance measured.

If we need a greater accuracy for our longer measurements, we need to use different equipment. Equipment that help us make accurate measurements. Obviously, these instruments will have to be carefully made, adjusted, and checked often to ensure the quoted accuracy. This is when you need to have the instruments calibrated.

What accuracy can you get with a spirit level?

A spirit level can provide accurate levels over a short distance. The spirit level I have quotes an accuracy of 0.5mm per metre. The length of this spirit level is 1m. Spirit level can really only be accurate over its own length. Trying to take level measurements over a greater distance will lead to a greater inaccuracy. The best accuracy achievable with this spirit level would be 5 millimetres over a distance of 10 metres. And this doesn’t account for any incremental errors built up over that 10 metres. To be able to obtain levels of a greater accuracy over a greater distance, we need a different instrument.

Spirit Level Accuracy
The quoted accuracy of the Spirit Level I use.

What accuracy can you get with an Automatic Level or a Dumpy Level?

Being able to take accurate level measurements over long distances needs an automatic level. Most people still call automatic levels, dumpy levels. With automatic levels, or dumpy levels, it is entirely possible to get an accuracy of 2 millimetres doing a single sight of up to 60 metres. This accuracy though is only available from the very best automatic levels. And being used by trained and competent levelling team. Using level using automatic levels that are under 200 pounds, you could only expect to have an accuracy of 5 millimetres per single sight over that 60 metres.

Accurate Auto Dumpy Level
Automatic Level. Topcon AT-B3

For a review of the best automatic levels under £200 have a look at this article here on LSSLTD.NET

What accuracy can you get with a rotating laser level?

Rotating Laser Levels are now ubiquitous on most construction sites. But what level measurement accuracy can be achieved with a rotating laser level?

The level accuracy that can be achieved with a rotating laser level depends on whether we are using visible light or invisible light, for which we need to use a detector.

Rotating Laser Levels can be very Accurate
Rotating Laser Levels can be very Accurate

Rotating Laser Level visible light accuracy. The best accuracy we could hope for a rotating laser level using visible light would be 7mm at a distance of 100m. This would be in a single sight.

Rotating Laser Level invisible light accuracy. The best accuracy that we could hope to achieve with a rotating laser level and using a detector on the finest setting would be 5mm over a distance of 100m.

The above accuracies can only be achieved with the very best of the rotating laser levels available on the market today.

Currently the best rotating laser levels on the market in 2020 are listed on this article here on LSSLTD.NET

Here is a list of the most accurate rotating laser levels currently available.

ManufacturerModelWorking Range
AccuracyWorking Time
(Single Charge)
Current Price
TopconRL-20011004mm at 200m100Hrs
TopconRL-H5A8005mm at 200m100Hrs
Topcon RL-SV2S800 5mm at 200m 120Hrs
TopconRL-H5B40010mm at 200m100Hrs
LeicaRugby 6809001.5mm at 30m60Hrs
Spectra PhysicsLL300S8001.5mm at 30m60Hrs
BoschGRL 500 HV5001.5mm at 30m25Hrscheck current price
Johnson40-65846001.6mm at 30m100Hrs
Johnson40-65415001.6mm at 30m33Hrs
Johnson40-65355001.6mm at 30m33Hrs
LeicaRugby 6105002.2mm at 30m60Hrscheck current price
LeicaRugby 6206002.2mm at 30m60Hrscheck current price
LeicaRugby 640G4002.2mm at 30m60Hrs
LeicaRugby 6405002.2mm at 30m60Hrscheck current price
Spectra PhysicsLL300N500 2.2mm at 30m 90Hrscheck current price
FukudaFRE-102AR6002.3mm at 30m25Hrs
Johnson40-6950L4502.4mm at 30m16Hrs
MakitaSKR200Z2003.0mm at 30m60Hrs
Spectra Physics LL100N3503.3mm at 30m50Hrscheck current price
PacificHVR 505R1523.3mm at 30mN/A
StabilaSTB-LAPR1502406.0mm at 30m80Hrs
HiltiPR 2-HS A12600N/A16Hrs

What accuracy can you get with a digital level?

Do you have a requirement for taking level measurements at an even greater accuracy than 2mm over 60m in length? Well, you are going to need a digital level. For more information on a digital level read the review of the Topcon DL-502 here.

With a digital level, a competent levelling team should be able to achieve an accuracy of 0.1mm over a 60m single sight. At this level of accuracy, the levelling team need to be very skilled and experienced.

How to check a laser tape measure.

With any accurate measuring device, it is important to be able to check the instrument for accuracy. Checking tape measures will be a simple check for damage and wear and tear. Checking laser tape measures will require a known distance. Laser tape measures should also be calibrated yearly.

Checking a Laser Tape Measure
Checking a Laser Tape Measure

How to check a spirit level.

Spirit levels can be checked by placing on a horizontal or vertical surface. Noting where the bubble lies in the vial and then rotating the spirit level through 180 degrees and noting where the bubble lies in the vial once more. The bubble position should be the same in both directions.

How to check an automatic dumpy level.

The standard way to check an automatic level is to do a two peg test. Here is a guide to doing a two peg test on a dumpy level. Remember that you should have your automatic (dumpy) level calibrated every year at least.

How to check a rotating laser level.

Checking a rotating laser level for accuracy is done using the same method as a two peg test. It is slightly different to checking a dumpy level. Here is a guide to checking a rotating laser level. Remember that you should have your rotating laser level calibrated yearly or before any accurate work is going to be undertaken.

How to check a digital level.

The standard way to check a digital level is to do a two peg test. Here is a guide to doing a two peg test. Remember that you should have your digital level calibrated every year at least.

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