Setting out on construction sites always involves either taking measurements or giving measurements from points or base lines. But whenever a measurement is taken or given for the purpose of setting out on a construction site it will be subject to errors. These errors may be negligible, or they might be significant. For the purpose of setting out in construction a negligible error would be one that would fit within the tolerance of the structure, road, feature or what ever we are trying build.
What are the 3 Measurement Errors in Construction Setting Out?
Whenever measurements of length, angle or level are taken they are never entirely free from some form of measurement error and thus should not be thought of as true values. Therefore, it is important to understand how errors can occur, how we can minimise them and to what extent we can accept these errors as being negligible.
The 3 type of errors are:
2). Systematic Errors.
3). Random Errors.
Mistakes are from the user of the measuring device and are often only done on a single occasion. These can often be difficult to find as there is usually no way of checking the mistake. For example, reading a tape measure incorrectly. This often happens when people are distracted or interrupted mid task.
Systematic Errors are from the instruments or equipment we use to take the measurements. Systematic errors are either due to following incorrect procedures or damaged or badly adjusted equipment. These types of errors can accumulate every time a measurement is taken. Systematic errors can usually be identified easily if proper checks are carried out.
Random Errors are errors that are usually beyond our control. These errors are ones that we can’t control and can’t “see”. If we have eliminated both mistakes and systematic errors, then we can identify random errors by repetition of taking the measurements.
These different types of errors can have a varying impact on the overall quality of the setting out on construction sites. For this reason, the more accurate the requirement for the setting out the more time is required for reducing these errors and therefore the longer the process will take.
How to Avoid the Measurement Mistake Error.
The measurement mistake error will happen from time to time. You can’t help it; I can’t help it. I’ve made loads over the years. They usually happen due to a lack of concentration, being distracted or being given to much work to do.
The only way to avoid the errors like these is to either check or repeat your measurements. If these measurements are critical then you can always ask someone else to check your work. Don’t be to proud to ask for help, a mistake can be more costly than your pride.
How to Avoid the Systematic Error.
Systematic errors are errors that are in the measuring equipment that you are using. Avoid these errors by checking the condition or the accuracy of the measuring equipment. The crucial step to avoiding the systematic error is to make sure that you have your measuring equipment calibrated if it requires a calibration.
As the user of the measuring equipment, you should check the condition and be able to check its accuracy. For simple measuring devices like steel tape measures, a simple check of the condition would be fine. Looking for damage or wear to the tape measure should tell you if the tape measure will take accurate, error free, measurements. For Laser Tape Measures the checks need to be more involved. Have a look at the procedure I use for checking laser tape measures here. For measuring equipment like automatic levels or dumpy levels then the two peg test is the best way to check for systematic errors. This type of test is also useful for checking rotating laser levels, this is a guide on how to check for rotating laser level errors. Theodolites used for measuring angles require more checks, but I do have a guide on how to check theodolites for errors here on this website. Then we have total stations. Here is a guide on how to check a Total Station for errors.
If you are not sure whether you have checked your survey equipment correctly or when checking your survey equipment, you are getting errors that are too great, then you should get your survey equipment calibrated.
How to Avoid the Random Error.
This is the problem one. You can’t avoid the random error. They happen in every measurement and they are beyond your control. You can reduce the affect of random errors by repeating measurements several times. This will enable you to either take an average or dismiss measurements that can clearly be seen as erroneous.
Random errors are more common in optical measuring devices than physical measuring devices. Optical measuring devices like levels, theodolites and total stations take measurements through another medium (Air) and that medium will create these random errors.
GPS Surveying and measurement errors in construction setting out.
As a side note, GPS surveying equipment is accurate because we can calculate and then eliminate the random errors that will affect the GPS signals as they travel through Space and then the Atmosphere before reaching the equipment. As we know that these random errors exist before the signals reach the GPS surveying equipment, we cannot be able to calibrate in the conventional sense. All we can do is check to see if we are getting the results we are expecting from the GPS Surveying Equipment.
As there is no formal way of calibrating GPS surveying equipment, it is important that we can prove that the positions being given by the GPS surveying equipment are accurate and correct to known positions on the ground.
As we know that GPS Surveying Equipment is always subject to Random Errors, we need to follow best practice when using GPS Surveying Equipment. On these pages I go through 4 ways that you can achieve accurate GPS Positions. But even following these best practices may not always yield accurate results and we need to pay attention to the real-world accuracies of GPS surveying equipment. This is an article on the real-world accuracies of Network GPS surveying Equipment.
What measuring equipment will require a calibration?
We already know that there is no formal way for calibrating GPS Surveying Equipment, but what Surveying Equipment should be Calibrated and how often?
Any measurement equipment that you look through or takes an indirect measurement should be independently calibrated. Having any measurement equipment calibrated ensures that the accuracy of the equipment is a close to the true measurement value as possible. Thus, negating the systematic error as much as possible.
For the surveyor or site engineer the equipment that should be calibrated: –
|Survey Equipment||Type of Measurement||Frequency of Calibration|
|Automatic (Dumpy) Level||Optical Measurement||12 Months|
|Digital Level||Optical Measurement||6 Months|
|Total Station||Optical Measurement||12 Months|
|GPS Surveying Equipment||Indirect Measurement||Daily Accuracy Checks|
|Measuring Wheel||Direct Measurement||12 Months (If Needed)|
|Rotating Laser||Laser Measurement||12 Months|
|Laser Tape Measure||Laser Measurement||12 Months|
|Steel Tape Measure||Direct Measurement||Replace if Damaged|
|Decibel Meter||Sound Measurement||12 Months|
|Supa Rule||Sound Measurement||12 Months|
|Theodolite||Optical Measurement||12 Months|
|3D Scanning Station||Laser Measurement||12 Months|
How often should Surveying Equipment be Calibrated?
Suggested calibration interval for Surveying Equipment is every year. This would be the minimum requirement for most construction companies and sites that operate. However, this doesn’t mean that the surveying equipment you are using is accurate all the time. The calibration certificate only means that the equipment was correct on that day. You should always check the equipment for accuracy yourself.
If the work you are carrying out is critical then you might be well served having the surveying equipment calibrated every few months. I know of one company that have their automatic dumpy levels calibrated every 3 months. The reason for this is that a calibration is cheaper than putting the work right due to an inaccurate level.
How to check different types a Surveying Equipment.
There are a number of checks that can be performed on surveying equipment to ensure their accuracy that doesn’t require them to go in for a calibration. Follow the links below to find out how to check your surveying equipment. Remember though that these checks are in addition to the calibration requirements.