Being able to level accurately is one of the basic skills any Surveyor or Engineer should possess. But there are many things that can affect the accuracy that any surveyor or engineer can achieve when carrying out a levelling traverse.
The most accurate levelling technique is the double run method. Double Run Levelling is a technique where the Backsight and Foresight observations are taken twice, this aids the identification of errors. The Backsight and Foresight readings are taken from one setup position, the auto level is then moved and then the Backsight and Foresight readings are taken again. Think of this method as being able to do the entire levelling run twice, but quicker.
But even using the double run method will not yield accurate results if you do not heed the following advice.
In this article I will outline 5 ways a surveyor or engineer can achieve accurate levelling. These are important points to remember and more importantly to implement.
- Always close your levelling traverse.
- Keep you sighting distances short.
- Carry out a two-peg test at least once a week.
- Make sure you E-Grad Staff is in good condition.
- Don’t try to make your readings fit your closing calculations.
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1.) To Achieve Accurate Levelling, Always Close your traverse.
A surveyor or engineer should always close your levelling traverse. This will allow the senior surveyor or senior engineer to gauge how well the engineer or surveyor has done their levelling traverse, and see your closure error. A surveyor or engineer should be aiming for a closure error of less than 3mm over the levelling run being undertaken.
Most of the levelling errors that have been discovered at a later date reveal that the engineer or surveyor undertaking the task did not close their levelling traverse on a known point, so they had no idea that an error was present. This is negligent on the part of the engineer or surveyor and they have no defence against the error found.
All surveyors and engineers have made errors whilst levelling, but all of these are found when closing the levelling traverse. Some surveyors or engineers will perform the levelling traverse twice so that they can compare their results for potential errors.
Remember to always close your level traverse to achieve accurate levelling.
2.) Keep you sighting distances short.
Trying to cut down on the number of change points done during a levelling traverse can reduce the errors over the levelling run. However, trying to sight the E-Grad staff over long distances can also lead to big errors in the levelling traverse.
The main issues with long sights (over 50m from auto level to E-Grad Staff) are two-fold. The first being the difficulty in reading the staff with accuracy, and the ease with which the surveyor or engineer can read the staff depends on the magnification multiplication of the Auto Level. The higher the magnification multiplication, the easier it will be to read the staff accurately.
The second problem is that long sights can be subject to heat haze and refraction of light. This is particularly important on hot days but can also be dramatically affected if sighting close to the ground or an obstacle.
Remember to keep the sighting distances to a reasonable distance to achieve accurate levelling.
3.) Carry out a two-peg test at least once a week.
Checking an Automatic (Dumpy) Level should be done at least once a week or before any accurate levelling is going to be undertaken. The results of the two-peg test should be recorded in a legible manner and in a way to allow comparison to previous tests. The two-peg test is the minimum test that should be carried weekly. Remember that a calibration check on the automatic (dumpy) level comprises of further and more rigorous checks.
If you need to know how to carry out a two-peg test on the automatic (dumpy) level, then follow this guide for carrying out a two-peg test.
Closing a levelling traverse is also a good check on an automatic (Dumpy) level, but these must be documented clearly if included in a checking regime.
There are also a few other quick and easy checks that can be performed on the automatic (dumpy) level. They are explained the post quick and easy checks on automatic (dumpy) levels.
Remember to check the accuracy of the dumpy level to achieve accurate levelling.
4.) Make sure your E-Grad Staff is in good condition.
It doesn’t matter how good your automatic (dumpy) level if the E-Grad staff being used is in poor condition. An E-Grad staff that is difficult to read will lead to errors in your levels. Over time dirt and grit will wear away the numbers and symbols on the face of the E-Grad Staff. There is not a great deal that can be done to prevent this and the engineer or surveyor would be wise to consider the E-Grad staff as a replacement item.
Check the E-Grad staff for damage weekly or before any accurate levelling exercise. Check the base of the staff for dents or unevenness of the bottom. An uneven bottom will lead to different readings being taken on nails and small points of interest.
Also check the push buttons on each section for wear or damage. Poor condition of these push buttons will lead to sagging of the sections of the E-Grad Staff and when reading between the sections of the staff there could be an error within the staff. The engineer or surveyor can check this simply by looking at the push button on the back of the E-Grad staff and measuring across the sections for discrepancies.
If the engineer or surveyor suspect the push buttons on the staff to be in poor condition, then replace your E-Grad Staff.
Check out the latest price of a new E-Grad Staff on the Lichfield Survey Supplies Website.
Remember to check all your equipment for serviceability to achieve accurate levelling.
5.) Don’t try to make your readings fit your closing calculations.
When doing your levelling traverse, it can be tempting to judge what reading the engineer or surveyor need to get a good closure for the levelling run. Try not to do this. If the engineer or surveyor are judging the reading the engineer or surveyor should be getting throughout your levelling run then the engineer or surveyor are potentially covering up your errors, not finding errors that maybe present, or have been done by others, and covering up inadequacies in the equipment being used.
Remember to be honest with yourself to achieve accurate levelling.
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If the engineer or surveyor are not getting results that are expected or closing errors are too great, then repeat the levelling exercise. Most of the time doing this will identify errors in the levelling run. What the engineer or surveyor should be able to repeat the same levelling run time and time again and achieve the same results each and every time. If the engineer or surveyor can do this, then there should be no argument against their competence in using an automatic (dumpy) level.
Related Articles
How to carry out a two peg test
Quick and easy checks on a dumpy level.
