Why we need the Automatic Level.
There are numerous times that we need to measure levels accurately. Most people know what a spirit level is and how to use it. They can get an arbitrary, accurate level over 1m, 2m or even 3m with a spirit level. But what if we need to achieve accurate levels over much greater distances. Distances over 1000 times longer than we can measure with a spirit level, but still maintain the same accuracy. For this we need something better than a spirit level. We need an Automatic Level.
What is an Automatic Level?
An Automatic Level is a specialist piece of survey equipment used to obtain accurate levels over long distances. It is an optical device that can be setup easily with the line of sight perpendicular to the force of gravity. The line of sight observed is horizontal relative to the position that the automatic level is positioned.
The Automatic Level is most used by construction professionals and surveyors who need accurate levels. These accurate levels are usually over hundreds of metres.
I have taken apart an automatic level to help explain how it works. This level was at the end of its working life.
What is the difference between the Automatic level and the dumpy level?
The compensator in the automatic level is what makes it different to the dumpy level. The compensator makes using the automatic level much easier than a dumpy level. We often refer to the automatic level as a dumpy level, but they are two different instruments. They do the same job though.
The dumpy level does not have a compensator built into it.
What is the compensator found in Automatic Levels?
The compensator that is in the automatic level is a device that will move freely and will align perpendicular to the force of gravity. Thus, aligning with the horizontal plane of sight that we need to use to take accurate levels over long distances. The compensator allows the sight of the horizontal plane even when the telescope of the level is not truly horizontal.
You can test this by looking through the automatic level and turning one of the foot screws to purposely take it out of level. Once you have reached the end of the working range of the compensator you will notice that the reading on the staff changes. This is a good check to see if your compensator is working properly.
The compensator is a pendulum mounted optical device (prism) that both makes the image erect (the right way up) but also horizontal. The reflective prism can move freely as it is suspended within the pendulum. To prevent excessive movement of the prism, the pendulum is usually damped magnetically. This prevents vibrations from making reading of the E-Grad staff difficult.
The prism is suspended on strings. These strings are usually made from mylar. Mylar has several properties that make it ideal for the use of suspending the prism in an automatic level. For more information on Mylar have a look at this website page https://www.thoughtco.com/mylar-polyester-film-608929#:~:text=Mylar%20Definition,or%20biaxially%2Doriented%20polyethylene%20terephthalate.
Pictures taken during the dismantling of the Automatic Level.
Where are the Crosshairs of the Automatic Level?
The crosshairs seen when looking through the automatic level are on a lens between the eyepiece and the compensator. As our eyes differ, we need to be able to focus on the crosshairs and to do this we need to be able to adjust the eyepiece focal length. A good tip for doing this is to hold up the yellow level book in front of the telescope and to adjust the eyepiece until the crosshairs are crisp and clear.
How to Adjust the Automatic Level.
The only part that needs to be moved to adjust the Automatic Level is the lens with the crosshair on it. By moving this lens up or down moves the crosshair and therefore alters the reading seen on the staff. This lens needs to be finely adjusted to give the most accurate of readings.
The crosshair needs to be set at the correct height in relation to the horizontal plane. This will be set by the manufacturer and will be checked before being sold. But over time, this lens can move, and this movement will give inaccurate levels. This is because the line of sight is no longer horizontal. Have a look at the image below to understand what is happening.
The adjustment of this lens varies between manufacturer. In this Nikon Level, the lens is adjusted by loosening one screw and tightening the opposite screw. Correct adjustment of this lens can be difficult, and this is where the calibration service of a reputable survey equipment dealer is essential. Depending on your criteria you can adjust automatic levels yourself. This may result in varying levels of success.
How do I check my Automatic level?
The established way of checking an Automatic Level is by a two-peg test. The two-peg test is a straightforward method for comparing the results of two points viewed from different positions. The observed level difference of these points should be the same for a correctly working Automatic Level. The limitation of the two-peg test is about 50m. This is dependent on the accuracy of the Automatic Level being checked.
For a full brief on how to do a two-peg test on your Automatic Level have a look at this page. How to check an auto or dumpy level – The two peg test.
How accurate is an Automatic Level?
With most Automatic Levels, you should be able to get levels that are within 3mm over 30 metres. This is for automatic levels that have been recently calibrated. All recently calibrated automatic levels should achieve this accuracy. More expensive automatic levels can improve this accuracy.
Although, other potential errors may affect the accuracy you achieve. The E-Grad staff and the tripod being used needs to be in good serviceable condition to achieve this accuracy. And lastly the skill of the operator and their rod-man or assistant.
Why is the Accuracy of the Automatic Level Quoted in 1km Double Run?
This is the accuracy that can be achieved with the Automatic Level over the length of 1km using the double run technique. This is a technique employed by professional surveyors who carry out large scale levelling exercises. This technique of taking two foresights and two backsights for every setup eliminates errors and can be used to calculate the standard deviation of the readings.
For most people using an Automatic Level this is a pointless bit of accuracy information. Only professional surveyors will ever employ this technique. Only pay attention to this information if you are going to buy the absolute best Automatic Level available.
What is the Best Automatic Level?
The best automatic level is the one that fits your needs. I cannot suggest which one is best without finding out your requirements. What I can say is that you should buy the best one you can afford.
A builder would be happy with an Automatic Level that can be used to transfer levels around a single building. A surveyor would want an Automatic Level that can be used over long distances and may have an optical micrometre fitted.
What I do find though is the best Automatic Levels are produced by Topcon, Leica and Sokkia. In my experience these Automatic Levels maintain their accuracy for longer periods than other manufacturers of Automatic Levels.
I have the Topcon AT-B3 Automatic Level. I have had it for about 10 years now. Have a look at the review for the Topcon AT-B3 Automatic Level on this website here. Topcon AT-B3 Automatic Level Review. I think this Automatic Level is ideal for most construction site engineers.
On this website there is also an article on the best Automatic Levels for under £200. For most people wanting to know which is the best Automatic Level this article will fulfill the requirements. Read the Best Automatic Levels for under £200 here.
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