All About Auto Levels – AKA Dumpy or Optical Levels

August 11, 2016

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Auto Level otherwise know as Dumpy or Optical Levels

Auto Levels, sometimes known as Dumpy or Optical Levels have been around many, many years. In some respects, they were the forerunner to the builders’ laser level. If you were on a construction site and you needed to check levels then you reached for your auto level. As with all levelling technology they were once quite expensive and predominantly manufactured in Japan or Europe. Now they are extremely inexpensive and pretty much all made in China. Auto levels are not used as much as they used to but, they still make a viable levelling option. The pros and cons of this I will cover a little later.

So what is an Auto Level?

As the name implies it is an automatic levelling device. If you go far enough back in time, not all optical levels where auto levels. Some needed to be levelled manually by using a highly accurate built-in spirit level vial. These days they are all auto levelling.

How do they level?

Before I get stuck into this one its worth noting that a surprising number of users of auto levels do not fully understand that the instrument actually auto levels. This is due to the fact that all of these levels have a bulls-eye bubble vial on them. Many believe that the more accurate you get the bubble in the centre of the vial the more accurate the device is. This is not correct. What the bullseye vial does is get the device into self levelling range, this is indicated by a circle on the bullseye vial once the bubble is within or mainly within this circle then the devices levelling mechanism, called a compensator, takes over.

Now assuming the bullseye vial and compensator are correctly calibrated whenever the bubble in anywhere in this circle then level will be found. In other words, don’t waste your time trying to the get the bubble exactly central, it will not make any difference to the accuracy.

Compensator

So back to how do auto levels, level. They use a wire suspended compensator and based on the principle of a plumb bob and the constant of gravity. The compensator works by light entering at one end, it then enters a prism or mirrors on a floating platform (pendulum). The light then exits through more prisms and lenses. The floating platform moves like a plumb bob with gravity and through clever geometry compensates the exiting light to be always level. The light in the case of an optical level is the image being viewed through it.

With the addition of a crosshair and focusing lenses, you have your optical level. Note some older laser levels utilise this technology with a laser beam instead of a person looking through the compensator. The diagrams above represent the path of light as a red line, whether it is the image being observed by the user through an auto-level or the path of a laser beam in a laser level.

How to level using an auto level

Using an auto level is pretty straight forward, fist you roughly level the housing of the level using the bullseye vial and the adjustment feet on the base. Again all you need is the bubble to be within the circle and the compensator does the rest. Next direct the level towards the direction you want to level and look through the eyepiece.

Site Levelling

Normally for site levelling, you will have a partner holding a measuring staff. Locate this staff by fine-tuning the rotation of the level by rotating the small adjustment knob (or knobs) at the front of the level. You will also need to focus on the image of the staff by using the focus knob on the right-hand side of the auto level. Finally rotating the eyepiece will bring the cross-hair into focus. When selling auto levels it is amazing how many phone calls from the field you get about missing or fuzzy crosshair lines.

Now you can read a height on the staff. By getting your partner to move around the site you can level it by making sure the staff height remains constant. Placing the staff on top of a stake or peg the staff holder can knock in the stake or rase it from your directions to get the height right. Once all the pegs have been adjusted and the ground has been filled or cut to the top of the pegs then the surface will be flat and level. Dumpy levels can also be used to level for installation jobs such as windows by visually looking at the image in reference to the cross lines.

Angle estimation

Many auto levels can also be used for angle estimation. The angle is the angle of rotation of the head of the auto level. The angle reader or scale is normally located at the rear of the unit, just above the adjustment wheels. To use it direct the level to towards the starting point of the angle you want to estimate. Next zero off the scale using the large adjustment disc under the scale readout. Rotate the head of the level until it is directed directly at the endpoint of the angle you want to get. Now simply read off the angle. It can only truly be stated as an estimation in my opinion rather than a measurement due to the scale used. If you want precise angle measurement then you will need a different device completely.

Distance Measurement

Most auto levels can perform a distance measurement or estimation. Again not completely precise but can be a useful gauge for measuring large distances when you haven’t got a theodolite, laser distance measure, or large tape measure handy. The distance in question being from the centre of the tripod to the position of the staff. A plumb bob is often supplied, this is hung below the tripod to provide the centre point.

The distance can be calculated by viewing a measuring staff through the eyepiece. Then observing the distance between two stadia marks on the cross hair, one above and one below the central horizontal. The calculation you need to perform may vary from model to model. Generally, by multiply this measurement by 100 and you have the distance between dumpy and staff. e.g. 5cm x 100 = 500cm which is 5m In other words whatever you read in centimetres is the distance in meters. The principles are similar in imperial best check the handbook for your optical level on this one.

Pro’s and Con’s of Auto Levels over a Laser Level

As mentioned at the beginning, auto levels are less prevalent today due to the emergence of inexpensive laser levels. But they still have a place in the levelling tool kit. Fist of all they are cheap, generally half the price of a reasonable quality rotating laser. Secondly, they are in theory at least far more accurate than a laser level. I say in theory in that once the human element comes into play the more opportunity there is for error. With an Auto level, there is a lot more potential for human error, in my opinion anyway.

So why are people using laser levels? The main reason is that a rotating laser is a one-person operation and using an auto level is two. One to look through the level and one to hold the staff. A second reason is time, it’s probably about twice as fast to level a site single-handed with a laser level than it is with two people and an auto level. But on the other hand, an auto level does not need batteries or charging and is relatively mechanically simple.

Buying an Auto Level, Dumpy Level or Optical Level

So you have decided an auto level is the correct device for you. The next problem is which one? Features wise most auto levels offer pretty much the same. Auto levels come in a number of different magnifications; 20x, 24x, 26x, 28x, 32x and greater, but, what does this mean. The greater the magnification the bigger the image appears so the measurements on a staff is easier to read. But, and it’s quite a big but, the greater the magnification the smaller the field of view. So you see a smaller area through the eyepiece, this can mean that finding the staff to read can become harder. Most magnifications sold are between the 24x and 28x, this seems to be the sweet spot.

Now we know about magnification, what about the brand? The two main components in an auto level are the compensator and the optical lenses. From experience, I have found the compensator on most modern auto levels to be of very similar quality. I have also found that optical clarity can vary allot particularly on the very cheap units. Poor optical clarity means a milky image which can make focusing and reading measurements difficult. More expensive models in general have better construction and materials. Brands that stand out as being good quality in my experience are; Topcon, Sokkia (basically the same as topcon), The more expensive Leica models and Nikon.

Below are some links to purchase my preferred Auto, Dumpy, Optical levels. Note that purchasing one of these through one of the links may result in myself receiving a small commission.