By Victor Russell
Today let’s look at the simplest of the survey instruments, the automatic level. Although they have been around for many decades they really haven’t changed all that much. Automatic levels are very precise survey instruments with the ability to give you millimeter vertical accuracy over an entire jobsite whether it’s the addition of a deck on the back of a house or new high-rise building.
By Victor Russell
The main part of an automatic level is the compensator. This mechanism is the device located inside the level that gives the unit a small mechanical rattle when picked up and gently shaken. The compensator works with gravity and usually uses four fine wires that allow it to move freely, albeit within limits. These fine wires suspend a prism, which swings slightly when you look through the eyepiece. This corrects your eye’s sight path, which then gives you a true horizontal line. The compensator is a complex piece of equipment made up of the main housing, the wires and prism (the compensating or movable part) and a dampening mechanism to slow things down for a more accurate reading. This unit is made up of between 50 or more parts (depending on the brand). Any rough usage can not only misalign these parts but also stretch or damage the fine wires. Any misalignment of one part with any combination of the other parts could put the accuracy off and make the level totally inaccurate and unusable.
Now let’s think about the accuracy of an automatic level for a second because this is so very important. The accuracy of an instrument on the job is directly proportional to how much it is out of level and the distance between the instrument and the survey rod. Therefore, if an instrument is out of adjustment just a bit, say 1/16 inch over 10 feet, one might assume that’s it’s close enough and carry on with the levelling of the job. However, that small error relates to 2/16 inch over 20 feet, 3/16 inch per 30 feet and so on, as it is proportional to the distance. So when you are on a larger job and you are 160 feet between the rod and the instrument you can now be out a full inch, and that could have disastrous results and would not be tolerated on any job site.
There is an international method for comparing the stated accuracy of automatic levels and that is “standard deviation per one-kilometer double-run levelling.” That means you run a level and rod step-by-step out one kilometer and back to the same point. If all is perfect (the instrument, the survey methodology and the operator) then the last reading should be the same as the first – zero elevation difference.
The accuracy of a level is stated in millimeters. A good level will be within plus or minus two to three millimeters accuracy from its starting point. If the accuracy is stated another way, such as “accuracy per 100 feet,” then I would suggest the manufacturer might be trying to hide the true number which could be much higher and therefore less accurate. Beware!
Now for the twist.
I know there are claims out there that certain instruments don’t need to be checked because they were adjusted at the factory and they have a nice name on them and we have been told that they are OK. Believe me, in my over 40 years in this industry I would estimate that a good percentage of all new instruments (regardless of brand) need some adjustments. And a percentage of those need extra work because the accuracies are outside acceptable tolerances. There are even the occasional ones that are so far out of adjustment or broken that they can’t be serviced at all.
To make sure that everything has been checked and adjusted as required, look for a service sticker inside the case. This service should be done by the local supplier before the sale to the customer and should be dated so that the customer has confidence that the level is job-site ready.
Victor Russell is a technical sales representative with Spatial Technologies in Richmond, B.C. He can be reached at firstname.lastname@example.org