Semi-automatic leveling. This category's name is a bit misleading. The only automatic thing these tools do is shut off when they get knocked out of level. You still use bubbles, knobs, and your eyes to level them, but they'll tell you if they somehow get thrown out of level. These tools' accuracy is often rated at something like 3/8-inch at 100 feet. That's a little less accurate than some similar models, but semi-automatic laser tools are purposely designed that way. If designers aimed for 1/16-inch at 100 feet, the tools would shut themselves off too frequently to be useful. Every tiny vibration in the area would mean a trek back to the tool to reset it. These tools range widely in cost, but average between $450 and $1,400.

Compensated leveling (narrow-range self-leveling). This is probably the largest, most popular category in today's laser market. Compensated tools use familiar bubbles and knobs to get the tool roughly close to level. Then a compensator, a small pendulum often suspended by tiny steel wires, takes over from there to find the exact level point.

Some compensators incorporate relatively heavy pendulums that weigh several ounces. Their top ends are secured in complicated ball-bearing arrangements. The weight overcomes any bearing friction to make the compensator hang plumb.

Dependability is one big advantage of this system. Center the bubbles once in the morning, and the tool projects an accurate line all day. The compensator handles any small movements in the tripod. If the tripod moves enough to keep the compensator from leveling, the tool lets you know by shutting off. These tools drop in price all the time, but currently sell for between $600 and $1,700.

The accuracy of compensated laser tools is generally listed as 1/4-inch at 100 feet. One drawback of these tools is they can't shoot a sloped circle, which can be useful for grading or setting drainage pipes. If you need this function, step down to a manually leveled tool, which will send the line where you point it, or step up to our next category.

Servo leveling (wide-range self-leveling). Now we're talking about the Cadillacs. Servo tools are sometimes also called fully automatic because tiny servo motors, which act like tiny winches, do all the work. Basically, you turn the tool on and walk away. Its sensors detect how far out of level the tool is and tell the servo motors what needs to be done. The servos reel in or ease out slim steel cables to achieve level.

Of course, you pay plenty for this marvelous convenience. Servo models sell for between $1,400 and $2,000. Since pendulums don't control the action inside these tools, some high-end models have slope features. Users can dial in a fraction of a degree or several degrees of slope.

There's also one type of rotating laser that falls somewhere between compensated and servo leveling. This hybrid type uses servo motors to roughly level the unit, and then a compensator finishes the job. These tools cost a little less than fully servo-leveled types because the level sensors and motors needn't be accurate enough to fine-tune the tool, just get it close enough for the compensator to swing. This hybrid category doesn't really have a name yet, but the tools sell for between $1,000 and $1,600. Like compensated tools, they can't project a sloped circle for grading.

A Word of Caution

Most rotating lasers are mounted on tripods for use. Pay attention to tripod materials, because not all are created equal. Differential heating is the bane of tripods. If two legs heat up in the sun and the other one is shaded, there could be enough movement to trigger auto-shutoff on a semi-automatic tool. With a manually-leveled tool, you won't know unless you check the bubbles periodically. Wood is the most stable material for a laser tripod. Next-preferable materials are fiberglass and aluminum.