Understand Laser Levels

> He said that is not the way laser levels work. That it could just as well be within the 1/16" at 400'.

"Could just as well be" isn't the same as "will be." The Topcon RL-H4C, for example, is spec'd to +/- 10 arc seconds. I'm guessing that's the standard error, and at 100 feet, that's 0.005 foot or just about 1/16 inch. As a standard error, 2/3 of the time it's going to meet that accuracy; sometimes it'll be 0 seconds off level, most of the time within 10 seconds off level, and sometimes more than that. So yes, it *could* be within 1/16 inch at 400 feet, but you can't depend on it being that close.

Check the beam width when it's near dark. You'll think again about only 1/16" error at 100'. The beam is wider than that.

well, why not just test it out??

shoot some points in with a level, maybe every 50 feet. then try it with the laser and see how far out you get.

Yeah, I tried something like that.

I shot two points about 300' north and south of the level. Then went to the south point and shot the north one. I missed by 0.01'. Plenty close enough for the work I do, coz the only time I shoot more than 300' or so, is for dirt work.

But, like was mentioned above, just because I got that close that time, apparently does not indicate that I can COUNT on it.

That being said, I would FEEL better if I understood what was actually going on, in case I am asked about it by one of those 22 year old engineer guys.

thanx,

gzr

(22=1/3 gzr)

If the spec is as Jim suggests (+/- 10 seconds), it is 0.005' in 100' like he says, and at least 0.02' in 400' if the error increases linearly (proportionally with distance). In reality, it is probably not linear, and degrades over longer distances, perhaps greatly.

at around 300' I would guess 0.03' to 0.06' for repeatable accuracy using a laser level.

The accuracy degrades linearly as long as it is a quality self leveling/compensated laser. If it is .005 at 100 feet, it will be .05 at 1000 feet. The beam width has little to do with the accuracy, as the sensor that clamps to the pole is finding the CENTER of the beam. Different sensors have different "dead band" settings that determine how picky they are in giving an on grade reading. Most quality sensors have 2 or 3 user selectable settings available, ranging from 1/32 of an inch to 1/4 of an inch. The dead band setting is a constant value and does not vary with distance. Some sensors designed for higher accuracy have a zero (or nearly zero) dead band setting that makes it almost impossible to get an on grade display on the sensor, but you know you are on grade when it constantly jumps from an up then down then up, etc., display when you are not moving it. Other sensors designed mostly for machine mounted applications have a .05 to .1 dead band setting to make them easier to read when doing less accurate tasks. As with any instrument, the accuracy depends on the current calibration accuracy of the instrument as well. Self leveling lasers are easy to field check for accuracy and calibration. Simply set one on a fairly level tripod with one of four sides facing an stable object a few hundred feet away, level it (or allow it to level itself), then go make a mark on the object where you get an on grade reading. With out moving the tripod, rotate the laser 90 degrees, level, mark, and repeat for all four sides. Your accuracy is half of the variance in the marks at the distance the object is from the laser. The two axis of the laser are adjusted independent of each other, so it quite normal to have one axis with better results than the other. If the variance of the marks are greater than the accuracy specification of the laser, then it normally just needs calibration, unless it has a damaged compensator. More surveyors and engineers could make good use of lasers for most tasks that they currently use optical levels for, and be more efficient. Lasers also reduce the odds of blunders as they are operated by one person and therefore the communication variable is removed.