questions about laser distance measuring devices

[peter]

Does anyone know more detail on the accuracy of the laser distance measuring devices? E.g. something like this:

It says the accuracy is ± ¼" at 100 feet. Does that scale proportionally, so that at 50 feet, the accuracy is 1/8", and at 25 feet, the accuracy is

1/16"? Or is the accuracy going to be 1/4" independent of the actual distance being measured?

Furthermore, is the error a consistent error, or random error? In other words, if I use it to measure a 100 feet distance 10 times, would I get 10 different readings distributed in a 1/4" range, or would I get the same reading 10 times that is within 1/4" of 100 feet?

[Stubby]

The surveyor I used told me the law requires a standard accuracy is 0.1' . A high accuracy measurement gets things to 0.01' .

GPS errors are random. They compute distances by subtracting the coordinates of the end points. The usual statement people in USAF (who developed GPS) used to make was "95% of readings will be within 10 meters of the point."

[Craven Morehead]

Stubby,

Who the hell said ANYTHING about GPS? Lasers and GPS systems are ENTIRELY different technologies. Duh!

>

[Jim Yanik]

Stubby wrote in news:v7idncVXOLY_cFnZnZ2dnUVZ [e-mail]:

You would have to ask the manufacturer,as it would depend on what sort of system they're using to make the measurement;phase shift or Time Of Flight. It depends on the reference clock crystal's accuracy/stability,and ambient temperature,for starters.

Maybe for professional equipment,but not for consumer tools. You get what you pay for.

GPS might be used for the measurement reference frequency instead of a crystal;a much better standard,at higher cost and complexity.

[nlbauers]

As was suggested, the manufacturer would know the answers to these for sure. It would be easy to set up a test of these questions for yourself. If your livelihood depends on these answers, you probably should test it yourself anyway, no matter what the manufacturer says, or buy a system that exceeds your needed tolerances.

If none of the above is possible, I would venture the following guesses:

*accuracy does not scale linearly with distance, and the worst case is not stated *the error is random

But this is purely based on what I would expect from statements about accuracy and error in such a tool.

Neil

[The Other Funk]

Finding the keyboard operational peter entered:

Being a handheld device, the error is probably for one reading. If you took

10 readings, while holding it in your hand, You will not get 10 readings +/- .25" If you mounted the instrument to something solid, it might get you 10 readings +/- .25" but this is not precision measuring device that is intended for absolute accuracy. Also notice that while the readout will show you 0.04 inches, the accuracy is 0.25 inches. I wouldn't use this to get me any closer then +/- 1 inch. Bob

[The Other Funk]

Finding the keyboard operational Stubby entered:

Stubby, can you get a clarification on that? It seems like getting 1.2 inches accuracy over a 100 foot (1200 inches ) is really tight and .12 inches (about 1/8") really really tight. If we take that to 100 yards (

3600,inches) then the level of accuarcy is 0.00027%. Just doesn't look right. Thanks Bob

--

-- Coffee worth staying up for - NY Times

[OldRoads]

What would you be measuring with the laser? Last time I looked at them (maybe a year ago) they all had really poor accuracy and were really only good for rough room measurements (as a real estate agent would use). If you're building something I'd measure it the old fashioned way.

Vin - Menotomy Vintage Bicycles

[spamTHISbrp]

Accuracy vs. repeatability rears its ugly head...

D

[Pete C.]

I think you're confusing the cheap ultrasonic measuring devices, most of which have a laser pointer for targeting, with the newer laser measuring devices which do not use ultrasonic ranging and actually measure with the laser.

The units mentioned in the Stanley line run from the $100 consumer unit with 1/4" accuracy and 100' range, to a $450 unit with 1/16" accuracy and 600' range.

Pete C.

[The Other Funk]

Finding the keyboard operational [e-mail] entered:

Add in resolution to the mix and we have got ourselves a mess.

--

-- Coffee worth staying up for - NY Times

[Al Bundy]

"OldRoads" wrote in news:1153842024.870936.132440 @h48g2000cwc.googlegroups.com:

Yea, closing one eye and giving it a good gander.

[Andy Asberry]

That is how some of the old railroad surveyors did it. Throw a rock off a cliff and write down the elevation.

--Andy Asberry recommends NewsGuy--