On Tue, 15 Nov 2011 09:30:58 -0500, Ed Huntress wrote:
....
I have one of the Stanley laser tapes, made under license from Leitz,
for which I paid around $80 or $90 at HD, on sale a few years ago. It
claims accuracy of +/- 1/8" at, I think, 100'. I've only checked it
against steel tapes out to around 20', because that's as far as I trust
them, and from what I can tell, it's around twice as accurate as the
guarantee. I do get readings out to around 100' with it but a steel tape
at that distance has catenary sag all over the place, so I haven't
checked accuracy at that range.

It takes a whole bunch of sag to make an appreciable difference in
measurement accuracy. For example, if a 100' steel tape sags one
foot, measurements will run about 0.02" (about 0.5mm) long.

To compensate, just use the steel tape at an ambient temperature
about 32F higher than its calibrated temperature, and thermal
expansion will cancel out the foot-of-sag effect.

--
jiw