Folks:
One instrument that may be useful to dermine plumb-ness of a tower is a
surveyor's tribrach. This is a universal, precision mount that accepts
theodolites, distance meters, or other instruments. It is the mechanical
interface
to the tripod machined flat top, and allows high precision levelling of the
attached instrument.
See:
http://cgi.ebay.com/Arise-Tribrach-Optical-Plummet-for-Sokkia-Topcon-Leica_W0QQitemZ290198354505QQihZ019QQcategoryZ55808QQcmdZViewItem#ebayphotohosting
or Google Tribrach, in images to see one.
The tribrach has a bulls eye level that is used for roughing in the tripod base
on setup, close enough to allow the precision leveling screws
to refine precision levelness of the theodolite or other instrument, to say,
better than 1-3 seconds of arc range.
A tribrach is about $80 now in Ebay. The tribrach has a small optical
telescope, called an optical plummit, to allow the user to see the true
vertical optical path, with internal crosshairs falling on the ground,
establishing a vertical line from that point, assuming the tribrach reference
horizontal plane is precision levelled. The bulls eye level itself might not
be able to be used to sufficient accuracy for precision levelling, It may be
accurate enough, for plumbing a 100 ft tower, I do not know the desired
precision for that job.
If someone knows a good standard for vertical accuracy over say, 70 ft, let us
know, but I would think .01ft or .02 ft would be very accurate.
Turns out that .01 ft (about 1/8") over 70 ft, is about 30 seconds of arc.
The precision level vials in the instrument, and careful thumb screw
adjustments do the precision leveling of the instrument. Some level vials have
a 20 second bubble, which means if the end of the bubble moves 1 full hash
mark from center symmetry, then that error is about 20" of a degree, or 1/3 of
one minute, or 1/180 degree. Such vials are not cheap to buy, or to have a
metrologist calibrate, but may be available to the industrious ham searcher in
a form that can be attached to a tribrach. There are also ways to accept a
small optical alignment error and make multiple measurements to cancel the
error in the field.
If the top plate of the tower was reasonably perpendicular to the vertical
axis of that section of the tower, and if the surface was quite smooth, you
could simply place a tribrach on the top plate of the fully extended tower,
level it, and view a reference center point marked on the concrete base of the
tower. Do this three times, with the tribrach rotated 60 degrees each time,
and you could box a center point pretty accurately, establishing how far off
of plumb, the top of the tower is. Depending on required accuracy, the bulls
eye bubble may be able to be used.
There are up-looking optical plummits, too, that install on a tribrach, but
they are are not cheap. If you had one of these, and could set it up centered
in the base section of the tower, you could use that.
You could experiment with a digital level, laid across the tribrach plate
edges, to see if that establishes a true horizontal.
Of course, if you had grampa's 60 year old transit and it was still in
precision calibration, you could do a surveyor's setup away from the tower from
90 degree different locations and check plumb. Operative words here are "still
in precision calibration", as a transit or theodolite will not take much of a
hit, without going out of precision plumb, itself.
The question you have to ask, is, if the level vials on the transit are
"level", is the alidade (base) level, and is the horizontal axis of the
telescope, level, and the crosshairs of the telescope truly sweeping a
vertical path. Lots of "gotcha's" between the level vials and the crosshairs.
If you have one, even in rough precision condition, you can still use it, by
taking special, redundant measurements to dial out any instrumentation errors.
A tribrach could easily be carried up a tower, if it was in a cushioned case,
to check plumb. And I am astonished that one can be bought for $80 today.
In 1972, they (Operative word.. Swiss-Made, Wild-Heerbrugg Company) were around
$200. Wild was the "Collins" of surveying equipment back then.
All the best,
Pat Barthelow
aa6eg@hotmail.com
Jamesburg Moonbounce Team
> From: K8RI-on-TowerTalk@tm.net
> To: TOWERTALK@contesting.com
> Date: Mon, 21 Jan 2008 13:17:42 -0500
> Subject: Re: [TowerTalk] Digital Angle Gauge
> >I plumbed my 100-footer while hauling a 200-lb mast up the center on a
> >steel
> > cable by winch ;o)) 73 Dave N3HE
>
> That's a wee bit dirfferent than a plumb bob<:-)) and on a calm day should
> work if attached to the cable properly. OTOH even a 200# mast on 1/4" cable
> will move suprisingly far in just a 5 MPH breeze. Of course the plumb bob
> will move several inches in a breeze almost to slight to feel. <:-))
>
>
> 73
>
> Roger (K8RI)
>
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