[TowerTalk] Plumbing Towers

[Pat Barthelow]

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 at hotmail.com
Jamesburg Moonbounce Team


> From: K8RI-on-TowerTalk at tm.net
> To: TOWERTALK at 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) 
> 
> _______________________________________________
> 
> 
> 
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk