Pat Barthelow wrote:
> 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.
One can also look at discount mail order suppliers like www.pqsa.com.
>
> 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.
I would think that one could fabricate some sort of bracket to mount a
standard tribrach upside down. Obviously, the bullseye level isn't
going to work, but you could put some vials on the back side.
> 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.
Most of the older surveying texts (like Davis and Foote) tell how to
compensate for these errors. Mostly a matter of taking two sights,
reversing the telescope between, and taking the average. Heck, for the
great trigonometric survey of India they had to transport everything
through the tiger filled jungle, build huge towers, carry the stuff to
the top, reassemble, etc.
Likewise virtually all the surveys done by the U.S. in the western US in
the 1800s and early 1900s were done with equipment shlepped around on
mule back, etc.
An older theodolite, unless it's actually broken or seriously bent, can
be (tediously) readjusted and brought into calibration by a dedicated
amateur without special equipment. It's mostly a matter of taking
repeated sights without moving it, and adjusting until you get the same
reading both ways. If the bearings are shot and have a lot of play,
you're out of luck, but if it's just that things are out of alignment,
that can be fixed.
Newer total stations, on the other hand, while much easier to use, and
more precise, in general, are built differently, and depend much more on
the precision and tolerances of the bearings, etc. (e.g. don't fling a
modern surveying instrument up on your shoulder mounted on the tripod..)
A older Wild T2 type instrument with the optical micrometer might be a
different story. The basic theodolite is the older design and pretty
darn rugged. It's an amazing piece of precision gear, but I can see
lots of ways to screw up the optical reading paths that might not be
trivially fixable. However, if you latch onto one of these, there are
copies of the service manuals online at some army.mil sites. (but not
the user's manual... you'll have to track that down elsewhere)
Frankly, though, a T2 might be more tedious to use for plumbing a tower
than a simpler transit. Setup takes a bit of time, and if you don't use
one regularly, there's a fair amount of fiddling around.
>
> 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.
Covered in Davis&Foote, any edition.. so you can use one of those
editions from the 1920s.. Probably any older surveying textbook would
cover this kind of thing (anything pre-"total station" would be
appropriate). Even the total stations do the same thing when you first
turn them on, except it's automatic, and all the math gets done inside
the box. Mind you the real new stuff, with the built in CCD cameras,
robotic drives, and such, are pretty darn amazing.
> 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.
Wild/Leitz/Leica still is the ne plus ultra in precision optical gear.
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