[TowerTalk] Transit Accuracy Statement Retracted

[Bill Ralston]

Back on 11/26 I made the statement that I was able to achieve "accuracy of
1/2 inch in vertical and horizontal" when laying out guy anchor points for
my tower. When asked how I knew this, I realized I don't.  I was mistakenly
thinking that, since I could see the "stick" scale 80 feet away clearly
through the transit sight that I knew it was that accurate.  Although I may
have that much _resolution_, the _accuracy_ is a function of how well the
transmit is pointed in azimuth and elevation.

For the particular transit I rented, the azimuth scale was calibrated in
degrees. Although it has some kind of vernier scale, I never figured out how
to read it. Hence, my azimuth accuracy was probably about a degree, or 17"
at 80'; a lot less than 1/2" (but fortunately well within the requirements
of the tower!)

For elevation, the level was quite sensitive.  Based on the repeatability of
my measurements when I moved the "stick" I think it is probably accurate to
better than a foot at 80'.  This is supported by previous (tedious)
measurements of the elevation differences that were done with conventional
level and agreed within about 6".

I have a newfound appreciation for how it is that you often find several
surveyer marks at the corner of a property, none of which seem to agree on
where the actual corner is!

(By "stick" I mean the big ruler that is held vertically by one person and
is viewed through the transit to read off elevation difference by the second
person).


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