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I was looking at the theodolites used by the USC&GS in the 1890s. I am quite curious about field observation procedures and field note recording.
Aa the instrument has 3 microscopes to read the plate, D and R readings, and given 16 sets, how were the data handled on paper.
In the Transcontinental Triangulation of the 39th parallel, the section labeled Abstract of Horizontal angles, I am curious just what was averaged.
I couldn??t find examples of field note recording, assuming they used forms.
And/or manual of how to operate and read the instrument.
The angles are reported to 0.001? and numerous correction values. Just curious to better understand a bit more of what is what.
That is an amazing instrument to look at.
Does D and R mean direct and reverse?
It would seem the telescope is too long to invert given the height of the standards.
should be four microscopes (one to read each quadrant) scope does not invert as you are reading all four quadrants
The documentation of the 39th parallel and photograph of the instruments used clearly describe and show 3 microscopes.
and that Direct and Reverse readings were always taken.
see the photo above. And this photo from the 1900 report. 3 microscopes 120° apart
Here is one from that time period, hard to tell if it has 3 or 4 microscopes. Not sure if it could be inverted. Also had mirror signaling attached to transmit instructions to attendants at distant points
That appears to have 3 microscopes. However there are instruments of differing designs. The 1900 report clearly stated 3, and the USC&GS photo shows 3.
While old, I’m not old enough to have any experience with this instrument. I believe it is the one designed by Fischer. A link to the NOAA Photo collection with some information (including reference to four telescopes) is:
https://celebrating200years.noaa.gov/theodolites/fin.html
As for the question of booking observations, observer read off readings the recorder transcribed them into a survey book. Observations were then abstracted with mean values for pointings then transcribed into a List of Directions.
Be sure to read the text on the page linked that describes issues related to the cost of instruments and how this was addressed.
The NGS facility in Corbin, Va now know as Instrumentation and Methodologies Branch was once the Instrumentation and equipment branch. The branch fabricated a lot of equipment in their own machine shop.
Contrary to the photo site’s description, Joseph Dracup’s information history of the USC&GS see link in image we see this
BTW, the Fischer was replaced by the Parkhurst that in turn was replaced by the Wild T3. Lots of info on the Parkhurst is included in the Manual of Geodetic Triangulation available as a PDF under the Publications link on the home page.
obvious point: observer recites the reading and the booker records it. Target reading minus initial reading. Yeah that’s a bit ….
My question is about the microscopes. Were all 3 read on every pointing? Were the 3 micro readings averaged.
in the 1900 report there’s a very complicated example with 132 sets, 683 observations.
the maths employed to get that into the abstract are far and away more complex than I’d seen anywhere. Were all 3 microscopes read on every pointing.
I’m a t3 guy from way back. The recording of 3 microscope reading is not obvious, and maybe they read only one, rotate the plate and read another one, successively using them one at time with each set.
its such a fantastic survey I am curious about the recording which I could not find in the report.
And regardless of 4 micro instruments in existence the reports cites 3 microscope instruments and photo of same. So perhaps later a 4 scope was introduced. But the narrative says 3 microscopes were the norm.
Pgs 36-46, section C. The abstract of directions.
skim that. It’s a handful.
https://geodesy.noaa.gov/library/pdfs/Special_Publication_No_4.pdf
The point of having 3 or 4 microscopes is to be able to average out centering error in the angle scale and variations in the graduations themselves. So for the most accurate work the average of all of them would be used.
