CONTEXT OF QUESTION
- I am determined to learn how to survey, even if I am 71. It is considerably different from my expectations and hence it is taking me awhile to get on board. My textbooks are Surveying Instruments and Methods for Surveys of Limited Extent by Kissam and another text book with Kissam one of the three authors: Surveying Fundamentals and Practices
I just bought a used Leitz TM-20c and tried it today.
WHAT I OBSERVED
What astonished me was that with the lower clamp tight, the lower tangent screw moved the telescope aiming point beautifully but did not change the horizontal reading when I looked through the circle and vernier reading microscope. With the upper clamp tight, the upper tangent screw DID moved both the telescope aiming point and the horizontal circle
QUESTION #1
is my above experience with the tangent screws the experience of others?Ā
QUESTION #2
What is the connection between the lower tangent screw and the horizontal circle?Ā
It seems the lower clamp is working as the telescope moves when the lower tangent screw is moved. Perhaps what is happening is that the horizontal circle, which appears to have an outside ring available for me to touch, is moving with the lower tangent screw and hence there appears to be no visual change on the horizontal circle reading
Any insight will be appreciated.
Replies
Dear Mr. Burling,
As you study some more you will realize how much of surveying is NOT about the instruments of measurement, but about the analysis of evidence, much of it from measurements.
Theodolites come in two flavors. Those with a tangent screw on the lower movement like yours are called repeating.Those without a tangent screw on the lower movement are called directional. Both may be operated in the manner of a directional theodolite with superior accuracy than the typical way a repeating theodolite is used.
The repeating theodolite was a compromise product for the American market that had grown very used to the Vernier transit. The method of "winding up an angle" involved repeatedly using the lower movement toĀ again aim at the backsightĀ and then turn to the foresight using the upper movement. Doing so several times effectively multiplied the precision of the Vernier thereby increasing the precision of the measurement.
A theodolite doesn't use a Vernier so adding up the angles does not increase the precisionĀ at all, but because so many were in the habit of doing so itĀ was good marketing to accommodate the buyers and add a tangent screw to the lower movement.
The proper method for increasing the precision of a theodolite is to take several sightings using different parts of the index circle, thereby compensating for any eccentricity in the circle. One does not "set" a reading on the backsight. One sets the reading near (not purposely) on zero. Record whatever the backsight number is.Then turn to the foresight with upper movement. Take the reading and subtract the backsight from the foresight. Then repeat the process with the backsight somewhere near 90Ā° and again somewhere near 180Ā°, and again somewhere near 270Ā°.Ā
Take those four differences. Average them. That becomes your observation. Trust me. If you use that method with any optical theodolite your results will be superior to using the lower tangent screw in the repeating manner of a Vernier transit. I've seen the closing ratio of a significant traverse improve 50% to 100%.
JAC
"the analysis of evidence"
If I understand what you mean by "analysis of evidence" then the above phrase is actually the reason I began my quest to learn surveying.Ā
I understand analysis of evidence to actually mean analysis AND synthesis.Ā To me analysis of evidence means to define deed declarations against the ability of physical earth to support the deed's claims. The evidence is how well the earth supports the deed.
But synthesis comes into play, I suspect. What happens when the physical earth does not support the deed's declaration.Ā I suspect the surveyor is burdened with the responsibility of finding a solution that not only approaches the intent of the deed but also fits with other declarations (other deeds, landmarks, man made features, many of which were intended and unintended encrouchments).Ā
My wife paid for an A2 survey back in 1992.Ā The survey revealed that the deed's measurements from A to B to .... were not accurate, or at least not reproducible with whatever earthen changes have occured since the deed origin (I live adjacent to tidal water so land/water changes do occur).Ā So the Surveyor's survey revealed the outcome of his attempt to synthesize a solution that might be agreeable to all parties if any potential contest should go to court.
So...regarding "anaylsis of evidence"....have I understood what you were saying to me?
Yes, I think you have understood. Performing a boundary survey properly is not dissimilar to a proper genealogy study or even a detective's pursuit of discovering the purportator of a crime.
The technician's job is to gather evidence. It is the surveyor's job to understand the evidence and, if synthesize a scenario that fits the evidence and leads to the most reasonable conclusion.
JAC
WOW
Thank you for taking the time to explain the history here.Ā So interesting.Ā It seems just about all fields are plagued with a similar issue - marketing perpetuates an old habit because it sells, not because it is better.
I very much appreciated your comments and suggested procedure.
I look forward to testing your recomendation out !!!!Ā I can look at my closing ratios and see whatĀ happens.
thank you once again.
bil
William, the manual shows you how to set and read it,again hope this helps. Maybe it's not broken.
Hi William,
I have a manual to your gun, See if this is it. it will explain everythingĀ on what Ed told you, if this is it.
It has a micrometer window , the TM-20C Theodolite has a remarkably accurate optical system.
Hope this manual will help you with it.
Thanks for the TM20c manual online. I have one that I have owned now for 20 years or more, and have never had a manual for the gun.Ā This is great!!
Harry Girdler, KY PLS2075
Hi Harry,
Thanks for the shout back, glad i could help.
I have always like LIETZ instruments .sorry you had to wait so long.
Now you have one to go with it. It is a Jewel.
bb
Thank you very much, Billy
I downloaded the document and am currently looking it over. Ed talked about needing a magnifying glass to read the vernier, but i think i will be needing a mangnifying glass to read some of the fine print (diagram numbers!!!)....just trying to joke. sorry about that.
Hi William,
I like to Joke myself, but about a year ago a surveyor told me he had run a transit before. We where looking at some old Plats, and Maps and I reach in my pocket and pullout a Transit magnifying glass.Ā He ask me what kind of magnifying glass is that.Then I ask him, you never seen one of these. Then what kind of Transit did you use. No your instrument was a upgrade from a transit you no longer had to look at a Vernier with a magnifying glass and some of the more modern Transit had a fixed magnifier . There is a art in reading a Vernier on a Transit,but yoursĀ has a micrometer window that takes in light from mirrors.
And it is also sit up to work like a transit that is why it is called a Double Center, because it has a upper and lower plate.Today modern instrument do not use this system, I have seen a Total Station with two plates,but you will not find one today. Your instrument was great for doing Sun Shots because of the speed it could be read and it is treasure to have one now. So you want need a magnifying glass and the PDF does have a magnifier at the top,just bump it up and you can see everything. I keep a glass on my desk. lol.
bb
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