@john-nolton ?ÿ
While I hope that everyone reading my posts will carefully read them; sometimes I do not do so myself. Your explanation is fine. I did get confused by your use of the 45 degree angle as this is a circle setting. I now see you use ??45d23m56s? and not 45d.
I had wanted to show how/when the plate settings changes for multiple readings take place. Your example was for one set.
Lest anyone think that each circle starts with telescope direct, I attempted to show that (in the four position case and using degrees only) we make circle setting changes after returning to our first target.
Ignoring pointings to the other sites and only showing values to the first target and only to degrees, we will have:
set 1
D 000d
R 180d
change circle to 225d
set 2
R 225d
D 045d
change circle to 90d
set 3
D 090d
R 270d
change circle to 315d
set 4
R 315d
D 135d
BTW, I do not see the difference between your reference for circle settings and SP 47. Of course, I did get a note from my optometrist saying I am overdue for new glasses??
@geeoddmike?ÿ ?ÿI like you BTW remark. I gave 2 references and can give many more.?ÿ
Back in the early 70's I was working for a Large Civil Service Org. I found out that doing night work with the Wild T3 they would always turn the scope back to the direct position to start the next set. They had never been schooled on the easy and fast way; of which I did school them. Before I taught them the easy and fast way I offered them a bet; I can turn faster directions than anyone of them. I had NO takers. I only used one set to show how to record and turn directions which Shawn said he had not done before. The next lesson is to show?ÿAS YOU HAVE DONE how too turn many sets with the different circle settings and how to record them (the term "book them" is used in some places), mean them. I will also show how to turn Zenith angles that I came up with in 1963 that was not put in the manual till the middle of the 70; that I know of.
@geeoddmike?ÿ Thanks for that.
A question for John Nolton. In your procedure for measuring one angle, you start in Face I, point to the back sight, turn clockwise to the fore sight (both times the slow-motion tangent screw is turned clockwise (into the spring); followed by inverting the telescope (Face II) and turning counter-clockwise to the fore sight and counter-clockwise again to the back sight.
I'm probably doing it all wrong by always turning clockwise whether I'm in Face I or Face II.
I'd appreciate your thoughts on why turning the direction theodolite clockwise for direct readings and counter-clockwise for inverse readings is the preferred method.
P.S. Thanks for pointing out that the last movement of the slow-motion tangent screw should be clockwise. As explained by an old mentor, that is preferred because the spring in the tangent screw is being compressed. If the last motion is counter-clockwise then the spring is being released which can produce less accurate pointings.
I just ran the inflation calculator from 1972 to 2020 (I don't think we really know what the current inflation is, so I backed up a couple of years). In 1972, $4,000 would be equal to $25,000 in 2020. Likewise in 1972, $12,000 would be roughly $75,000 today.
THRAC alert!
It is 2022, so you need to pump those numbers up quite a bit!
If a Chevy Blazer cost $4000 at the time, an equivalent in power, off-road and capacity...which you cant buy it anymore...so standing in is the cheapest new 4x4 Chevy pickup I could find in the USA... costs $34k.
A top of the line, brand new Leica robotic total station, FTL lists at $23k...FTL has a Leica GS18 I and controller for $30k.
Perhaps top of the line total stations are deal now!
A question for John Nolton. In your procedure for measuring one angle, you start in Face I, point to the back sight, turn clockwise to the fore sight (both times the slow-motion tangent screw is turned clockwise (into the spring); followed by inverting the telescope (Face II) and turning counter-clockwise to the fore sight and counter-clockwise again to the back sight.
I'm probably doing it all wrong by always turning clockwise whether I'm in Face I or Face II.
I'd appreciate your thoughts on why turning the direction theodolite clockwise for direct readings and counter-clockwise for inverse readings is the preferred method.
P.S. Thanks for pointing out that the last movement of the slow-motion tangent screw should be clockwise. As explained by an old mentor, that is preferred because the spring in the tangent screw is being compressed. If the last motion is counter-clockwise then the spring is being released which can produce less accurate pointings.
Gene the theory about turning clockwise and then counterclockwise is: when you turn one way with the theodolite?ÿtorque might build up. So turning the opposite way you will relieve the?ÿtorque.
@john-nolton It was explained to me that turning the instrument only in a clockwise direction would create a uniform "torque" in the tripod and/or tribrach that would eliminate this small systematic error.
In this case, we have an avid new owner wanting to turn some angles. So if Shawn is up for it, he can turn 16 or 32 sets of angles with clockwise/counter-clockwise rotations for the direct and inverse readings (you have to do this with a parasol or at night Shawn) followed by the same number of sets with all angles being turned in a clockwise direction. This does not mean that you have been voluntold without notice, Shawn! 🙂
If I were to guess, I'd say that either method will produce similar results on a T2. If there are any differences, my guess is that you'd need to use a T3 to see the small differences.
@gene-kooper?ÿ I can only tell you what the?ÿTheory?ÿsays. I have no data to show you. I can say this that from my post above to now (in time) I looked in my Wild T2 manual ( new style T2) and on page 31 says "As only two targets are sighted measuring time is short, thus systematic errors from residual changes in the verticality of the standing axis and twisting of the tripod can practically be avoided".
I have old journals that the theory was presented in, but I do not have the time to look for the article. I will find one about different oil/grease used and errors that can happen. I will post that soon.
Thanks John.
Congratulations Shawn, that was a good looking T2. Yes, good meeting face to face again, well attended conference.
SHG
Wow.?ÿ What a blast from the past!?ÿ ?ÿI learned to survey on these instruments in the early '90s at Ft. Belvoir VA.?ÿ If I remember correctly, the bubble on these things were so sensitive that the sun played havoc with it.?ÿ ?ÿ?ÿ
Should you take up the suggestion to actually turn multiple sets of angles, you might want to observe to intersection stations published by the NGS.
While your profile only identifies your location as east Texas, I took the liberty of retrieving some information for the city of Houston.
You can readily find these intersection stations via the NGS Map and get an overview of the geodetic control using the diagrams available in the link below.
While there are many intersection stations in the data base, remember that Houston is in a subsidence area, and that tanks, radio masts, church spires, etc could have been changed or destroyed then first positioned. Also carefully read the description to determine the point observed.
Given good geometry, observing at least four sites (and watching for ??swingers? ) you can determine your position with checks. Follow the example in the USC&GS SP 47.
Links follow:
https://www.ngs.noaa.gov/data/survey_maps/gcdmap/ - page showing CONUS searchable for your area of interest
https://geodesy.noaa.gov/library/pdfs/Special_Publication_No_247.pdf ?ÿ- see the section titled ??Special Angle Computation? digital page 194 of 361.
https://www.ngs.noaa.gov/data/survey_maps/gcdmap/pdfs/Texas/Houston_Texas_NH_15_7.pdf ?ÿ ?ÿ - geodetic control diagram showing Houston area
Even more interesting would be to eccentrically occupy an NGS monument and do the reduction to center computation.
Working with ??real? data helps validate your work.
@beer-legs I just checked. It's a DM-S2. I don't recall which model Dad had, I think a 3. It had slope reduction, I do recall that. This S2 has slope reduction also, but it doesn't look quite like what I remember Dad's looking like. I haven't tried turning it on yet. I wonder if I can re-cell the batteries for it.
You are correct, I had forgotten that each even round starts with the scope reversed, and should be 135 instead of 45, for example.?ÿ
you might want to observe to intersection stations published by the NGS.
Just be careful about interpreting the results. Depending on where you are, you may find the intersection stations don't match GNSS results.
I'm ~40 miles from the nearest triangulation station with GPS constraints in the adjustment. I find the two surviving and easily visible intersection stations near me, and a tri station that had no GPS data, to be most of a foot from matching GPS coordinates. That's great accuracy in parts per million, but unusable in a GNSS world.
@bill93 To reinforce what you said...
1) the intersection stations were not adjusted on NAD83 (2011)
2) they are third order (1:10,000)?ÿ
3) guyed antennas, for example, often have the guy wires re-tensioned periodically, lights are changed, etc
4) the accuracy of their positions depend on the geometry when they were observed to since they are angles only.?ÿ
I found them useful when running traverse since if you observe to any of them from more than 2 traverse stations they will add strength to the traverse (not using the published coordinates but rather computing new coordinates in the adjustment) and can serve as backsights from the traverse stations in future occupations.?ÿ
In Pittsburgh one of the intersection stations is the University of Pittsburgh Cathedral of Learning (PITTSBURGH CATH LEARN ANTENNA, PID KX2072), visible for many miles in all directions. It is about 550 feet high, and very distinctive...
?ÿ
If you are ever in Pittsburgh, go to see the nationality rooms in the building...
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Here is a picture of the inside on the first floor...
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Back in the late 80's, we did a few control projects for a proposed highway from Morgantown, WV to Pittsburgh. At the time, one of the requirements was that all stations had to have an azimuth mark. The antenna on top of the Cathedral of Learning was visible from many stations. So, I was able to get permission to set a GPS station on the roof, a few feet away from the antenna. I was using a Trimble 4000SL, which was susceptible to interference. There was a microwave antenna which connected the campus with the mainframe computer located about 15 miles away. My first setup the microwave wiped out the signals. Then I set the tripod lower so that the stone wall blocked the microwaves, and got a good solution. After that I used a T2 and backsighted another station and turned to the center of the antenna and measured over to it with a tape. At the time there were no HARN or CORS, so we used first order triangulation stations for control in GPS networks.?ÿ
The coordinate I computed for the center of the antenna, which had a diameter of 18" or so, was within 0.3 feet of the published NAD83 (86) value if I recall correctly.?ÿ
My suggestion was that instead of merely turning angles using multiple plate settings to quantify instrument performance the OP consider instead using intersection stations to compute a position. A more interesting exercise.
You both make valid points about the low quality of intersection stations. Nowadays positioning at the level of 1:10,000 is trivial using GNSS.?ÿ
As John Hamilton mentions, the inclusion of observations to these points during a traverse is quite beneficial. In fact, in urban areas the existence of multiple intersection stations allowed the use of resection methods that were quite efficient when positioning targets for photogrammetry.
WRT the datum for computations, I would use the superseded NAD83(86) positions. Computations use inverse distances for the known points.
BTW, when observing be sure to check for the point observed e.g. spire, flagpole, center. When observing a smokestack or similar large structure consider observing the L and R tangents and use the mean of the tangents.
Some information about intersection stations captured from NOAA Professional Paper 2 follow:
Several years ago, when we first started using static GPS receivers, I noticed several radio towers in our work area. One or more were often visible from our jobs and I thought it would be handy to determine their location as azimuth marks in cases where we couldn't establish pair points that were sufficiently far apart to get a good azimuth.
In my spare time, I would setup pair points relatively close to the antennas (within a mile) and I would turn sets of angles to strobe at the top of the tower. I didn't have least squares software at the time nor an understanding of least squares, but from the multiple turns I made, I felt confident that I knew the position of the strobes to within 3cm horizontally and 5cm vertically.?ÿ
Later I learned the math behind the three-point resection and as it so happened eventually we landed a job where I could see four of the towers I had located. We were using a Topcon 5" total station. I turned multiple sets (around 12 as I recall). The towers were 2- 5 miles away as I recall and well spaced around the site. We also performed static GPS observations of my total station observation point. Again, I did not have least squares software at the time, so I picked the best spaced three stations to perform my resection. As I recall, horizontally the resection solution was within 7cm of the GPS derived position, and 8cm vertically.
I agree that this would be a really good way to test this old T2 and it would probably be best if I determined my own targets for all of the reasons John mentions regarding legacy marks. I had figured that with my procedures I was probably able to get 2 second accuracy from that 5" Topcon. With least squares software I could definitely create a much better test bed and perform much better analysis. The downside is that I had much more spare time back then and seem to have a deficit of it as of late.
I will confess, I've had a difficult time concentrating on work because I keep wanting to take that T2 out and turn angles. Thank you for the great suggestion regarding triangulation. I think it would be the best and perhaps only way to really nail down the accuracy of this great instrument.
@shawn-billings?ÿ ?ÿUsing ISO 17123.XX will tell you how to test your instrument and show the Math to see if your instrument is working according to manufacture spects. I do not have the correct number in front of me but YOU can find it by internet search.
Electronic tacheometers is 17123-5.3?ÿ (this was the number when I worked on the specification, way back.)
JOHN NOLTON