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You guys have helped me out with a lot of terms, and I have yet another one. Came across this today, while arm-chair road-tripping:



What's a "turn-point" station? I mean, I'd assume it's where a line 'turned', or changed bearing?




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That would be my guess also. This is a USGS station, and they probably were filling in control for their maps by running lines between known stations as well as doing triangulation networks.


The term "turning point" is most often used in a level run to mean a point where the rod is placed for a foresight from the prior instrument setup and a backsight from the next setup.


I can see them using the same term for a point in a horizontal control traverse.


It might also be an alternative term for an eccentric point, near a major control point but offset for ease of visibility or access.

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Not sure what USGS means in this instance but a turning point (TP) can be a permanent or temporary point used in the course of a level run. When running levels they use temporary turning points which they drive into the ground to hold the elevation while the instrument is moved up to the next setup, then they back sight the TP and set a new one ahead. Once the new one ahead is calculated and everything checks, the TP back is pulled and the rodman moves up ahead to set it again. In the course of the survey they often will use a permanent point so the can break and come back at another time and continue on. I assume that's what they did in this case.


So generally speaking a Turning point is a intermediate point in the level run either temporary or permanent used to hold an elevation but normally not intended to be for record.

Edited by Z15
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Once the new one ahead is calculated and everything checks, the TP back is pulled and the rodman moves up ahead to set it again. In the course of the survey they often will use a permanent point so the can break and come back at another time and continue on. I assume that's what they did in this case.
This sounds like it's possible - given the point is on top of a grain elevator in extremely flat land, it'd be a prime spot to sit atop of and be able to sight in more than one direction easily.


Thanks guys. :sad:

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Our experience is much less extensive than that of some of the above responders, but we're not quite ready to be persuaded that this bolt is a turning point for a leveling run. Triangulation seems much more likely. Our understanding has been that leveling involves considerably shorter sight distances than those set up during triangulation. The challenges of reading a leveling rod at extraordinary distances over flat open country could be insurmountable in most cases.



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I also know the word to refer to a point or object used to set a rod on during a leveling run. Often something with a rounded top. In this case they may have used a physical object 'used' for turning points as a the referred to station object.


If the point is a horizontal mark on top of a grain elevator or such then in context I guess you could say it meant something else, but this description is a bit confusing. Center bolt of what? Occupiable? Anyway it may be moot if you can find this bolt.


Note the actual data sheet describes in more detail "





So I would guess a bolt set in the center of a concrete slab. What they meant by turning point may remain an ambiguous mystery.


- jlw

Edited by jwahl
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I would like to point out that AH2323, is a horizontal control station, so a "leveling run" would not be done to or from it. Furthermore, for the life of me I can't imagine how one would level to a point on the top of a grain elevator unless you had a leveling rod 400 ft long!


In the context of this situation, I would guess "turning" refers to turning angles, and most likely simply means it's a point where the theodolite was set up to observe other stations. Starting with the azimuth mark (somewhere out there) they would assign that angle 0, then turn first clockwise to the other stations being observed and then do the same counter-clockwise. Often this was repeated a number of times to reduce the average errors.


Since there is reference to a "turning point" and a "station mark", this is most likely what is called an "eccentric". An Eccentric was used for observations because of better sight lines, whereas the station mark was further away from the edge closer to the building. The station mark was generally used to observe TO the station (from other stations) and had the flag or heliotrope visible from the other stations. The turning point OTOH, was away from the building ("head house") and was used to observe FROM. The difference was accounted for mathematically in a process called reduction.


That's my take on this.


Here's a diagram for an early triangulation in NY City. The station is the middle of a tower which was observable from other stations. (see KU3532). But they could not set up an instrument on the roof over the finial, and if they set up over the exact point on the platform under the finial (which they had in fact measured) they could not see the other stations through the narrow windows. So they observed various other stations through the windows. In this case there were 5 eccentrics, positioned near the various windows. Complicated, but that's how it was done.





Edited by Papa-Bear-NYC
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Fine, but all of us who have been involved in surveying all our lives say that the term NORMALLY applies to a leveling point. As I tried to say... context may change that.


So in this case a "turning point" may mean an object that the describer understood to be of a "type" that would 'normally be' a turning point" i.e. some kind of round topped object.


Or perhaps the describer has enacted some novel term about a station where angles were turned. But that would be a rare application. Virtually every horizontal station before 1990 or pre GPS would have been determined by angles "turned".


It seems like a unique reference that can at best be described as ambiguous because the TERM means one specific thing!!


Yes it is obvious that is probably what this station IS. But what was in the head of the man in the party that described it? And what was that person's experience?


I still say that the description MAY possibly be of the the 'point' monument 'type' as being similar to what would NORMALLY be a good turning point i.e. a bolt or object with a rounded top surface.


I just don't think the term was normally used to refer to any station where "angles" were turned. That would be on almost every horizontal point between 1760 and 1986" wouldn't it? And do you see that anywhere?


- jlw

Edited by jwahl
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Like I said, I have no idea what USGS is referring to by a "Turn Point Station" but I suspect USGS might have used that term to mean a point that was never occupied but a sight was given to one or more stations afar. In other words, a point who position was calculated from other stations.


This is from a list of Surveying Terms and is the general usage of the term.


Turning Points – Temporary points of known elevation.


The following is from a BLM list of terms


TURNING BENCH MARK – A bench mark set during continuous leveling and used as a turning point.

TURNING POINT – A temporary point on which the rod is held, after a foresight has been made on it, while the

instrument is moved to another station so that a backsight can be made on that elevation.



I have read thousands of descriptions over my 30 yrs as a Survey Tech and never seen that statement on any data sheet before.

Edited by Z15
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Most of the discussion here seems to be about terminology. I think that is NOT where to start.


When I look at the data sheet, the first thing I see is it's a horizontal control station, the second thing I see is it's on top of a grain elevator with a "head house" and the station mark is at the center of the elevator. I also note it's a USGS station and it's third order and the setting is a concrete slab. That's the context and the details of the description should be understood in that context. Terminology should come next.


Two points are described. What were the function of these two points in the context of the occupation of a horizontal control station? There are not a lot of choices.


My best guess it that the "station mark" was not occupied, but was observed, and the "TURN-POINT STATION" was occupied, roughly the opposite of what Z15 said.


BTW: As Foxtrot-xray points out, it's called a "TURN-POINT STATION", not a "Turning point".


I have not read thousands of data sheets as Z15 has, but hundreds yes, and can't remember seeing the term before for either horizontal OR vertical control. The similarity to certain terms used in leveling is seductive, but in the end, I think, irrelevant.


I think if we want to go farther than speculation on this. we need to understand the practices and terminology used in 1966. Compared to today, we know the practices have changed totally. I see people doing jobs along the streets where I live with whom I have often conversed. Typically they would not know a theodolite if it hit them on the head (and that would hurt :laughing: ) - at least the ones under 50. Has the terminology also changed - you bet it has. Anyone know a retired surveyor who worked on a horizontal control team in the 1960s? Preferably in Texas?

This is where George usually comes to the rescue. :P George?

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