Scan it.
This is so exciting; I can actually afford an opinion!?ÿ I was the hired forensic surveyor in a situation where a military helicopter dropped low into a canyon, cut a powerline and caused a fire which?ÿ caused major damage to private property.?ÿ The crux was he was too low, proved by my?ÿ survey which showed?ÿ even at?ÿ maximum line tension?ÿ there was no way he should have gone below the lines safely according to airflight maps.?ÿ
Since it was forensic we did the big survey and recorded every line balllup,?ÿ debris fields,?ÿ likely fire startups, pole damage, etc.?ÿ Got paid for two weeks work, $22,000 bucks.?ÿ Everybody accepted our?ÿ survey as the facts,?ÿ so the payoff was in the millions; I made my bucks?ÿ by simply providing the facts.?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
?ÿ
HC mentioned the term "catenary" in his post and I thought I would add?ÿsomething I was a party to some time back.
A few years ago a?ÿyoung and very smart facility engineer with a power company took some of my field observations and?ÿcompared them to a calculated sag with some of their high-brow software.?ÿ Predictably the calc'd sag and the field data were very much congruent?ÿto one another.?ÿ The differences were usually in the hundredths of a foot; acceptable in the real world; and?ÿfew exceeded a tenth or so.?ÿ
At first?ÿwe chalked these up to the error found in observation and left it at that.?ÿ But the engineer checked a number of instances and found some differences that appeared to him to be in predictable spots.?ÿ Over a period of time he continued to scrutinize the differences.?ÿ?ÿ Of course he was far more interested in this than I was.?ÿ From time to time he would call me and pick my brain about field procedures and the equipment until he discovered his "aha" theory.
As in all mathematical formulae we depend on "constants".?ÿ Some of the initial "fixed" values utilized by his software was (of course) the beginning and end of the sag.?ÿ He found the field data and calc'd sag could better corroborate each other by?ÿ"fixing" the mid span values and allowing the formula to solve for the end points.?ÿ The differences were not outside of?ÿacceptable error, but did show me some haunting predictability.?ÿ The sags were always a little longer than I reported and the "attachment" elevations wound up a little higher.
He brought some typical attachment hardware out to a site and we compared what I was looking at through the TS and what was really going on.?ÿ It was a good learning moment and eye opening for both of us.
Long story short, the beginning and end of a catenary sag are critical point and possibly more critical to the observations than the sag shots taken along the conductors.?ÿ The hardware varies greatly with the type and size of the conductor.?ÿ The "beginning" and "end" of a sagging span needs to be located just as precisely as mid-span points, if not more so.
Lived and learned.
The manual is quite useful for explaining Survey Pro's nuances.
https://www.trimble.com/globalTRLTAB.asp?nav=Collection-67418
[Survey] [Plane and Vertical Angle Offset]
The Plane and Vertical Angle Offset screen allows you to define a vertical plane by measuring distance and angles to
two points, and then store any number of points on that plane by measuring angles only to those points. A common
usage of this routine would be to measure the location of two power poles with a prism or reflectorless observation, and
then observe the power lines between the poles with angles only.
Scan it.
Taking a sledgehammer to a nut surely!
HC mentioned the term "catenary" in his post and I thought I would add?ÿsomething I was a party to some time back.
A few years ago a?ÿyoung and very smart facility engineer with a power company took some of my field observations and?ÿcompared them to a calculated sag with some of their high-brow software.?ÿ Predictably the calc'd sag and the field data were very much congruent?ÿto one another.?ÿ The differences were usually in the hundredths of a foot; acceptable in the real world; and?ÿfew exceeded a tenth or so.?ÿ
At first?ÿwe chalked these up to the error found in observation and left it at that.?ÿ But the engineer checked a number of instances and found some differences that appeared to him to be in predictable spots.?ÿ Over a period of time he continued to scrutinize the differences.?ÿ?ÿ Of course he was far more interested in this than I was.?ÿ From time to time he would call me and pick my brain about field procedures and the equipment until he discovered his "aha" theory.
As in all mathematical formulae we depend on "constants".?ÿ Some of the initial "fixed" values utilized by his software was (of course) the beginning and end of the sag.?ÿ He found the field data and calc'd sag could better corroborate each other by?ÿ"fixing" the mid span values and allowing the formula to solve for the end points.?ÿ The differences were not outside of?ÿacceptable error, but did show me some haunting predictability.?ÿ The sags were always a little longer than I reported and the "attachment" elevations wound up a little higher.
He brought some typical attachment hardware out to a site and we compared what I was looking at through the TS and what was really going on.?ÿ It was a good learning moment and eye opening for both of us.
Long story short, the beginning and end of a catenary sag are critical point and possibly more critical to the observations than the sag shots taken along the conductors.?ÿ The hardware varies greatly with the type and size of the conductor.?ÿ The "beginning" and "end" of a sagging span needs to be located just as precisely as mid-span points, if not more so.
Lived and learned.
Do you mean that the cables did not form a perfect parabola (catenary)? Maybe this is due to some stiffness of the cable at the insulator ends. so that there is a bit of "hogging" ie it doesn't change direction instantaneously like a parabola.?ÿ
This was a trick question, and all of you missed it.?ÿ The real answer:?ÿ you don't locate the sag in an electric line between 2 telephone poles.?ÿ They're telephone poles, so there are no electric lines between them.
Now, if you want to talk about joint utility poles or power poles, well, that's a different matter.
Ah but if there is an electric line between two telephone poles then super important to survey it!
Posted by: squowse
Do you mean that the cables did not form a perfect parabola (catenary)? Maybe this is due to some stiffness of the cable at the insulator ends. so that there is a bit of "hogging" ie it doesn't change direction instantaneously like a parabola.?ÿ
A cable that is very flexible forms a catenary curve.?ÿ A parabola is a quite different mathematical equation.?ÿ Ignoring scale factors and letting x be distance +/- from center,
Parabola y= x^2?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ Catenary y = e^x + e^-x
Departures from the catenary could indeed be caused by stiffness, particularly at the end connections, causing the curve to be more of an inverted bell shape than the mathematical catenary.
Posted by: squowse
Do you mean that the cables did not form a perfect parabola (catenary)? Maybe this is due to some stiffness of the cable at the insulator ends. so that there is a bit of "hogging" ie it doesn't change direction instantaneously like a parabola.?ÿ
A cable that is very flexible forms a catenary curve.?ÿ A parabola is a quite different mathematical equation.?ÿ Ignoring scale factors and letting x be distance +/- from center,
Parabola y= x^2?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ Catenary y = e^x + e^-x
Departures from the catenary could indeed be caused by stiffness, particularly at the end connections, causing the curve to be more of an inverted bell shape than the mathematical catenary.
fair enough.
every day's a school day
Posted by: squowse
Do you mean that the cables did not form a perfect parabola (catenary)? Maybe this is due to some stiffness of the cable at the insulator ends. so that there is a bit of "hogging" ie it doesn't change direction instantaneously like a parabola.?ÿ
A cable that is very flexible forms a catenary curve.?ÿ A parabola is a quite different mathematical equation.?ÿ Ignoring scale factors and letting x be distance +/- from center,
Parabola y= x^2?ÿ?ÿ?ÿ?ÿ?ÿ?ÿ Catenary y = e^x + e^-x
Departures from the catenary could indeed be caused by stiffness, particularly at the end connections, causing the curve to be more of an inverted bell shape than the mathematical catenary.
A catenary weighted at equal intervals with equal size weights will form a parabola.
In addition to vertical offset method, It also can be solved by recording the vertical and horizontal angles from three known points. There is a formula to this method.
2 angles only observations from 2 known points will do it but the difference between this and a roof?ÿ crown (for example) is - how do you find the same points on the cable again?
Vertical offset method also leaves a lot to be desired as it's time-consuming and inaccurate to attempt to position a prism exactly underneath the cable. Might not even be safe or possible.
?ÿ
I agree with Mr. Marks. Remember the coefficient of expansion for a steel tape? While doing 500KV line rebuilds the inspector told us the line can sag as much as 9' between?ÿtowers (800' between towers). We did all the lines running to/from a coal fired plant, set up about 90 degrees and shot the distance/elevation to the lines with the reflectorless EDM. It's important to note the temperature AND time of day so the electrical engineer can see what the load was on the line because it will cause sag also. We mapped the line at the hanger wire on the insulator and the sag. The elevations at the insulator points it will be a straight line between them so the sag point can be determined as the low point. The sag will not be midpoint if the towers/poles are at different elevations because of the load, temperature, and gravitational pull. It was one of the fun, interesting jobs I've done.
If you can get it with reflectorless than that is the best way. If not, then assuming there isn't much wind (note: not only can a wind blow the conductors sideways, but it will also cool them - so wind speed and direction also need to be recorded as well as air temperature, steel temperature - taken off the tower legs with a magnetic thermometer on the shaded side, and exact time of observation, ideally recorded automatically with the survey observations.) you can get a good approximation by intersection.
The way to do this is to use two stations to first intersect suspension points and the clamps at each end of the insulator strings on adjacent towers. This gives you two pairs of points which should be the same distance apart and orientated in the same direction. If the directions are significantly different then the wind is blowing the conductors sideways.
On a twin or quad conductor you can't rely on intersecting the spacers, as any movement of the conductor between instrument set ups will nullify the results. On a single conductor there is nothing to identify (birds don't sit still for long enough!)
Therefore the best intersection is to take a number of shots HA/VA along the conductor from ONE station, preferably nearly square on the the line (for twin and quad lines visually take the centroid), working from one end of the span to the other. Repeat in the reverse direction.
You then have two lots of intersections onto a plane between the two end clamp points. Calculate each set separately - they should give good agreement. Combine the pair, graph out and curve fit: you will have the best answer you can get without reflectorless. The operation is very fast (assuming your fingers are!) so the conductor load is unlikely to change much during the observations. If in doubt, pick the observation time to be between adverts on the most popular TV programme showing in the region as this is probably the most stable load moment. At advert breaks everybody switches on the electric kettle and the load peaks.
I've done this in conjunction with a "live line" team (out on the arm taking direct temperature readings from the conductor - better them than me) as part of some experimental work on a 440kV line and our results came in within a couple of cm. of the predicted load calculations.
?ÿ
