We do a lot of transportation work, and locating low wires are normal for us. We typically use the vertical angle offset routine. We have a crew member look for the lowest point in the line, and puts the rodman on line, and the rodman will "eyeball" themselves under the line. We'll take a ground shot, and then turn the vertical angle up to the line. We note date, time, and approximate temperature, and it seems to work fine. There are safe clearances worked into the design, so this is typically what most people do around here.
We do a lot of transportation work, and locating low wires are normal for us. We typically use the vertical angle offset routine. We have a crew member look for the lowest point in the line, and puts the rodman on line, and the rodman will "eyeball" themselves under the line. We'll take a ground shot, and then turn the vertical angle up to the line. We note date, time, and approximate temperature, and it seems to work fine. There are safe clearances worked into the design, so this is typically what most people do around here.
Must be a southern thing Jimmy. I remember when I first started doing electrical utility surveys, all the techs were using hand held lazer?ÿ distance meters. I blew them out the water.?ÿ
I know in trimble access I think it is points on a plane you shoot the two ends reflectorless and then just turn angles to the other points on the cantenary. But by the time you do that you might as well shoot them all reflectorlessly. We have to shoot all the attachment points as well. Make sure to record time and tempature.
The way I used to do this was to locate each point of attachment with a unique identifier code, then shoot the sag reflectorless with the same code.?ÿ Drew automatically.?ÿ Alternately, if it was too far for reflectorless, I would shoot a point with an obviously incorrect distance using "angles offset" then turn and store the correct HA and VA.?ÿ The line would draw automatically, but to the wrong location.?ÿ Then I'd draw a line through the two connections - draw a 3D line from the point the TS was on (must add HI) and through the incorrect shot, then exten to the line between the attachments - move the point to the end of the new 3D line.?ÿ A perfect location...
Yes a catenary curve is what you have and it has a formula
Yes a catenary curve is what you have and it has a formula
As a rhetorical comment on field procedure I would like to relate the following observations:
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Years ago at the highway department we always calc'd "low wire" clearances (with a TS) by taking a shot "under the conductor" and then turning a vertical angle up to the conductor.?ÿ While this method seemed to work there were a few times things didn't turn out so well.?ÿ
It is not so easy at times for a rodman to guess at a spot "exactly" underneath a cable crossing.?ÿ And if the instrument is fairly close, the vertical angle can be sharp enough to produce a good few tenths of a foot (I've personally seen a half a foot)?ÿor so of vertical error in a simple triangle solution.?ÿ If nothing else?ÿanother crew member?ÿshould view the span from one pole to the other and give the rodman a good "on line" location under the span.
'glass height poles or a reflectorless instrument are so much better for that application.
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The formulas I employed many years ago to do sag checks on EHV transmission lines allowed me to actually use the hyperbolic functions on my trusty 41CV.?ÿ
I don't remember where it came from but my calculations were based on an obscure paper written by some rocket scientist type guy named Donald Dean.?ÿ
Some of the inputs required were span length, weight/ft., temperature, elevation difference for attachment points. I wrote a series of programs that allowed me to sit at a convenient point offline and sight a point on the belly of the wire or to sit under one of the towers and observe the sag along the line.?ÿ
Some of the contractors used a return wave method which although a sort of flat earth approach seemed to work pretty darn good.?ÿ
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