Yes, I learned to respect power lines after hearing the story of how one of the founder's of a local survey firm was lucky to be alive. They were bringing in a benchmark and the founder had the level rod fully extended. Keeping his eye only on the IM, he touched the sagging hydro line and enough electricity seared through him that it blew him back from the rod. The safety toes in his boots "burst" the leather.
He was lucky to be alive.
It taught me to always double check above me before I raised the rod--no matter the rod height.
The contractor asked us if we could measure the sag in the power lines.
There was a vertical offset program on one of the old TDS cards that might suit your purpose. You shoot the prism under the sag, then turn a vertical angle to the sag. The sag, technically called catenary, is necessary for different reasons.
If you can reliably measure to the cables reflectorlessly then simply do that.
Otherwise you can measure each end of the cable (with distance) and then angle observations to intermediate points on the cable will yield a coordinate if you process it after. I have previously used a spreadsheet to do this.
Actually since then there has been added to trimble access?ÿ a "measure points to plane" function in the measure menu.
This allows you to measure the insulators (or similar) at each end of the cable and define a vertical plane. You can then record points "angles only" on that plane. Access will output coordinates with no further processing required.
See extract from Access manual attached
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Only if you cannot sight the cables should you need to start thinking about scanners. That's such an expensive and long winded way round a simple problem.
I measure these powerlines sometimes with TLS. The pic below is 4 scans from a reigl vz400i. 10 minutes on site. It was to measure the distance between 2 crossing lines.
<a href="https://ibb.co/V3zRVPw"><img src="https://i.ibb.co/PtPKxfD/2019-08-16-10-37-36.jpg" alt="2019-08-16-10-37-36" border="0"></a>
To clarify, exact time is essential - load changes can cause substantial movement in seconds. I once observed a 275kV line drop by over 0.3m in a ten second period just as a very popular TV programme finished (everybody switched on their kettles!).
As far as temperature is concerned, unless you have a way of measuring the conductor temperature you need to record two figures - open air temperature and shaded steel temperature. the latter is measured by using a surface clamp thermometer on the shaded side of a steel tower leg. Just remember to pick up the thermometer as you move on! The supply company can then use these to calculate corrections to the observed sags.
Unless you can plumb up from a prism pole accurately, the results using that method are poor. If you can't scan and there is only light wind then good results can be obtained by treating the conductor as a straight line from insulator to insulator and intersecting points along it - has the advantage in that you can zap a span of up to 400 metres in about 30 seconds. Ideally I set about a span length off the line and near square to mid-point.?ÿ This self-corrects for any curvature/refraction as the points along the span are all relative and you tie the heights into separate observations of the suspension points. Suspension points can be fixed either by resection of by non-contact - not always easy by non-contact as the eye can adjust for bolts and clamps which get in the way.
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that's right. I've given details of the time, temp on the ground (no one in the masts) , clouds and humidity at the time of scanning to the client.
You said you are working on a powerline where you staked the poles. To me, that means new construction. Therefore no current is running through the lines.?ÿ
The measurement of the sag is the way a contractor and the engineers are able to get the proper amount of tension on the conductor(s) also the static wires if any are installed.?ÿ
Normally the conductors are pulled in along the new line through pulleys and then secured at a deadend structure at one end. The other end then in pulled by winches or for smaller lines hand come-alongs until the proper tension is reached. That tension is usually determined by sag measurements.
A common method used by many contractor for decades is called the "return wave".?ÿ A rope is tossed over the wire and a loop formed at the bottom. The man doing the measurement will jump down on the loop and start a stopwatch at the same time. The bounce on the wire sends a wave back and forth between the suspension points and the time it takes is used to figure the sag.
If you Google "return wave wire sagging" you can see an actual demonstration on YouTube.?ÿ
I wrote a program for my HP41 using hyperbolic functions and all the variables such as weight/ft. of the wire, temperature, span length, etc., that allowed me to do the sag checks with a theodolite.
It's likely the contractor is looking for real time answers, so Lidar and scanning probably will not work.
Roughly how much time was neded to process the scan data to the point where it could be used for engineering design?
10 minutes of scanning, 5 minutes processing, 20 minutes of data export for client.
We use this or a ground based scanner. Usually when conventionally locating we shoot it reflectorless. If you use the plane method in trimble access remember if you adjust your travers afterwards it does not adjust those points. And as others have said record the temperature.
You also need the time and temperature, the electric company will need this along with the load at the time of observation.
