Activity Feed › Discussion Forums › Strictly Surveying › Measuring power line sags.
Measuring power line sags.
Posted by david-livingstone on August 14, 2019 at 3:04 pmSo we are working on a project where we staked some power poles. The contractor asked us if we could measure the sag in the power lines. I assume this is for minimum clearance reasons and we need to know the ground elevation and the low spot on the wires. There are about 48 poles on the project. I can set control points with our GPS and use a Trimble S5 on these control points.
I’m looking for ideas for the best way to do this. My original idea would be to do it like I would a water tower over flow. Turn a vertical angle to it. Problem is the corridor for the power line is fairly narrow, less than 100 feet wide. Instead of being say perpendicular to the low spot and a few hundred feet away I might be set up near the lines themselves. I’m not sure if this fact creates a lot of error or not. I could also maybe setup on a parallel road but that might be between a half a mile and a mile away.
I’m not sure of the exact number but the lines are about 50′ high.
party-of-one replied 4 years, 7 months ago 21 Members · 27 Replies- 27 Replies
The most modern way to do would be with a form a LiDar and from the air. A few passes with aerial LiDar and you would have more than enough data, however that may be overkill. Alternately, from the ground, a terrestrial LiDar would probably still be the most accurate measurement tool. Outside of that a good narrow beam reflectorless total station should be sufficient. If it is possible to rent or demo some scanning equipment, now might be a good time to break into a new market.
We have scanning equipment within my company. Hadn’t thought of that.
I’ve done this using the total station shooting reflectorless. Shoot the prism beneath the line at it’s lowest point. Turn to the line and shoot it reflectorless. We didn’t need to do this 40+ times though. Just a few spots near entrances. Are you sure the client just wants it for clearance?
How accurate do they want it to be? If they only want clearance information it shouldn’t matter if there is a little error?
I wasn’t sure it would shoot the line reflectorless. No spec on the accuracy give to me yet. I would assume nearest foot for something like this.
This is a perfect scenario for a scanning total station. SX10, MS60 or equivalent. Set up on control points or resection, no need for targets or tons of overlap for registration. Window the wires on the collector screen, run a higher-density scan, and then if you need ground shots take a couple under the low areas (or scan if possible). Import scans directly into post processing software, pick the low point, calc the catenary if needed, and export out along with ground shots.
When I didn’t have a scanning TS, my preferred method was to position the rod and prism under the low point (preferably with a spotter to guide the rodperson), take a ground shot, then use vertical angle offset routine to tie the wire with another observation. Easy and ensures there are pairs of shots to calculate clearance. Reflectorless can be (literally) hit or miss.
But it looks like you are going to be set up close in and it may be difficult to distinguish between wires, or target the lowest point. Scanning will be much more reliable and confirm that you have the lowest point and the correct wire.
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil PostmanThey need the data for planning of future lines that cross under or over those wires.
A major transmission line will already have been topo for design and the length and pull of every part between poles is a part of the design.
The first time I saw anyone measuring the height of wire was done by using a rubber ball tied to a braided cord and thrown over the wire and then the length of cord was measured by tape and half that distance was the height.
Reflectorless instrument is the answer, it may require you to have an offset eyepiece to do so at such a close distance.
Low conductor elevations are a breeze with a reflectorless TS. Take a series of shots though, ‘eyeballing’ the low point can be elusive. EEs always like to have the outside air temps, date and time of the measurement also.
Be sure and note the time and ambient temperature for each shot. Powerline sag varies greatly depending on how much juice is going through the wire, which can change in a matter of minutes, and ambient temperature has an effect too.
One thing to keep in mind with power line sag is that it’s not constant, but a function of several factors from ambient air temperature, the amount of current flowing through it, ice, wind loading, on and on, so the conditions the sag was measured under are just as important as the actual measurements themselves. I’ve seen long span high voltage lines fully loaded to capacity around here in winter sag nearly to the ground whereas, in summer they’re upwards 30’+ off the ground.
Me .02′
WillyI believe your Leica Cyclone software will automatically detect and extract the low points. I have used the P40 for this purpose before.
Cyclone also has the cubic spline option to create a pline that matches the actual very closely
I did clean the data several times by trimming edges
Sorry, my feeble brain needs a bit more explaining on this procedure. The crew had a rope with a rubber ball that they threw over the hydro wire from the ground? Then they pulled that rope down off the wire while still on the ground, measured the length, and divided it in half?
This method just seems like a liability nightmare. Surely, I’m missing something here.
I wouldn’t throw a rope or anything else over a power line. Even small lines may be 14 K volts, and it doesn’t take much moisture in a rope to conduct a lethal shock from that voltage.
.In the early 1970s the same crew attempted an invert elevation to the point of crossing with another power line using a Philadelphia rod and the rodman was electrocuted immediately.
Using a braided fishing line/cord was their next process. It was kept in a dry box until they needed it.
That crew consisted of 3 very reliable and trustworthy retired men that were learned on the job and may have graduated HS or not, not sure, and any of them could outwork most anyone I ever worked with since. Their thinking was as simple and as country as it gets and their needs were little (food, alcohol, clothes, bath and sleep).
In college, one of the first field exercises with transit and tape was to compute the height of the flagpole at Tyler Junior College.
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.
- Posted by: @david-livingstone
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.
MH Use a pocket laser measurer
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 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
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.
@jewel-chadd
Or that, but LiDar and Drones are cool!
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>
Log in to reply.