Colleagues-
We are returning to a site after a few years to do a topo using GPS (Z-Max) for a drainage basin and part of the basin is under a triple set of hydro transmission towers.
We experienced no difficulty in finding a few other existing control stations and getting reasonable results in X/Y/Z.
But, in searching for a known control station beyond the tower lines from our known base, the co-ords of this known station did not match the ground conditions on the location sketch and we could not find same even with a reasonable line of sight condition.
I am asking if anyone has had any experience with interference due to the high tension hydro lines while doing GPS measurements.
FWIW we have had excellent results with obtaining co-ords within 1 foot of huge wind turbines on a repeat basis.
No my friend Kent, I'm not looking to wear my AFB as this is a serious question, honest.
I just don't see how the electric induction etc. could affect our GPS'n)
Thank you in advance.
Cheers,
Derek
On my Leica 1230, which has a display of predicted accuracy, you can watch the accuracy decrease as you walk under power lines. The effect is real.
About a year ago we lost a mile (6 structures) of 138kV transmission towers due to a tornado. We had to scramble to get the replacements in place. We did static sessions with the control points directly underneath the towers that weren't destroyed, with no obvious interference.
We do transmission line work regularly and I can tell you from experience, satellite reception is not affected by transmission lines.
The flip side of that sometimes your proximity to the lines can wreak havoc on your RTK comms and bluetooth. The lower the relative humidity, the higher the interference. We try to keep our control points (base) geometry 90 degrees to the route, and a mile away. Composite rods on the rover seem to help also.
GPS Signals Are Electro-Magnetic Waves...
...and yes they are affected by high tension lines. I have had to set up under them and while watching a sky view you can even see satellites disappear. Orient the screen to North and you can determine which line it is.
The wires and towers can act as antennas and can suck the GPS signals out of the sky above you or bend them around. It all depends on the orientation of the wires. A favorable orientation and you won't know they are there.
You pays your money and you takes your chances.
Paul in PA
Yes, GPS is affected by transmission lines. Your total station is affected by the sun. The redundant belt and suspenders deja vu all over again approach that we were all taught steered us away from side shots on control work.
Multiple checks will provide the answer to the quality question, and guide you to deciding which results are acceptable. Doing GPS work for AEP in the 1990's I found that working under lines required longer static observations and mission planning. But then we had maybe six satellites tops. I wouldn't trust RTK under major lines. I'd get out the total station.
If you are doing static work under lines, you should revisit that lost art of mission planning and plan observations for times when adequate coverage exists outside obstruction from the lines.
First GPS data I had to deal with had a worse than expected elevation. It was located under high tension power lines. Someone else ran and processed it, I followed with the total station and ended up disregarding the GPS point.
They say you can light a florescent bulb when standing under the lines on a muggy day. This is an awful youtube of the phenomenon, skip around...
How about all that talk about living near power lines causes cancer?
There is stuff going on there, why wouldn't it mess with little tiny signals making their way down from space?
I've never tried GPS under transmission lines but I've had the static shocks from leaking electricity cause havoc with a total station and DC connected by a cable on several occasions.
We find that the local detail work is generally best done with a total station (tree cover, etc) so we use a combination of GPS to set a baseline, at right angles to the OHL if possible and then detail (or intersect in the case of tower geometry investigation) using a total station.
If you always measure the GPS base with the total station you'll have a good idea if there is a signal problem on the GPS. It is not uncommon to get apparently very good GPS results, but find the total station results differ by several centimetres (0.03'), presumably because of some interference from the line - it can also happen on a switched out line. A triple station base should enable the poor station to be identified.
We either mount the GPS on a total station target, or on the total station itself, using an adaptor. (See our website www.SCSsurvey.co.uk - go to Services-powerlines for an illustration).
Yes!
You should never rely on GPS when near 161Kv or 500Kv lines, Switch yards or transformer yards.
We work for TVA and try to keep our GPS control as far away from these areas as possible and we also try to keep the control intervisable so we can verify by the total station our GPS control.
When mapping the underground anomilies in these areas GPS is not used. I have had data collectors to loose data memory in these areas so we set our total stations out in the open away from any static electric interference
"Light up my Room"
Here's what the Bare Naked Ladies have to say about that!
Colleagues-
Thank you all for your replies.
Very much appreciated.
Cheers,
Derek