Surveying point on a power line pole

[Naggud]

I am a structural engineer in Canada and for the record my survey skill are limited so please excuse my terminology and knowledge.

We have just won a job where we need to measure the height differences on an electrical pole. At the top there are high voltage power lines, and around mid height on the poles there are mobile phone lines. We need to find out if the space between the upper power lines and the mobile phone lines is sufficient to run another power line.

I have used EDMs many moons ago and at first thought we could use one to determine the heights. We cannot do that though because we risk our guy with the prism getting fried on the pole! I am wondering what other options do we have? Perhaps measure the horizontal distance to the bottom of the pole and then sight the points on the pole that we need to measure and get their heights based on the vertical angles?

I would be very thankful for your help.

Thank you,
Naggud

 

[IRstuff]

Why would you need to put your EDM up a pole? The British Survey of India in the late 1800s measured the height of Mt. Everest to within 30 ft of the official height without getting anywhere near the mountain.

I suggest you review your trigonometry. Assuming your poles are indeed vertical, a measurement to the base and the two points in question are sufficient to determine the height difference.

TTFN
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[brandonbw]

IRstuff: Its been a long time since I shot something in the field. But are you saying to shoot a point on the bottom of the pole. And then aim the laser at the top and get an angle reading from the laser to plug and chug the basic trig? Need to brush up on some of this stuff myself.

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[fel3]

You may not even need survey equipment if you don't need high precision results. If the ground is level or nearly so, you can do this with the length of the pole's shadow (including shadow length from the base to the cross-arms), the altitude of the sun, and trig (or a cad drawing). If the ground is not level, but you have surveyed ground elevations from which to develop a profile, you can solve this in a cad drawing. For the first case, this is a set of SSA right triangle solutions: the second S represents the various measured shadow lengths, the A is the sun angle, and the first S represents the various heights you want to solve for.

Several years ago, I had to find the height of a roughly 80-foot-tall high-mast light pole for which no record information existed, so I used its shadow. The pole needed to be moved, and the height was one of the pieces of data I needed for designing the new foundation and anchorage. In addition, from the width of the far end of the shadow, I determined the diameter of the top of the tapered pole so I could estimate weight and wind capture. I also used the shadow cast by the lighting array to estimate it's size for the wind load calc and to find a comparable lighting array to estimate its weight. Here is how I described the procedure in my Mathcad document of the calculation:

Using trigonometry, estimate the height of the pole from the length of the pole's shadow and the altitude angle of the sun. First, measure the length of the shadow cast by the pole from the face of the existing concrete foundation to the apparent top of the pole. Second, look up the altitude angle of the sun for the location (lat/long) and time of the measurement. Finally, estimate the height of the pole using simple trigonometry and including adjustments for the radius of the existing foundation and its height above the parking lot pavement. The altitude of the sun was obtained from the U.S. Naval Observatory website

http://aa.usno.navy.mil/.

Latitude and longitude were determined at

http://www.topozone.com

(Lat = N 36°44'02" & Long = W 119°46'55"). For verification, the shadow was measured twice — on two different days at different times.​

In my case, I used this equation: Hp = ((Ls + Rf) * TAN(A)) - Hf

Where:
Hp = estimated height of pole
Ls = length of shadow from the face of the existing foundation
Rf = radius of the existing foundation
A = altitude angle of the Sun
Hf = height of the existing foundation above the parking lot
​

BTW, my two results were 83.0 feet and 83.7 feet, so I used 84 feet.

==========
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[IRstuff]

If you can already measure angles, then you don't need to shoot the base.

distance*(arcsin(top)-arcsin(middle))

TTFN
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[dik]

I'd have chained off a distance from the pole and set up my theodolite, and simply measured the angle from the base to the point of intersection of the two upper sets of wires... measure the height of instrument and done a quick CAD drawing... faster than figgurin' the mathematics... I carry my laptop with CAD... everywhere...

Or if a cloudy day and you can see a laser beam, use my Hilti measuring device and shoot upwards from the base of the pole to the cable...

Dik

 

[Naggud]

I think we'll simply measure the distance to the base, shoot to the higher point on the pole and get the angle. From there as you say we can get the height differences using simple trig.

Thanks to all of you for your replies, I appreciate you taking the time to help.

Regards.

 

[eea]

That routine should be built into the total station and/or your data collector.