3 wire

I hope I can explain it. I was taught by a fellow surveyor.

On the level you have the top stadia line, the middle line that you always read, and the lower stadia line.

You read the top stadia line, and record that reading. You do the same for the middle line, and the lower stadia line.

You take the top reading and add it ot the bottom reading, and divide by two, and you should have your middle reading.

The way ot was explained to me was this helps yo make sure your readings are accurate, and it "should" keep you from having to close back to your starting point, because you are checking yourself as you move along.

I have never really used it, as my loops are generally pretty small for my work.

I welcome any clarifications or corrections if I have remembered incorrectly, or mis-spoken.

I hope this helps.

Like you, I normally don't use 3 wires for leveling, but I know there's a lot more to it than just blunder checking which is all I have used it for when I have used the method.

With the 3 wires you can also get distances between stations (stadia) and use this for some more sophisticated adjusting of misclosure, etc.

Most text books have significant information on the proper way to use 3 wires.

3-wire leveling is how I used to always run levels for tight work, it really doesn’t take much more time, and is a more accurate way to use the equipment.

As Jimmy stated above, read all three wires and write them down, then add them together and divide by three to get your answer. That answer should be within .005’ of the middle number.

As you probably have noticed you often need to “pick” a number when you read the middle wire, do you round up or down? Or maybe you try to read to the nearest .005’ say 5.105’ when you are between 5.10 and 5.11.

Using three wires helps to eliminate rounding issues with each turn and it is a good check to see how you are doing balancing your turn distances. You multiply the difference between the lower and upper wire by 100 and you have your distance between the level and the turn point. It is also a very good blunder check, each reading will should be consistent with the other ones.

I often left the level at the office back in the day (cause it may be needed elsewhere) and used my instrument to run levels, to do this meant setting 90 in the gun reading it and then 270 and reading it, then meaning the two numbers, and for a more accurate reading it didn’t take much more time to read all 6 wires and average all of them. When I would do that, I never had a bad closure. You will learn quickly that a little more time during the run will save lots of time because you won’t have to rerun the levels.

As an aside, have you have been taught how to "peg" the level?

Jimmy is right. That's how it works. The average of the two stadia readings should equal the middle reading.

The differences between the stadia readings will tell you whether or not the distances in your loop are balanced. If you have a large difference between the upper and lower stadia readings that means the rod looks smaller through the telescope because it is farther away. Conversely, if you have a small difference the rod is closer. You can actually calculate what the distance is based on the stadia readings.

I think the principal of closing back or into another known point is not negated by taking precise reads, either three wire, flopped rod, FT/MT or digital.

I have read that a running three wire loop is nearly as accurate as running the same loop three times.

One more thing

We use 3-wire all the time, but don't forget that when you are leveled up, sight to the rod, pay attention to the center hair, lightly touch the end of the scope and make sure it bounces back to your original reading - this makes sure the compensator isn’t stuck.

> ... You can actually calculate what the distance is based on the stadia readings.
No kidding? Every surveyor who did a topographic survey prior to the 1980's and reads that line will spit his coffee on the screen.

> I often left the level at the office back in the day (cause it may be needed elsewhere) and used my instrument to run levels...
With a total station, set it to read Vertical distance, fix the target rod height, and shoot the BS, then the FS, in the usual manner for levelling. Note the readings in level note form. Be sure to account for the negative readings. Balance the BS & FS lengths but no need to worry about "running out of rod". It works.

:good:

wasn't so good with my T2 however;-)

I wasn't born until 1982, and this guy seems like he's probably younger than I am....

You can find forms for booking the readings, but I like my format shown in this spreadsheet.
https://dl.dropboxusercontent.com/u/25124076/Albia3wire.xls

I was checking whether a suspicious-looking bench mark had been moved. It turned out to check very well. I concluded that the street widening was not symmetrical, and the railroad track had been raised (not uncommon). I was using a topo rod and estimating 0.01 between 0.05 ft marks, and then carried the average to 3 decimals. It must have been luck - I couldn't have done as well as the result suggests.

> Trying to learn everything I can now make life easyer later.

Personally, I wouldn't bother learning to run 3-wire levels these days. Any leveling worth running more accurately than can be accomplished with a good auto level and rod can be run more easily, much faster, and with fewer blunders using a digital level and barcode rod. I'd invest the time in learning something you might actually need to know in a modern survey practice.

I disagree. There are some basic surveying, mathematical, and geometric principles involved that could prove beneficial. By understanding how and why 3 wire leveling works and how distances are derived from stadia readings and why it's important to balance distances just builds on the foundation of having a strong ability to interpret measurements. At least I think it's better than learning what button to push and how to do side shots and turns in a digital level.

> If you have a large difference between the upper and lower stadia readings that means the rod looks smaller through the telescope because it is farther away. Conversely, if you have a small difference the rod is closer. You can actually calculate what the distance is based on the stadia readings.

dan, the stadia is calculated as (upper-lower)x100=approx hd for a single measurement, so you have to compare a back dist to a fore dist to monitor leg lengths in a 3 wire run. one measurement vs the other

So my first surveying gig the old timer I worked with called this 3-wire leveling, but reading responses sounds like not...It was a huge site 6400 acres we had to run across, he ran with two turning points at each turn. I had a blue and red screwdriver I'd place about foot apart in the ground and preferably with some elevation diff. Then I'd show him rod on one then other calling out red or blue. He did this so we didn't have to turn around and come back, running two seperate loops. Seemed to work, but I've never tried it.

I have done a lot of 3 wire leveling over the years, but none in the last two decades due to digital levels. Why anyone would NOT use a digital level is beyond my comprehension. But, you asked, so...

3 wire is very good at picking up any blunders. Except...I had a guy running for me that was only checking the thousandths place, and blowing the hundredths and tenths. He had to go back and rerun many miles of levels. So it is important to check the data, not just read it.

Before digital levels, the most accurate way to run levels was using a micrometer with a double scale rod. Even in that method we used 3 wires, but the two other wires were only used for the distance. If there is no micrometer, you read all three wires to the thousandths of a foot, and take the mean, then you will be a bit more accurate, it is similar to reading the rod three times, so the accuracy improves by sqrt(3). It is important to make sure that the upper and lower intervals agree.

I used my HP41 to collect, check, and store 3 wire readings when I used to run many miles of 3 wire.

> I am woundering exactly how to run a 3 wire. My party chief has explained to to me but he is horrible at teaching . Trying to learn everything I can now make life easyer later.

The old K&E Ephemeris probably has the best example of 3 wire leveling. I've ran many miles of levels using this format and a Ziess Ni2 level.

> > If you have a large difference between the upper and lower stadia readings that means the rod looks smaller through the telescope because it is farther away. Conversely, if you have a small difference the rod is closer. You can actually calculate what the distance is based on the stadia readings.
>
> dan, the stadia is calculated as (upper-lower)x100=approx hd for a single measurement, so you have to compare a back dist to a fore dist to monitor leg lengths in a 3 wire run. one measurement vs the other

Moe,

Mostly correct. However, There are levels with quarter stadia hairs and there are levels with a stadia ratio of 200:1 and 300:1.

Also another reason to run 3 wire is to average all 3 readings when running a closed loop. This mathematically reduces the reading error introduced into the loop.

B-)