Primary Control Level Loop For Field Engineers (Construction Elevation Control Basics)

Primary Control Level Loop for Field Engineers (Construction Elevation Control Basics)

In this blog, I’m going to talk to you about level loops or level circuits is what a surveyor might call it as it relates to primary control.

So let me start out by saying that primary control is permanent and it surrounds the job site and it lays out everything within. Secondary control is something like a baseline that controls the building and it’s semi-permanent. And then working control would be like after grid lines laying out a wall or a column and laying out the actual column or wall lines or the offsets for the wall or the column meaning it’s component specific and it’s temporary.

So primary, secondary, and then working control. We always have those different types.

Establishing the Complete Network (Northing and Easting Only)

In the last blog, we talked about if you have your basis of bearings that might be out on the roadway here and you want to build a building, let’s say that we’re just looking at the foundations and the slab on grade that you will create points in your site and these points will surround the job site like we talked about and they will be permanent. And what you’ll do is you’ll take distances and angles and connect these. So, we’re on the same page and then you will have a complete network.

From there, if you’re going to go ahead and lay out a baseline, you would, for instance, set up here, backsight a longer backsight, and you’ll radial stake from that point. Then you’ll do it from another side of the traverse. Set up here, backsight. And then what you’ll do is you’ll radial stake that one and radial stake that one. And then you might set up here and backsight a longer backsight than your foresight. And you will lay out that one. And you will lay out that one.

And what happens is the points at the end are now laid out and it’s a beautiful thing. And then you can do a forward and back for your baseline and do direct and reverse shots to establish your distances. And so, you have a really nice tight control network.

Now all you have is basically your northing and easting for each of these. Your northing is your y and your easting is your x. And I’m using civil coordinate geometry basics. You might run from the x-axis, but in survey and in civil, you go from the y-axis 360 degrees around the circle in four different quadrants. So, you have a northing and easting, but you don’t have an elevation.

The Design Benchmark (Where Elevations Come From)

But one of these bases of bearings, these monuments inside the roadways will likely have what I call the design benchmark. That means the benchmark from which the surveyors working for the design team took as-built elevations of the adjacent roadways and the sidewalks and the topography of the site for the purpose of designing the building. That means the building is based on that benchmark.

Let’s say it’s right here. What you want to do is level loop. And a level loop has a couple of key things. Number one is you are going to do what’s called three-wire leveling. Number two, you’re going to estimate to the nearest thousandth. Number three, you are going to close your level loop.

How to Do a Level Loop (Backsight, Foresight, Close)

And here’s how you do it. You take a calibrated, well-functioning automatic level with the proper specs. And what you’ll do is set up and you’ll pace forward to this point and then back and then get the exact middle. You don’t have to go all the way back on the second. And you will set up your automatic level and you will come sight your level rod and you’ll take your benchmark plus your backsight which becomes your instrument height and you will come over here and you will shoot to your foresight and that instrument height will be minus your rod height will establish the new elevation of this point but not yet.

And so let me make this point that you will backsight, foresight, backsight, foresight all the way to your original point. And what will happen is that you will in a nice format in your field book log each of your elevation shots.

Three-Wire Leveling (Top, Middle, Bottom Readings)

Now, here’s the deal. When you’re doing these shots inside in your scope, you will read your top number, your middle number, and your bottom number. Those other horizontal crosshairs are called stadia hairs. They’re used for two different purposes.

One is you can take the top reading minus the bottom multiplied by 100 and that’s how far away from your rod you are which is really cool how they did that. The other thing it’s for is if you take your top, middle, and bottom, add them up, divide by three, the average should match your middle rod reading, or else you read the rod improperly.

I don’t know about you, but I don’t want to go traverse a massive 200-acre site only to find out I made a mistake here. I’m going to three-wire level as I go. The other thing is you’re not going to get good enough numbers to be accurate by the time you close if you don’t do that.

Estimate to the Nearest Thousandth (Not Hundredth)

The other thing is you’re not going to read these as the nearest hundredth. You’re not going to be like, “Okay, 252.02.” No, you’re going to be like “252.023 feet.” And somebody would be like, “Well, Jason, you’re guessing on that third number.” But if you round up to the nearest hundredth, you’re intentionally guessing to the wrong number. So, if you guess to the right number, that’s much more accurate than rounding.

So, you’re going to estimate to the nearest thousandth and you’re going to close back.

Closing the Level Loop (Within Tolerance)

Now, what happens when you put this into an Excel sheet and you actually look at your linear error of closure and it’s within the tolerance, you can then adjust the elevations ever so slightly around this thing to match and to close that linear error of closure or that vertical error of closure, what you could also call it, as long as you’re within the tolerance.

Now, in the third version of the book, the Construction Surveying and Layout on page 716, I just had that memorized over the years, there’s first, second, and third order accuracy depending on your acreage that will give you a little mathematical calculation to tell you how far you can be off when you close. If you’re within that tolerance, now you have elevation on all of these points. In addition to your northing and easting, you have your elevation.

Why You Need Elevation on Primary Control

And the reason you need your elevation is because you want to reference it for any additional control. And if you ever want to bring in another benchmark inside the site, like let’s say I want a temporary benchmark in here, you will go backsight, foresight. Then you’ll do backsight, foresight, backsight, foresight. Include two benchmarks in that level loop from known elevations and close. And only then can you know the elevation of your new point. You cannot just do a side shot and spray over there to it.

So, this level looping is how you get elevations on your primary control and then how you would later bring it in through this process to your secondary and working control.

Here’s the level loop process:

• Start from design benchmark: One of the bases of bearings monuments will have the design benchmark (elevation used by design team to as-built topography and design building). Start from this benchmark.
• Backsight, foresight around the network: Set up automatic level at exact middle between points. Benchmark plus backsight equals instrument height. Instrument height minus foresight equals new elevation. Backsight, foresight, backsight, foresight all the way back to original point.
• Three-wire leveling (top, middle, bottom): Read top, middle, bottom on stadia hairs. Add them up, divide by three. Average should match middle reading or you read rod improperly. Top minus bottom times 100 equals distance to rod.
• Estimate to nearest thousandth (not hundredth): Don’t round to nearest hundredth (252.02). Estimate to nearest thousandth (252.023 feet). Rounding to hundredth is intentionally guessing wrong. Estimating to thousandth is guessing right.
• Close within tolerance and adjust: Put into Excel, check linear error of closure. Page 716 of Construction Surveying and Layout has first, second, third order accuracy formulas based on acreage. If within tolerance, adjust elevations slightly to close. Now you have northing, easting, and elevation.

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A Challenge for Field Engineers

Here’s what I want you to do this week. Run a level loop on your primary control. Start from the design benchmark (one of the basis of bearings monuments). Set up automatic level at exact middle between points. Backsight, foresight, backsight, foresight all the way around and close back to the original point. Use three-wire leveling (read top, middle, bottom). Estimate to the nearest thousandth. Log each elevation shot in your field book. Put into Excel, check linear error of closure against page 716 formulas. If within tolerance, adjust elevations. Now you have northing, easting, and elevation on all primary control points.

And remember: you cannot just do a side shot to set a new benchmark. You must go through two known benchmarks and close. That’s how you know the elevation of your new point. As we say at Elevate, level loop for primary control: backsight/foresight from benchmark, three-wire leveling, estimate to nearest thousandth, close within tolerance, adjust elevations. That’s construction elevation control basics.

On we go.

Frequently Asked Questions

What’s the difference between primary, secondary, and working control?

Primary control is permanent, surrounds the job site, lays out everything within. Secondary control is semi-permanent (like a baseline) that controls the building. Working control is temporary and component-specific (like laying out walls or columns).

What is the design benchmark?

The benchmark from which the design team’s surveyors took as-built elevations of roadways, sidewalks, and topography to design the building. The building is based on that benchmark. You must use it for your level loop.

What is three-wire leveling?

Reading top, middle, and bottom stadia hairs. Add them up, divide by three. Average should match middle reading or you read rod improperly. This ensures accuracy over large sites and provides distance check.

Why estimate to the nearest thousandth instead of hundredth?

Because rounding to nearest hundredth (252.02) is intentionally guessing wrong. Estimating to nearest thousandth (252.023) is guessing right. You won’t get accurate enough numbers to close within tolerance if you round to hundredths.

How do you set a new benchmark inside the site?

Go through two known benchmarks and close. Backsight, foresight, backsight, foresight. Include two benchmarks in that level loop from known elevations and close. You cannot just do a side shot and spray to it.