Secondary Control For Field Engineers (How To Expand Construction Survey Control)

Secondary Control for Field Engineers (How to Expand Construction Survey Control)

In this blog, I’m going to talk to you about secondary control and the proper ways of bringing it in from primary so that we have it right on the project site. I love this because if we get this wrong, your grid lines will be wrong, you’ll be chasing a quarter inch, three-quarters of an inch, three-eighths around the building. It will be a nightmare. So let me talk to you about this right now.

The Foundation: Primary Control Network

We’ve already talked about how every building will have probably a basis of bearings and then what you can do is create primary control points around the building that are permanent, surround the job site, and are the basis for all other survey shots on the project. Now, let’s say that this is your slab on grade and foundations. I’m just drawing it that way to make it easy. And of course, everybody knows you have grid lines, typically in the United States, at 30-foot intervals. It can be lots of different things, but it should match from drawing set to drawing set. And you want to lay out this building. Thankfully, you have a primary control network where you have a northing, an easting, and an elevation. So that’s awesome.

The Worst Option: Corner Offsets (Don’t Use This)

Let me tell you the different types of secondary control. And I’ll tell you my most favorite. Sometimes you’ll have offsets for a secondary control network. And this is probably the most used, meaning a surveyor will come out and do offsets from the corners. I hate this. This is the most worthless thing ever. And it’s because you can’t check if a point is wrong. It’s very much not helpful to maintain control. It will help you set up batters with your sticks and strings, but it’s not very useful in the long run. You’ll have to trust me on that.

Why You Need Four Points Per Line (Not Three)

The other thing is I have seen secondary control networks end up basically being offsets on a grid pattern all the way around. I like this just fine. I think this is fantastic. And I’ll tell you why. It has corner hinge points so you can check the perpendicular of the lines and you have more than four points to a line. Let me explain why that’s so important right now.

This is all from Wesley Crawford, by the way. If you have three points and they’re not in a perfect line and they’re not matching for distance, which one is wrong? Well, this could be the line. That could be the line. That could be the line. You don’t really know. But if you have four points and it’s like this is what’s going on and you notice that this one is off. Well, these are all on their increment and they’re all in line. Those are the right ones and that one is wrong.

So, in a four-point line, you can always find out if a point is wrong because you can compare them within themselves. And so, a baseline must have four points to it. So that’s what, again, why I don’t like this. You can check 90, you can check the line, you can check every point. Here you can’t check anything. You can’t hit those points and they better be right from the start. So, I really don’t like that one.

Different Ways to Set Up Baselines (Ranked from Worst to Best)

The other thing you can do, which I really love, is you can do a single baseline and then turn 90s. It’s not as good as if you have the situation where you have two outside intersecting baselines. That’s a little bit better. My favorite, believe it or not, is inside intersecting baselines.

And here’s what I mean by that. A lot of times most of the activity and damage is going to be outside of the building footprint. And so if you sleeve the slab on grade for key monuments or all of them, I don’t care, and you are controlling the building from within where there’s a lot less traffic, logistics, generators, pump trucks, cranes, blah blah blah, rebar. I freaking hate rebar everywhere. Or at least me when I was a field engineer. I could keep this baseline protected and center all of my accurate control at the cores where it mattered most. I love intersecting inside baselines.

So, you can design your secondary control however you want. I would say this is definitely a no for me. I hate that the industry does this. And I would say this is probably the least effective. And then I’ll write a happy face on these because these are the jam. These are absolutely fantastic.

This just proves that most of what we do in construction, just because we’ve always done it doesn’t mean that it’s right. Most of what we do is wrong in construction. That’s absolutely horrible thought process there.

How to Bring in Secondary Control from Primary (Three Sides of the Traverse)

So if we create a baseline and we’re like, “Hey, we want to design this.” So, let’s say we design that in AutoCAD and we want to bring it in from primary control. This is how we’ll do it. I have a beautiful traverse that’s very precise. I will set up on one point. Let’s say I set up here and I’m going to backsight longer than my foresight. That’s a rule. And so, I’ll set up and I’ll backsight and I’ll lay out this point and this point.

I’ll usually want to do it from three sides of a traverse. So, I did it here. I’ll probably want to do it here and I’ll probably want to do it here. I’ll show you why in a minute.

Then I will go ahead and set up, backsight farther than my foresight, and I’ll lay out and I will lay out. And then I’ll set up here, backsight longer than my foresight, and I will lay out and I will lay out.

The Three Dots: Instrumental Error and the Real Point

Now when you look at this monument, if you do this right, we’ll end up with three beautiful little dots. Now why are they not the same point? It’s because of instrumental error. You can say human, but it’s mostly instrumental. In a total station, unless you buy those really fancy $50,000 ones, you’re going to have about 3/16 of error, roughly around 0.015 of error inside the EDM, the laser inside the total station.

And so, what you’re seeing is that the real point is the average of those three. And so when I mark my point, I’ll mark the middle. See how beautiful that is?

How to Check Distances Between Baseline Points

Now, here’s the key point. When I’m setting up my secondary, I don’t want to lay these out from my primary. I’ll just go from here to here. And then I’ll verify a couple things that my distance between the points is accurate.

What I’ll do is I’ll mark a line forward, mark a line back, and I will then shoot a distance forward on these and then set up over here and shoot a distance on these back.

Let’s say I have a point here and a point here. I will set up backsight and I will pound a hub and mark a line, mark a line, mark a line, and then I will shoot a distance and shoot a distance and shoot a distance. Then I’ll set up here and backsight here, confirm the line, and then shoot a distance and shoot a distance and shoot a distance.

If I zoom into this, it will look exactly like this. It’ll show here’s my hub and here’s my line and I’ll see a line and a line. Why did that happen? That’s the instrumental error. Again, the point is right in the middle.

And so, what I can do is mark the middle of these points and then I’ll check it with a tape measure and you’ll be hitting flat. You’re not going to be chasing three-quarters of an inch or a half of an inch or a quarter of an inch or three-eighths or an eighth. You’re not going to be chasing that around your grid system. You’re going to have a perfect solid baseline here.

And if you wanted to do these other patterns, you would simply do the same thing on those other patterns. And then also like for this one, you could check the diagonal. But the bottom line is that’s how you’re going to bring it in from your primary control.

What Secondary Control Enables

Your secondary control should allow you to set up and backsight along the baseline and have the radius be longer than any foresight, and you should just be able to turn 90s into that building. And that is secondary control. It’s building-specific and it’s semi-permanent.

Here’s the secondary control ranking from worst to best:

• Corner offsets (worst, don’t use): Surveyor comes out and does offsets from corners. Most worthless thing ever. Can’t check if a point is wrong. Not helpful to maintain control. Will help set up batters with sticks and strings, but not useful in long run.
• Single baseline with 90s turned (least effective): Not as good as other options. Limited checking capability.
• Grid pattern offsets all around (good): Has corner hinge points so you can check perpendicular of lines. Has more than four points to a line. Can check 90, can check line, can check every point.
• Two outside intersecting baselines (better): Good option for checking and control.
• Inside intersecting baselines (best, my favorite): Most activity and damage outside building footprint. Sleeve slab on grade for key monuments. Control building from within where there’s less traffic, logistics, generators, pump trucks, cranes, rebar. Keep baseline protected. Center accurate control at cores where it matters most. This is the jam.

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A Challenge for Field Engineers

Here’s what I want you to do this week. Design your secondary control as inside intersecting baselines. Sleeve the slab on grade for key monuments. Bring it in from primary control by shooting from three sides of the traverse. Set up, backsight longer than foresight, lay out points. Do it from three sides. You’ll get three beautiful little dots. Mark the middle. That’s the real point because of instrumental error (3/16 in total station EDM).

Then verify distances between baseline points. Mark line forward, mark line back, shoot distances. Set up on other end, confirm line, shoot distances back. You’ll see two lines. Mark the middle. Check with tape measure. You’ll be hitting flat. You won’t be chasing three-quarters of an inch or half of an inch around your grid system. You’ll have a perfect solid baseline.

This just proves that most of what we do in construction, just because we’ve always done it doesn’t mean that it’s right. Most of what we do is wrong in construction. As we say at Elevate, secondary control for field engineers: inside intersecting baselines best, four points per line, shoot from three sides of traverse, mark middle of three dots.

On we go.

Frequently Asked Questions

Why are corner offsets the worst option for secondary control?

Because you can’t check if a point is wrong. Not helpful to maintain control. Will help set up batters with sticks and strings, but not useful in long run. You can’t hit those points and they better be right from start.

Why do you need four points per line instead of three?

If you have three points and they’re not in perfect line, which one is wrong? You don’t know. But if you have four points and one is off, the other three are all on increment and in line. Those are right, that one is wrong. Four-point line lets you find if a point is wrong by comparing within themselves.

Why are inside intersecting baselines the best option?

Most activity and damage outside building footprint. If you sleeve slab on grade for key monuments, you control building from within where there’s less traffic, logistics, generators, pump trucks, cranes, rebar. Keep baseline protected. Center accurate control at cores where it matters most.

Why shoot from three sides of the traverse?

Because you’ll get three beautiful little dots at each monument because of instrumental error (3/16 in total station EDM). Real point is average of those three. Mark the middle. That’s how you get perfect solid baseline without chasing three-quarters inch around grid system.

What is secondary control?

Building-specific and semi-permanent control. Allows you to set up and backsight along baseline with radius longer than any foresight, then turn 90s into building. Brought in from primary control points.