Primary Control Traverse For Field Engineers (Construction Survey Control Basics)

Primary Control Traverse for Field Engineers (Construction Survey Control Basics)

In this blog, I’m going to talk to you about how traversing really ties into how we should be setting up a project as a field engineer. And I’m going to draw it the way I understand it. And I’m going to encourage you to check out the book Construction Surveying and Layout by Wesley Crawford if you want to know more.

Let me talk about this.

What Is Control in Construction?

When you have a building in construction and you want to properly control it, the term control and I’m not talking about toxic, like abuse of spouse control, but like control we’re trying to control the location of every component on the project site. So, control is like we’re keeping control of that building. If we want to lay out the foundations and the slab on grade and then start going up for this building, we need to get there from the basis of bearings.

So typically, we’ll have like, let’s say out here on the road there’s a city monument, and then over here on the road there’s another city monument, and they’re like, “Hey, go build this building.” Okay, great. Well, you’re going to have to go research with a surveyor or with a civil engineer the northing, easting, and elevation for these monuments and make sure that these are the ones that were used in the design.

See, now let me make sure I’m doing a good job here. Whatever basis of bearings and design benchmark is what I call it. It’s basically a city monument with an elevation that was used to as-built the elevations and the topography on the site to design the building, is what you now need to use to lay out the building. At least to double-check it.

Setting Up Primary Control (Surrounding the Building)

So, if I want to build this building and I’m like, “Hey, I want some primary control around the site in concrete monuments that are permanent that will control the building,” I will make sure that it is surrounding the building and the entire project site, and I will make sure that it ties to the basis of bearings. So, what I would do is I would connect a traverse from here to here to here to here to here. Let’s go that direction and then back.

How Traversing Works (Distance and Direct/Reverse Angles)

And what a traverse basically is and let me just do it with three points. When you do traversing, you set up your total station and you shoot a distance, then you turn an angle: direct, reverse, direct, reverse at a minimum, and then you shoot a distance. Then, depending on which direction you’re going, you’ll set up again and you will shoot a distance. And you’ll do direct, reverse, direct, reverse, and then shoot another distance so you have two.

Then you’ll set up over here and you’ll shoot a forward distance so you have two. Direct, reverse, direct, reverse, shoot another distance. So now every leg has two distances, and it has direct and reverse shots for the angle.

And what happens is those numbers should create a perfect geometrical object. But they won’t because of human and instrumental error. So, this is what I like to do when I’m teaching field engineers. That triangle should hit flat. But because of human and instrumental error, once you move the measurements into AutoCAD, it will have a gap. And that is called your linear error of closure. So, what it does is if you’re within the right accuracy, it will close and adjust the data. So you have accurate coordinates at all of the different points.

So traversing is essentially networking these points together in a precise way. Accuracy is, am I in the right location per the world? Precision is, do I have a good relationship from point to point within a network? So, precision and accuracy are two different things.

The Biggest Mistake: Not Comparing Traverse to Survey Coordinates

So, when I do this, I will network these points together. And here’s a really important point. If these points already have survey coordinates, you can plug those into AutoCAD. But let’s say that that’s your points. And in AutoCAD, you plugged them in from the survey data. And then you go do this traverse and you’re like, “Oh my gosh, we closed with greater than 1 in 50,000, which is what I typically recommend.” Which means that you did such good work on this traverse that it would take you 50,000 linear feet of traversing to be off a foot. That is the ratio that they use to make sure that you have an accurate traverse.

So, let’s say that you’re above 1 in 50,000. You’re like, “Oh, my points are good.” That’s the biggest mistake people make. Just because you did a good traverse doesn’t mean you know these points are right. What you have to do I hope you like and subscribe but what you have to do then is take your traverse coordinates and overlay them using your basis of bearings as your basis and then overlay it over the points. And I’m just doing a little demonstration here.

Let’s say the black marks on this sheet were the coordinates that you got from the surveyor, and the blue is what your traverse came up with. Well, obviously you can see you’re pretty close on these, but this one’s way out of whack. And we find this all the time. And so, the traverse has to be overlaid with the survey coordinates. And that’s why we go back to that other blog that it’s important to do that in AutoCAD or Civil 3D to make sure they’re correct.

Now, if you see this problem, you call the surveyor back out and you reconcile it and then you align and make sure that everything’s pretty close. The other thing you can see is if you’re within acceptable tolerances. Like if this is 0.01 off, 0.01, 0.01, 0.015, 0.02, 0.02, 0.025, and this one’s 0.05, and this one’s off like by 0.80, you’re probably going to want to adjust that one too and talk to the surveyor about that one.

Why Accurate Traverse Points Matter (Laying Out Baselines)

The traverse points are very crucial to get the network tied in the way it needs to be. And let me tell you why. If you just take these random I am going to call them random because typically surveyors will just go spray points with a 4-foot prism pole and it’s garbage they will not be accurate.

If you go take these points and you like set up over here and you’re like, “Hey, I want to lay out the baseline for this building.” And you set up backsight farther back than foresight, and you lay out these two points. And then you set up over here and you backsight a backsight that’s longer than this building longer backsights than foresights. And you lay out these two points. And let’s say you do it from a third point, which is typically a good practice. And you actually look in here at this monument and I’m just going to draw this real size. This is what we see.

You’ll see a point here. You’ll see something like that. You’re like, “What in the world is that? What’s the real point?” You’re introducing a lot of error, and you are also not maintaining consistency.

Once you get the surveyor and you align and you use the coordinates from your traverse and you lay out this end point of the baseline from one side having a longer backsight than foresight and then from this side and then from this side, and you actually go zoom in to one of these and I’m telling you this happens every time you’ll see exactly what I just drew right there: three perfect little dots, and the real point is right in the middle, taking the average of that, and that’s getting rid of that instrumental error.

And so that 3/16 of a problem inside the total station that’s going to get rid of it. So you can literally position any building within this network whenever you want as long as you’ve done this right.

Key Rules When Doing a Traverse

Now a couple of rules. When you do the traverse:

• Use tribrachs with prisms for backsights: Not 4-foot prism poles which are garbage and inaccurate. Use tribrachs with omni prisms for precise work.
• Use greater than 3-second gun: You need really good equipment for construction control. 3-second total station or better.
• Shoot forward and back, direct and reverse: Every leg has two distances and direct/reverse shots for the angle. This gets rid of instrumental error.
• Close the traverse: Do the traverse computations to get rid of error. Must close better than 1 in 50,000 (meaning it would take 50,000 linear feet to be off a foot).
• Compare traverse with actual survey points: Take traverse coordinates, overlay using basis of bearings, compare to surveyor’s coordinates. Call surveyor to reconcile if points are out of whack.

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What Happens When You Do This Right

And if you do that, this is what you’re going to do. You are going to end up with beautiful tied-in, aligned to the basis of bearings beautifully accurate, beautifully precise points from which you can go lay out your secondary baselines and anything else where, if you hit them from three any layout that you’re doing from three sides of a traverse you’ll get a perfect little triangular pattern and you’ll be able to take the average and have the exact right point.

And this matters because if you’re like on big airports, if you don’t have that, if you don’t do it from different sides of a traverse, you’ll have this building over in this direction, that one over in that direction, and then when you connect them with bridges or other connections, it won’t be accurate.

So, you’ve got to be able to do this. Traversing is great. The next step is level loop. We’ll cover that in a different video, but if you follow these practices with Wesley Crawford’s book, you’ll be spot on.

A Challenge for Field Engineers

Here’s what I want you to do this week. Set up primary control surrounding your building. Tie to basis of bearings (city monuments with northing, easting, elevation used in design). Run a traverse: shoot distance, direct/reverse angles, close the traverse better than 1 in 50,000. Then overlay your traverse coordinates with the surveyor’s coordinates using AutoCAD or Civil 3D. Compare. Call the surveyor to reconcile any points that are out of whack.

When you lay out baselines, do it from three sides of the traverse with longer backsights than foresights. You’ll get three perfect little dots, take the average, and have the exact right point. That’s how you position buildings accurately within the network. As we say at Elevate, primary control traverse: tie to basis of bearings, shoot distance and direct/reverse angles, close traverse, overlay coordinates, compare to survey points. That’s construction survey control basics.

On we go.

Frequently Asked Questions

What is the basis of bearings?

City monuments with northing, easting, and elevation that were used to as-built the topography and design the building. You must use the same basis of bearings to lay out the building to maintain accuracy.

What’s the difference between accuracy and precision?

Accuracy is: am I in the right location per the world (tied to basis of bearings). Precision is: do I have a good relationship from point to point within a network (traverse closure).

Why overlay traverse coordinates with survey coordinates?

Because just because you closed the traverse well doesn’t mean the surveyor’s points are right. We find points out of whack all the time. Overlay, compare, call surveyor to reconcile discrepancies.

Why shoot from three sides of the traverse when laying out?

Because you’ll get three perfect little dots instead of scattered points. Take the average and you have the exact right point. This gets rid of the 3/16 instrumental error in the total station.

What accuracy should a traverse close to?

Better than 1 in 50,000. Meaning it would take 50,000 linear feet of traversing to be off a foot. That’s the ratio for accurate construction control traverses.