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Back to Basics: Intersecting line layout.

September 3, 2002
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In construction, setting points can be accomplished by layout with a chain—or two chains—using the 3/4/5 method, by radial layout with a total station, layout with GPS, by intersecting the line of sight of theodolites or total stations, as well as other methods. Although all methods have their strengths and uses, radial has become the predominate method of layout on the construction site today. It is fast and accurate if proper techniques are used to check it. On some jobsites, however, superintendents do not allow radial layout use because of bad experiences with sloppy people who made mistakes. So, they require that control be placed by some other method.

One of the best alternatives to radial layout is using intersecting lines (see Figure 1). Once established, this method of layout is quick and accurate and easily used by anyone who can set up an instrument over a point. With intersecting lines, layout can even be checked by simple observation since straight lines are involved. Adequate time must be taken at the start of a job to design control so that baselines surround the project site. Initial setup of the points required for intersecting lines with a theodolite is indeed time-consuming, but if performed properly, can be very rewarding with buildings that are square and easy to control during construction.

Figure 1. With intersecting lines, layout can be checked by simple observation since straight lines are involved.

Planning

Extensive planning is the key to being able to locate points by intersecting lines. A building site must have control completely around it and targets must be placed to create an efficient system of baselines around the site.

Review the Plans
Determine first if intersecting lines will work for the project. Projects most suitable for this method are those that are rectangular or square and have columns that basically line up. Projects that have numerous walls and unusual dimensions to columns, or have curved lines, etc., are adaptable to this method of layout, but will require more initial layout work. Review the site plan to see if there is room to place the control all around the structure. Will haul roads destroy the control? Will materials be stored in such a way that the control would be unusable at times?

Communicate
Check with others who will be building the structure and let them know that intersecting lines is the method of control to be used for the project. Talk to all the crafts people represented on the job to obtain their input. Discuss the plan and encourage them to point out weaknesses that may have been overlooked. Obtain their ideas on a good offset from the structure for the control to be located. Find out where they will be storing materials, building roads, setting cranes, etc. Tell them how important the control points will be so they will be careful not to disturb or destroy them.

Figure 3. There should always be four permanently monumented points per line to establish control.

Establishing the Control

Surrounding the site and locating control points on every column line will require that more points be placed and monumented. Ensure that points are located outside the limits of construction and materials storage areas. Build monuments so that the control will be available throughout the length of the project. Look for areas where targets can be placed to make the use of the control easier.

Monumentation
Once the control lines have been established, monumentation of all of the control points can begin. Each side of the structure will have many points as shown in Figure 3. Some of those should be permanent control points marked with concrete monuments while others can be temporary hubs. The rule of thumb is that there should always be four permanently monumented points per line. Select those points that are most likely to survive the construction activities to monument. Determine the frost depth for the area and also the expandability of the soil to determine how deep or big to build the monuments.

Establish targets
Having a backsight available on each line is the key to efficient use of intersecting lines during layout. Place targets on hubs, adjacent buildings, sidewalks, etc., by painting or using sticker targets on walls, window sills, windows, parapet walls, etc. Obtain permission from the owner first before placing targets on a building. If necessary, place targets immediately behind the control point itself by building a mini batterboard and putting a nail on the line. Well-placed targets are important to the successful use of layout by intersecting lines.

Figure 4. Once targets are available on a site, establish a line between points.

Using Intersecting Lines

Assuming that control has been established around the site and that targets are available, use the following method to locate a needed point. The illustration in Figure 3 represented the control around a simple building. Control points and target locations have been established and are indicated. The superintendent has requested exact location of an anchor bolt template at the intersection of lines “B” and “2.” Two instruments are available. A carpenter has approximately located the template but needs the exact location to drill holes in the template for the anchor bolts.

Figure 5. Establish a second line using the same method.
Set up an instrument on line “B” (see Figure 4). Backsight onto a “B” line target across the building site. Signal to the carpenter at the anchor bolt location to mark on the template the “B” line. A point is marked on one side and then the other, and a straight edge is used to connect the points with a line.

Figure 6. The two lines intersect to indicate the center of the template
Set up an instrument on line “2” as shown in Figure 5 on page 61. Backsight onto a “2” line target across the building site. Signal to the carpenter at the anchor bolt location to mark on the template the “2” line. A point is marked on one side and then the other and a straight edge is used to connect the points with a line.

Figure 7. Other points can then be located on a project.
The intersection of lines “B” and “2” shown in Figure 6 marks the center of the template and is indicated on the template. The carpenter can now use these lines to measure and locate the position of the anchor bolts.

Repeat this procedure for any point that needs to be located within the structure. Check the layout of the anchor bolts before the concrete is placed (see Figure 7).

Summary

Using intersecting lines is a simple process. The critical part of using this method is in establishing the control points and targets around the site. If time is available to establish control points and targets that surround the site, it can be one of the best methods of establishing the lines needed for building a structure.

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