EVERYTHING YOU NEED TO KNOW ABOUT CONSTRUCTION SITE CONTROL POINTS
The 4 ‘W’s of Control Points
Have you ever stopped and given control points a second thought? Probably not. However, we felt obligated to dedicate a blog to the importance of control points because without them, quite frankly, there would be nothing more to the job site. So, let's dig into the 4 Ws of Control Points and give them the attention they deserve!
WHAT is a Control Point?
We cannot talk about what a control point is without first defining what traversing is. A traverse is a series of points established on the job site connected through the measurement of angles and distances. These measurements are used in a process called traverse computations, which determine the relative location of the points to each other. In construction, traversing is used to establish control around a job site to lay out the structures. A control point is a point on the ground or any permanent structure whose horizontal and vertical location/position is known. Control points are used as a starting point of the surveyor's CAD.
WHEN do I need to have Control Points in place?
Immediately! Control points are the foundation for everything on the job site. Many times jobs are held up waiting for control points to be established. If your control points are wrong or inaccurate, everything else will be incorrect. You could've done everything else right, but your entire job site will be off if your control points are wrong.
For this reason, traversing is typically one of the first surveying activities performed since control is needed right away to see where the structures will be located. It is good practice to run a traverse around the site and include the property corners as points in the traverse. This allows for the information on the site plan to be double-checked before beginning construction. This control will only be as good as the precision of the traverse.
WHERE should the Control Points be placed for my GPS equipment?
Surveyor working with control points on GPS equipment
There should be a MINIMUM of 4 points surrounding the job site. They should be placed on the ground in a hub, drill hole, pk nail, or iron rod - pretty much anything that can be in the ground that you can get the point of the GPS rod on. They should surround the site as much as possible. If you draw a line from point to point, it should close around the site. They should also be placed in open sky - meaning not under trees, next to tall buildings, or high fences. The GPS communicates with satellites and radio signals. If you have a GPS in your car and go in a tunnel, it doesn't work either. Finally, the information the contractor should receive from his surveyor is the X, Y, Z coordinate information of the point. This is also called the Northing, Easting and Elevation of the point. This can be given to your virtual construction engineer in a variety of formats such as a Cad file, TXT, ASCII, CSV, XML, all of these would work. Even a pic of the coordinates in the survey field book work too.
WHY are Control Point coordinates critical?
The control coordinates must match the design CAD coordinates. In GPS terms, 'Localization' (also called Site Calibration, Transformation, or other terms depending on the software) is the mathematical process used to relate local grid coordinates to the GPS earth-centered, earth fixed ellipsoidal datum. In short, setting up the GPS and being in the same universe as the proposed design location.
Using control points on construction site
If you can imagine control points as pins, the pins in the proposed CAD, and the pins in the survey, CAD must line up for everything to work. This lining up of the control (pins) is known as the process of aligning the proposed design to the survey CAD. The control points, which are in the survey CAD, have to be in the same coordinate system as the proposed design file. It would be best if you always asked for the survey CAD right away because they have the correct physical locations of the control points with the correct dimensions. We are often presented with a survey file containing the control points in one coordinate system; however, the proposed design is in an arbitrary system. This typically happens because the proposed design was done independently of the surveyor's CAD file. Before GPS days, the CAD was usually only used to create the PDF's that would generate the plan set. This is no longer the case because, with the advent of GPS, the CAD is physically placed in a real-world environment. The design must be in the same coordinate system.
ECI has expert Virtual Construction Engineers who are able to ensure your project gets started and is executed smoothly and efficiently. Let us put our knowledge to work for you. Contact us with questions about control points and other aspects of your earth moving projects..