Many contractors regularly use single point site control as a quick and convenient way to leverage their GPS machine control systems for projects that do not require formal engineering. Typically small building sites, agricultural designs, stockpile calculations, or as an ad hoc laser solution. However, single point site control has limitations that can lead to accuracy and repeatability issues if not implemented properly.
If you're not entirely clear about how site control affects your grading accuracy, read our detailed blog post on The Four Principles of Site Control to ensure you're making informed decisions on your projects.
Common Pitfalls of GPS Single Point Control
Here's an avoidable scenario that Quantum Land Design has encountered when a contractor is new to single point control:
Initial Topographic Survey:
The contractor sets up GPS with single point control and performs an initial site topo. Watch our YouTube video to learn how to perform a topographic survey.
Model Creation:
They send the data to Quantum, where we produce a precise 3D grading model for their design. It might be a building site, drainage improvement or fishing pod. So far, so good
Return for Grading:
Returning days, weeks, or months later, the contractor re-initializes their GPS system using the original single control point, only to find that the site model no longer matches the existing conditions—it’s rotated or tilted in an odd orientation.
The problem isn't the design or initial survey; it's that a single control point isn't stable enough to reliably align your 3D model repeatedly. Unlike engineered sites with multiple control points, a single point allows your site to shift or rotate around that point, compromising accuracy significantly. Â
Re-read our site control principle 3 blog post for a refresher on why you need multiple control points.
Reliable Single Point Control: Step-by-Step Guide
If your project isn't engineered, but you'd still like to use GPS for accurate grading, follow this reliable eight-step method to establish robust single point control:
Establish a Permanent Base Station:
Set up your base station in a permanent and stable location. If you use a fixed height tripod, mark the location so you can find it to set up in the future. Set up single point control as you normally would. Many contractors use 10,000, 5,000, 100 for N,E,Z coordinates - avoid negative numbers. Verify your GPS system initializes and works as expected.
Identify Reusable Control Points:
Find at least four, but preferably five, locations on the outside boundary of the site you can take topo shots and return in the future to find them (see this blog post to help determine where to place control). These areas might be a sidewalk corner, storm sewer drain, property pin, etc... You may need to drive a pin or hub in the ground if there are not any permanent features on the site boundary. It is best to mark these places with a lathe or at least paint them; you will return to them in step 6 and in the future. They must be in a location you can return to as the project progresses and are not disturbed. You will use these locations to set up new control points.
Record Coordinates Carefully:
Use your GPS rover to take a topo shot at those locations and save them in your controller. Write down, take a picture of or export to a text file the point #, northing, easting and elevation of those points.
 FYI - it is best record the shot over a 30 second or longer observation period. We would also suggest you use three decimal places for each point, to reduce rounding errors.
***Tip: Check with your GPS vendor—some systems allow you to directly record these shots as control points, simplifying the process significantly.***
Start a New Project File:
Create a completely new project file in your GPS controller. Do not reuse the single point file from the initial survey.
Enter Your Control Points:
Enter or import the northing, easting and elevation from the topo shots you took in step 3 as control points in your new project. Â
Measure Control Points:
Measure in the control on the points you topoed in step 3, just as if it were an engineered project. You will see higher residual errors than on control points placed by an engineering firm. If something is way off make sure you have good GPS and radio signal at the new control locations. The slightly higher residuals are acceptable in this case as the project is now "controlled" and won't tip or rotate over time. It just won't be quite as precise as a project where a surveyor placed control with survey processes and instruments. Â
Perform Check Shots on Hard Surfaces:
Take a few topo shots on hard surfaces and save them to your controller. When you set up in the future, in addition to checking into your control points, you can use them to verify that your system is set up right and matches your earlier work.
Verify Every Visit:
EVERY time you return to the site, set up your base and start the project with the control file you made in steps 4, 5 and 6. Stake out to your control points and the topo checks from step 7 to verify everything is working correctly.
This graphic should help you understand how how to layout your control points.
Why Avoid Single Point Control for Engineered Sites?
The above method should never be used on engineered sites. We always suggest that you hire the Project Engineer's surveyor to set up site control for engineered projects. Here's why:
Your machine control model must align to the Project Engineer's data.Â
Detailed explanation in this blog post. Â
Engineered projects require that your work align to the previous and following contractor's work.
Misalignment to the Project Engineer's coordinate system will lead to costly rework and delays.
The Project Engineer's surveyor has authoritative knowledge and can ensure accuracy.
Real-World Example: The Cost of Mistakes
One contractor client of ours learned the "Single Point" lesson the hard way on a pond project in North Dakota:
This contractor set up single point control without any additional checks. He returned a couple months later with a pond design file, started his job up on the old single point control and started grading.Â
When he was close to done with the dam, it became obvious that something was not right. The entire design was tilted nearly 3 feet from one side of the project to the other.
The design file was not the issue, single point control was. It took a redesign and significant re-work to fix the problem. Had he taken the time to set up his control properly, he would never have had to suffer through the expense and wasted time caused by the tilted site.
Ready to topo your projects and have Quantum help with the design? Email us or call 515-505-3510 ex:702. Our team is here to help ensure your grading projects run smoothly and accurately every time.