Single Point GPS Success
Many contractors regularly utilize single point control. It is a great way to use your GPS machine control system for small designs, stockpile calcs or even as an ad hoc laser. Unfortunately, due to the "single point" nature of the process, we have seen it cause problems that could have been avoided. If you need a refresher on how site control works, check out this blog post. It will help you understand why single point control is not used on engineered projects and why you need to be careful using it on yours.
When Quantum gets involved in single point control accuracy problems there is a recurring story line and it goes something like this:
Contractor sets up his GPS with single point control and performs a topo of a site.
The contractor sends the topo to Quantum for a custom design. Usually it's a pond, building site, parking area or drainage improvement.
Quantum designs the site and sends the finished design in a 3D machine control format for the contractor to grade the site.
Contractor returns to site a few days or even months later to complete the grading project. When they start the GPS system back up with single point control, the design does not match existing. It measures in fine at the one control point but the design is tilted or even rotated, often times by feet.
The problem here is not the design, or even the topo, but in that the job was set up with single point control. With single point control, there is only one control point placing your site in the real world. Unlike an engineered site with multiple control points, you only have one point to hold your site vertically or horizontally. The site can tip or rotate around that point as there is no other control holding the 3D model in place. All things considered, RTK GPS holds itself close in this situation but not close enough for repeatable and precise work. Re-read our site control principle 3 blog post for a refresher on why you need multiple control points.
Below is an eight step process many of our contractors have used in the field to set up their own control system. This process is not meant for engineered projects, just projects you are designing and grading yourself.
Set up your base station in a permanent 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. Verify your GPS system initializes and works as expected. Many contractors use 5,000, 5,000, 100 for N,E,Z coordinates. Use whatever you like but avoid negative numbers.
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. The key point here is that they are in location you can return to as the project progresses, and they are not disturbed. You will use these locations to set up new control points.
Take a topo shot at those locations and save them in your controller. Write down (or take a picture of) the 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, too. Quick tip - Depending on your GPS brand and software you make be able to record the point as an actual control point at this step. If so, you can skip steps four, five and six. Check with your GPS vendor to see if this is possible with your system, it's a timesaver.
Now, start a NEW project in your controller with a new control file. DO NOT USE THE SINGLE POINT CONTROL FILE YOU JUST MADE.
Enter the northing, easting and elevation from the topo shots you took in step 2 as control points in your new project.
Shoot in 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. The higher residuals are ok. If something is way off make sure you have good GPS and radio signal at the new control locations. The 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.
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.
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 understand how correctly setting up single point control works.
The process above should NEVER be used on an engineered site. You will need to rely on the Project Engineer' surveyor to set up your site control network, here's why.
Setting up your own site control network is more work than the usual single point control. The extra effort is well worth it when it comes time to grade the site.
Below is a brief example from one of our clients that had to suffer through issues with single point control. He started using the process above and has not had problems since.
Not very long ago, 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. 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.
Want to topo your projects and have Quantum help with the design? Email us or call 515-505-3510.