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Site Control - Principle 4

4. Trust, but verify

He never knew it, but Ronald Reagan's approach to dealing with the Soviets has a direct application to machine control. This leads us to site control principle four.

Trimble GPS Base and Rover

You hired the project Engineer to place your control, you surrounded the site with points and shot in more than five control points. What could go wrong now? A lot, if you don't use some independent checks to verify that your 3D model matches up to the real world.

Here are a few things the surveyor can do while he is onsite to place your control. Usually, the cost to place a few more stakes is minimal the surveyor is onsite anyway. Consider it insurance.

  • Set two or three building corners and mark cut/fill to finish floor elevation. This will allow you to check your site rotation and elevation at the building pad - one of the most important features on any project.

  • If your project is, or incorporates, a new road, ask the surveyor to set a few centerline stakes with cut/fill to finish grade. Again, you can reference your 3D model and linework against it to make sure you are in the right place and reading cut/fills correctly. A couple stakes on the outer edge of your working area at an intersecting road are a good idea, too. Don't run all of your check stakes down the centerline, for reasons discussed in our site control principle 2 blog.

  • Instruct the surveyor to place a benchmark on or near the center of the site that is easy to check. If the center of the site is not feasible, have them give you a benchmark that is easy to access and provides for good GPS reception.

  • If the surveyor stakes utilities at the start of the project check your GPS system to their stakes. The elevations and locations should match your file.

Topcon Pocket 3D Control Screen

Use information about existing conditions from the plans to check your model to the real world.

  • Take a look at the existing conditions plan sheet or grading plan. Look for existing manhole cover elevations, storm sewer inlet elevations, data at the site entrance(s), contour lines, property corners, etc. Any of this information can be helpful to back-check your GPS system to the actual site.

  • If your project ties into an existing street, parking lot, driveway or hard surface, make sure your finish grade matches at the tie-in. There should be elevations called out at the tie-ins you can check against. Reading elevations at hard surface tie-ins might not be perfect due to freeze/thaw or subsurface moisture but should be close.

  • If you have good GPS signal there, verify that your GPS matches the site vertical benchmark(s).

Once you are happy with site control and are confident that it matches up to the site you can get to work. Operationally, there are checks you should perform regularly.

  • Check your rover against a benchmark DAILY - before you start grading work.

  • Use your rover to check the elevation across your dozer blade, motor grader, excavator, etc… every day. Even better, grade out a pass and make sure the dirt is to grade across the entire cutting edge. The finished dirt/rock elevation is what really matters in the end.

  • Check your equipment against the benchmark stakes the surveyor placed for as long as they remain in place.

  • Ensure your cut/fills match up against where you left off the day before. This is a quick way to know if something is off from day to day.

  • Stake out to your site control points.

  • When other trades like paving, foundation or even electrical contractors call for staking check your system to those stakes and make sure your system agrees with them.

  • If you have measured in site control and are confident in your work, but your GPS system's reported residuals (errors) are high, talk to the surveyor.

Finally, take good care of your - expensive - high tech equipment. Good maintenance can go a long way. We have quite a few clients running 10 year old GPS systems every day.

  • Check your rover pole to make sure it is the correct length. You might have a worn or incorrect tip throwing your elevation off a little. If you measure in site control with a pole of the wrong length, your entire site will be shifted vertically. Consider this a reminder to double-check the pole length you have entered into your rover settings.

  • Make sure the bullseye level bubble on your rover pole is not damaged or off.

  • Is your software and firmware up to date? Some GPS suppliers will update it at no cost. Off season is the best time to do this.

  • Do you need to adjust for cutting edge wear on your heavy equipment? The answer is probably yes.

  • Is your heavy equipment "tight"? A motor grader with worn out linkages might measure in ok up by the job trailer but won't be on grade under load. Who would have thought greasing your equipment regularly would affect your GPS grade control systems accuracy?

Lastly, does your GPS system and model pass the eyeball or gut test? Does it seem right when you are onsite? Is site drainage working as expected? While not perfect, a good gut check can go a long ways in making sure your GPS system is working correctly. If something just doesn't seem right, it probably isn't. Remember, you can always dust off that grade laser or dumpy level and use it to double-check your GPS system, too

4. Trust, but verify

That's all there is to it. Four simple principles you can apply to any machine control project.

If you need to clear up any lingering questions regarding site control email us at or call 515-505-3510 ex: 702, we would be happy to hear from you.

Rather watch a video? This companion video will walk you through all four principles and give you a few pointers on base station setup and control point placement. Start HERE if you would like to jump right to Principle 4.

1 Comment

Aug 02, 2022

Great reading your blog

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