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GPS Vertical Accuracy

GPS Vertical Accuracy

GPS Vertical Accuracy

I have a question about gps vertical accuracy. I am an inspector on major highway projects in California. I work with alot of surveyors as they set grade stakes. Typically the surveyors will use GPS for setting the stakes and hubs in the horizontal position but follow up with a level or total station to set the vertical. They have told me that GPS is not that accurate in the vertical. Is that true, and if so why? Also how precise is the total station in the vertical since it uses trigonometry for angles.

RE: GPS Vertical Accuracy

Visit a jobsite and have the survey crew show you some elevations by total station and then what they get using the level. Then report back here on the findings. I will also ask my kid about that. He is a crew chief for MPLS.

Richard A. Cornelius, P.E.

RE: GPS Vertical Accuracy

It somewhat depends on what you are doing and what the accuracy requirements are on the job.

RTK GPS surveying has the ability to have approx. 0.1' accuracy at a 95% confidence level. However, there are many field conditions where you will not get this kind of accuracy. I have seen grading equipment use RTK to lay out the sub base and road base both horizontal and vertical. For this type of work, the accuracy of RTK seems to be sufficient combined with the grading requirements that are programmed into the machine control. I have also seen it used for grading ditches/ponds/earthwork on sites. However, the machine control usually has to be supplemented with total stations when tree cover or other obstructions become an issue and you are no longer able to get a fixed RTK solution or it becomes fixed but not at a high enough accuracy.

For setting drainage and sewer structures I have just about never seen RTK GPS used to set these grades. Very high accuracy and very strict slope requirements (at very shallow slopes, often under 0.5% where I live) usually mean using pipe lasers, laser levels, or traditional levels.

Total Station precision usually depends on weather conditions and distances in the shots. But when used correctly the accuracy is quite high, usually measured in millimeters (often specified in a fixed instrument accuracy in mm + part per million). As usual knowledge of how to use the equipment correctly is key.

RE: GPS Vertical Accuracy

I talked to my son. He is a surveyor for City of Mpls and Red Bull Div. He tells me that they use GPS as a method to make topo's for the Army. He spent 4 months in school at Ft. Leonard Wood to update skills. Also if you leave the machine alone for 15 minutes the accuracy is very good. It keeps refining the data.

Richard A. Cornelius, P.E.

RE: GPS Vertical Accuracy

Typical industry standard for reproducible measurements are 0.1' on dirt and 0.02' on rigid pavements.

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