How do I Troubleshoot Inaccurate Stationing and Control?

Check that the tool is calibrated properly

If you are doing any measurements with a total station, it is important to first make sure that the total station is properly set up to read measurements accurately.

Please perform the following checks to make sure your total station is prepared to read measurements accurately:

Set the correct PPM and acclimate to the weather (see this article)
Verify your prism pole (if being used) is truly plumb and level
Ensure the total station is stable and secure (see this article)
Run a field calibration of the tool (see this article)

If you have verified that all these three things are complete, your tool is in good condition to read measurements accurately.

Check your digital file measurement data (COGO)

Not being able to station the total station accurately is frustrating, especially when you don't know what is going wrong. Most often, the inaccuracies from control points have to do with the data that is imported into the tablet rather than the control points in the field themselves. So, it is always good practice to check your digital data set up before moving on to other troubleshooting methods.

Any of the following functions on the tablet can help clarify the control point dimensions your data, as imported, is asking for:

Distance between points
Line and offset
Length of line, arc or circle
Distance between parallel lines
Angle 3 pts

Feel free to use any of these data measurement features to check the data within your file on the tablet. If the measurements seem off digitally, they will certainly be off in the field.

If you notice that this is where your errors are likely coming from, try to re-import your data (as it may have been imported with the incorrect units), or, discuss with a surveyor, general contractor, designer, or someone in charge of your control to discuss why the digital file is not reading properly.

Check the in-field measurements with basic HCL applications

If the digital file seems to be reading correctly on the tablet, but you are still not able to station with the control points that have been given you, it is likely that the control points in the field are either the wrong ones, or are not placed correctly to what the digital file is expecting.

There are two main ways to use the total station to check the accuracy of the control points without having to officially station the tool with control points. You can just turn the tool on and start verifying various measurements.

Basic Method 1: Missing Line application

The "missing line" application is a glorified tape measure that can read not only horizontal distances, but also slope, height, and angles between two points.

After you have verified the distance (and/or angles) that are supposed to exist between your control points, simply open up "missing line" and start measuring between the points. As you measure, verify that the data "missing line" is giving you matches what your digital file says should be there. If it does not match, discuss your findings with the surveyor, GC, or perhaps the designer.

Basic Method 2: Use Free Stationing to see an "As Built" of your in-field control

It can be very helpful to create a new project and use the "free stationing" application to start measuring and recording points in an open space. It lets you visualize the existing in-field control points you have been given to work with and lets you develop a report, of sorts, that you can show stake holders. This process will help to identify any issues with control points between the digital design and what is provided in the field.

Create a new project on the tablet and go to "free stationing" to arbitrarily station the tool in your area.

Then, open the measure and record application to start measuring in the various control points (or anything of pertinence) in your jobsite area. You can then use the on-tablet data measuring functions mentioned above to check the measurements and angles between your points, and then compare that to what you have on the digital file.

Attempt to Station the Tool only with Control Points that are Accurate

Sometimes, you may be in a situation where you are certain some control points are correct, but uncertain of others. You can quickly check the accuracy of control points compared to the ones that are accurate.

Station the tool only by using the control points you are confident are accurate

For this troubleshooting step, begin by stationing the tool only with your accurate control points, or, the ones you are confident in. Then, use the measure and record application to plot the other control points on your jobsite. It will be very obvious how far they deviate from the way the control points were designed on the plan. Just compare the measured point to the designed point you see on your screen and share the info with your team.

Force an accurate station by drawing in accurate control points manually

This troubleshooting step will likely only work if you are able to find line work on the digital file that can also be found in the field. If possible, you can use the drawing applications on the tablet to create temporary control points within your CAD file. Typically, this would be a control point at the intersecting control lines that exist in the digital file as well as in the field. Then, simply draw in a second control point a certain distance away from that point, but ensure that it is following the line that is visible both in the CAD file and in the field.

Now, you can temporarily perform a station to do some troubleshooting checks of your control points. Just make sure that what you drew on your tablet is also drawn in the field (using the missing line application is often helpful). Station the tool on this "make-shift" set up, and do some general measure and record applications to see if the as-built report you build sheds light as to what may be the issue.

At minimum, this will provide basic insight to the as-built conditions in the field compared to the digital file to begin troubleshooting your control point set up.

How can I ensure I stay accurate once I have found the right control?

The best way to maintain accuracy is to set up several "backsight" points around your jobsite after you have stationed the tool accurately. These backsight points can be used as temporary accuracy checks throughout your work day.

Once you have completed an accurate stationing, go into measure and record to measure in a new measured point at a location of your choosing. This point will be your backsight point, or reference point, throughout the work day. After you perform layout for a certain period of time (perhaps 1 hour), simply turn your tool back to this backsight point and observe if you are experiencing any significant deviations.

When you station the next day, turn back to the backsight point again to see if there are significant deviations from the previous stationing.

By having this best practice, you can always check if you are maintaining accuracy in your layout day after day, and can remedy the situation in a timely manner by noticing problems sooner rather than later.