Control Line Stationing: How-To

Control Line Stationing Purpose

Sometimes, referencing control points is not practical, as your surveyor or general contractor has not established control points for stationing. Instead, as is common with most jobsites, it is expected that control lines, such as marked grid lines on the ground, be used as measuring references. This is when Control Line Stationing, rather than Control Point Stationing, is most practical.

In addition, sometimes it is preferable to use control lines rather than control points as a stationing reference because all of the sub trades on a jobsite are "pulling," or using, the same linework for their layout. 

The use-case is up to you, but since this could be a useful stationing solution for certain occasions, it is made available on the HCL software.

Remember that Control Line Stationing requires you to work with a CAD file, or linework, in your project, as your total station will be positioning itself within your digital design for your measuring applications.

If you are unsure if control line or control point stationing is better for your situation, perhaps consider control point stationing first, as that is the most common, and typically considered to be the most accurate option.

How-To Step 1: Open Control Line Stationing

From the home screen, simply open the Control Line Stationing application to begin the stationing process.

Notice that when you do so, the software warns you that since you are not stationing on control points, your layout accuracy now completely relies on the accuracy of the linework you are about to station on. More on how the total station estimates its location below.

How-To Step 2: Select your First Control Line and Measure

Find the control line in your CAD file that you will use as your initial reference line, as in the example below:

Now, spot a location anywhere along that line and prepare to measure. When you are confident that you are ready to measure a location on that line (ensuring the correct target/prism is selected), press the red measure button.

When completed, move on to another location on that line, as you prepare to measure a second point on that line. Best practice is to find a second point that is as far down this line as possible that is able to maintain a line of sight with the total station, and also be a location that you are confident is accurate to the line you are measuring.

Remember, your layout accuracy all depends on the accuracy of the measurements you are making on these lines. Giving yourself a long stretch of this control line for the total station to work with is in your best interest, but only if you know you can measure the line locations accurately. The longer distance you measure along the line between these two points gives you a wider layout area you can be confident to work in, as long as you are confident in your line measurements.

When you have found a second location along the line, follow the prompt on the controller, and measure:

How-To Step 3: Select your Second Control Line and Measure

Find the second control line you would like to use for station set up. Important to note: this line must intersect the first control line you selected. If it does not, the stationing process will not work. In my example, I selected a control line that crossed my first one at a 90° angle. Follow the prompt and measure the first point along this control line at a location of your choosing.

Follow the prompts again and measure the second point along this control line, following the same advice from Step 2, which was to measure as far down the line as possible that you are confident will be accurate. When you are finished this step, you should see 4 points in your measurements tab on the right side. It will express the angle at which the total station turned (HA & VA), as well as the distance away from the unit in which the points were measured (SD).

If your stationing set up was poor (either not accurate enough or the lines you selected are not intersecting), the total station will warn you that you cannot complete the stationing process and must try again with a different selection.

How-To Step 4: Select your Station Location

If you have completed the 4 required measurements of your 2 lines, it is now time to indicate where your total station is located. While you may know where it is located in relation to your line work, the total station does not. All it knows is that it measured 2 lines, and that it could technically be in one of two locations on your project.

In the screenshot below, I know that the location in relation to the linework in which I measured my lines is the "S1" location, which is the one I will click:

After clicking, it shows a visual of what the distances were to the lines that I measured:

The software provides as many visuals as necessary to ensure you achieve an accurate stationing, and that you are aware of each step you are completing along the way.

How-To Step 5: Verify your Stationing Errors

In addition to this visual, the "values" of your measured points indicate your potential inaccuracies in your measurements, as in the screenshot below:

Notice that there are no inaccuracies seen in my measurements until "Line2Pt2" - it shows that I have a northern deviation of 7mm from where that point should have been, if I was accurate to the way the lines were designed.

Understanding any Stationing Inaccuracies

It is important to note here that the software only calculates the inaccuracies of your lines in the final point measured. In practice, this means that in the calculation of this stationing, the software "assumes" that line 1 is measured perfectly accurate to the design, and that the first point from your second line is also measured perfectly accurate to the design. If there are any inaccuracies at all amongst the points, they will somehow show up in the final point measured.

So, put another way:

1. The first two points measured of your first line establishes your "benchmark line" - and it is considered, for calculation sake, to be accurate to the design in how you measured it.
2. When the first point of the second line is measured, the software immediately establishes where that second line is in space, and visualizes exactly how that line would intersect the first line at the angle the designed file suggest.
3. When you measure the final point (the second point of line 2), the total station is essentially saying, "assuming the first 3 points were measured perfectly, this line, at the point you just measured, is a certain deviation inaccurate to the design."

The three statements above is why using control points during stationing are considered more accurate, as control points have set coordinates and can be checked against each other in regards to their relative locations and distance from each other and the total station when measured. The errors in these measurements, if any, are minimized and spread across everything that is measured.

In control line stationing, the station assumes the first three points are accurate to the design and positions itself accordingly. The fourth point measured suggests there is an error amongst the control lines, but it is up to the end user on how to troubleshoot this, or, mitigate this. Accurate layout is certainly possible with control line stationing, but understanding how the stationing process works and what the data is explaining during the stationing process is critical to stay accurate.

At the bottom of this article, you will see several examples of how to interpret the stationing accuracy. In sum, remember that the accuracy of your layout depends on the accuracy of your stationing - find the best measurement locations on your lines that is practical. Optimize the layout area you want to work in, but also minimize the stationing errors to a tolerance of your choosing. This may mean deciding to layout in a smaller area than you preferred in order to protect the integrity of your layout.

So - How do you Troubleshoot the potential Inaccuracies?

There are several ways to check the accuracies of control points and lines - you can see this article for a robust overview.

For a basic troubleshooting option, simply use the COGO functions in the tablet to check the measurements of your objects in the digital file, and use the missing line application to check measurements, which you can do without being stationed.

COGO applications:

Missing Line Application:

How-To Step 6: Set your Station Height (if Needed)

If you are working with heights, you will need to set a station height. See this article for more help on how to set a station height. However, simply pressing the "set station height" button on the right side under the "Summary" tab should guide you through the process.

If you are not working with heights, simply go to the settings gear at the top and turn heights off (or on, if needed). With heights off, the station height step will also not appear.

Note, if you set a station height during this process, it is always possible to go back and adjust it later if you made a mistake. You can do this during the stationing process, or by going to the edit station height button from the home screen after you have completed your station. See screen shots below:

How-To Step 7: Consider "Shift Station"

As discussed in this article, shifting your station coordinate location may be useful. This option is only available in control line stationing, and can be used when you are aware of where the errors are in your control lines. Shift Station allows you to move the total station coordinates after stationing to hopefully mitigate the errors you discover between the designed control lines vs. what was measured in the field.

Please see the Shift Station article for more information.

How-To Step 8: Begin your Measuring Applications

Your station now has a coordinate location on your CAD file and you can proceed with your measuring applications.

Quick Helpful Tip:

Sometimes, you may prefer to station on control points, but no matter what you do, the control points don't seem to be accurate. One option as a troubleshooting method is to temporarily set up based on control lines to see if it can help you identify control points that may be positioned incorrectly. While you may prefer not to do any layout or official measuring applications after a control line stationing, it may be a good way to inspect potential issues that need to be brought to attention.

Examples of Stationing Errors for Understanding:

Remember that only the final point of the Control Line Stationing process will show any error of the Control Line Stationing process. Below are examples of what that error is expressing.

Remember, however, that even though the software only puts the error on the final point of this stationing process, it is likely that the errors are coming from any point in the stationing process, which will need to be observed and troubleshooted by the end user.

Line2Pt2 dN = 7mm:

A visual of how the total station calculated this:

The purple points (point 1 and 2 from line 1) are considered accurate, and a benchmark. Point 3 (blue) establishes where the 2nd line should be in space, and is considered accurate where it is measured. Point 4 is the final measured point from the second line, with a 7mm northern deviation from the red X, where the line should have been.

L2Pt2 dE = -7mm:

A visual of how the total station calculated this:

The purple points (point 1 and 2 from line 1) are considered accurate, and a benchmark. Point 3 (blue) establishes where the 2nd line should be in space, and is considered accurate where it is measured. Point 4 is the final measured point from the second line, with a 7mm western deviation from the red X, where the line should have been.

L2Pt2 dN = 4mm, dE = -4mm

A visual of how the total station calculated this:

The purple points (point 1 and 2 from line 1) are considered accurate, and a benchmark. Point 3 (blue) establishes where the 2nd line should be in space, and is considered accurate where it is measured. Point 4 is the final measured point from the second line, with a 4mm deviation north (higher than where the designed line should be) and a 4mm western deviation (further left from where the designed line should be).

Final Note:

The deviations from that final point is likely to be some combination of errors amongst all the measurements. For purpose of this stationing application, however, the error is only displayed in the final point. As you now hopefully understand how the total station is interpreting this error, you can decide how to move forward with your applications.