Control Point and Tool Setup - Best Practices

Best practices for ideal control point and tool setup include:

1. Utilize at least 3 control points, preferably 4-5 if possible
2. Give priority to control points that are secure and immovable
3. Keep control points at the extents of your layout area, surrounding the entire jobsite if possible
4. Keep control points on the same horizontal plane, with a turning angle between them between 45° and 120°
5. When using the prism pole for stationing, try to keep the prism as low to the ground as possible
6. Secure the total station from sliding or vibrating
7. Add back-sight prisms as additional control points around your jobsite

Use a minimum of 3 control points:

For total stations, the more control points at your disposal, the better. Especially with HCL, the tool is able to take the control points you measure and calculate the best assumed total station set up. It will calculate the most accurate control points to use, and will even recommend which control points are likely inaccurate. The more control points it has to analyze, the more likely an accurate station can be found.

While the tool is able to calculate a station position based only on 2 control points, having 3 or more control points allow the total station to recommend the best stationing option with the data that was measured. In addition, having 3 or more control points to measure in helps mitigate the potential risk of not being able to station the tool if one or the control points was hidden or moved.

Prioritize control points that are secure and immovable:

When in doubt of which control points are accurate or inaccurate, it is prudent to prioritize control points that seem to be secure and immovable, such as control points that are mounted or anchored in place. If these are measuring inaccurately, it is more likely that they are drawn incorrectly on the CAD or labeled incorrectly within your project. If these types of control points do not help station the tool, speaking with the general contractor or surveyor may be required.

Keep control points at the extents of your layout area, surrounding the jobsite:

The more consistent you are at measuring control points that go beyond your area of layout, the more consistent you will be with your layout. For instance, if your control point measurements are only 10m away from the head unit, laying out a point that is 30m from the head unit is unlikely to be highly accurate. Best practice is to establish mounted and secure control points that surround the jobsite, so that you are rarely laying out beyond them. 

By having the control points surrounding the jobsite, you also give the chance for the total station to make a full 360° turn during the stationing process. It measures each control point in succession, essentially turning in a complete circle to do so. By having it complete a full 360° turn, calculating its positioning is likely to be more accurate.

Maintain a consistent horizontal plane and a 45°-120° turning angle between control points:

Horizontal and vertical angles are a key part of how the total station calculates various measurements, especially its positioning on the jobsite. By keeping all the control points on the same horizontal plane, you reduce the chance of a positioning error, as the vertical angle remains fairly consistent among the points.

In addition, keeping the horizontal angular turns between control points between 45° and 120° makes it more likely you will get the most accurate positioning as possible. Maintaining this horizontal angular turn range prevents the total station from making positioning calculations from drastic differences of horizontal angles. For instance, calculating a positioning based of 5° and 145° angular turns is harder to get an accurate positioning if you were to calculate it based off of consistent 90° turns.

Keep prism low to the ground during stationing if possible:

If you are using a prism rod to station the tool, such as the image below, consider putting the prism on the bottom of the pole, rather than the top. Having the prism on the bottom of the pole reduces the possible error of the rod not being level when the control points are being measured.

Whether the prism is on the top or bottom of the prism rod, be mindful of keeping the prism rod level, especially when using it for stationing.

Secure the total station from slipping and vibrations:

Use creative measures to secure the head unit from vibrating or slipping. While the tool does run its own calibrations, making sure it is in a secure and stable location on the jobsite gives you the best chance of maintaining accuracy throughout the layout experience.

Add back-sight prisms as additional control points:

After the tool has been stationed, it is always wise to use the Measure and Record application to measure in additional control points that can be used in the future. Using back-sight prisms, such as the POA 103, are a great way to secure consistent jobsite control. The more back-sight prisms you save as control points, the more likely you are to mitigate the consequences of having control points get hidden or removed during the project.