Over Control Point Stationing - How To

Purpose of "Over Control Point" Stationing

"Over control point" stationing lets the end user tell the head unit where the head unit is without needing to measure control points to "calculate" the coordinate location. The user simply sets up the unit directly on the control point coordinate. This often is a very accurate method of stationing, but the end user must be very sure that the control point they use for stationing is indeed accurate as well.

Then, the end user can simply measure a second point (or an angle to that point) to orient the tool's angle on the jobsite.

For context of how this works, see below:

For a total station (or, layout tool) to know where it is located on a digital design, it needs  three things: a coordinate, an orientation, and, if working with heights, a height value. See more about each of these below:

1. A coordinate location within the design:
   • All CAD or point files have a coordinate system, and the total station will need to what coordinate location it is in reference to the file it is working in.
2. An orientation or, angle:
   • Once it knows at what coordinate it is located, it needs to know which way it is looking.
   • As an example, imagine you are blindfolded in a living room and are told to walk toward the kitchen. Even though you knew which room you were in, you would not know which way to start walking unless you knew the direction the kitchen was. Likewise, the total station is "blindfolded" until it knows which way it is looking on a jobsite
3. A height (optional):
   • If height values are required, it not only needs to know where it is and which way it is looking, but how high it's telescope is above it's coordiate's height value.

Which total stations can do this type of stationing?

For Hilti, only the POS 150 and POS 180 can perform this type of stationing. It requires a plumb laser to accurately shoot down beneath the tool to the control point below, which the POS tools can do.

The PLT 300 and PLT 400 will need to use another method, such as Control Points or Control Lines Stationing.

How-To Step 1: Open "Over Control Point" Stationing

Select the stationing option from the home screen. Note, you will need a project that has control points already imported into the project.

How-To Step 2: Select the Control Point your Total Station is Located on

Use the point list on the left side, or select the point from within the CAD view and press the green checkmark to indicate your total station's location. In the example below, the total station is located on point CAT-3. Press the green checkmark to confirm you have selected the correct coordinate.

Notice that the total station coordinates are identical to the coordinates of CAT-3 (my selected point) from the point list:

How-To Step 3: Set your Angle or Orientation

Tap on the other control point you are measuring to and measure. You'll notice on the right side of the screen that you have an option to measure the control point only as an angle (indicating that you are simply setting the total station's orientation), or, you can measure to confirm the distance of the control point you are measuring to as well. Below will explore and explain both options:

Measuring based on an angle only:

In the screenshot below, you can see that "NAGEL-1" is the point that is selected as the orientation point for this stationing process. It is the point I intend to measure. You can also see that the "measure angle" button is highlighted in red.

If I use this button, I am telling the total station that I'm simply orienting the tool on the jobsite based on the angle it is looking at when it measures "NAGEL-1." I am not necessarily checking any distance information from my stationed point to "NAGEL-1."

After clicking the "measure angle" button, you can see that the total station now has its orientation. There is no deviation data, as the total station is simply assuming that the information you have given it is accurate: the point it is on is correct, and the angle it is looking is correct.

Now, from here, you would simply indicate the height of the instrument, which we will review below.

Why use the angle option rather than the option to measure angle and distance together?

From a practicality standpoint, jobsites often have both control lines and control points. Measuring only on an angle allows you to set the tool on a control point, for instance, that falls directly on a control line. Since the line is visible, you can orient your total station to look directly at the control line. Then, you can select any control point that also falls along that line in that direction, and measure. This is especially helpful if the control points in your point list along that control line are hidden by jobsite conditions, but the line is visible.

As long as the control point and orientation angle are accurate, the total station will also be able to perform layout and measuring applications accurately. It's up to the end user to take the necessary precautions and perform the necessary troubleshooting checks to verify accuracy of set up.

Measuring based on angle and distance:

In the screenshot below, the choice to measure angle and distance is chosen again using point "NAGEL-1" as the example.

Notice that in the screenshot below, after measuring the "orientation" control point, the software gives me deviation data that I make note of and consider, to verify if my control situation is accurate. As the example indicates, the software is telling me that the point I have measured is technically 1mm too far to the North of the CAD design, and 6mm too far west on the CAD design.

Regardless of this deviation, if I continue with this stationing, the total station will still accept the orientation angle I gave it. It is common to have deviations, as in-field conditions are very rarely perfect, but it is up to the end user to troubleshoot and inspect potential errors before layout begins.

Note that if the point I measure is well beyond any practical error (in the case below, my measurement was almost 1m off where the point should be), the total station will not let you continue. Instead, it shows the orientation it was at when it measured the point (VA/HA), and also the absolute distance it measured (SD) to the point. SD means it takes not only the horizontal distance, but also the slope distance into account of how far away the point was when measured.

Another feature you can use to check your deviations is by comparing your measured deviations from the digital design's expected measurements. In the example below, I measured 6mm too far short of the point's expected distance.

How-To Step 4: Set your Tool's Height

Remember, since you are stationing over a control point, the total station is assuming that its coordinate height is also at the height of the control point you are stationing over. During the stationing process, it is going to set up the station and instrument height based on this point's coordinate.

Remember, if heights are not critical for your layout, you can always turn them off in settings. See this article for deeper information on setting a station height.

To get started with heights in this stationing process, click "Set Instrument Height."

Then, you'll notice that you are automatically taken to the "Station Height" screen where the only option is to set the "instrument height." This is because the total station is assuming the point height of the point we used during stationing (632.778m).

Instrument height is simply the distance from the ground where the point you are on is located to the center of the telescope, which is indicated by a cross hair on your head unit. It is important to indicate this distance so that the total station knows how high its telescope is off the coordinate point, and thereby making accurate height measurements as you work.

You can always go back later and edit the station height by going to the home screen. The other benchmark height options will be available to you when you do this.