What are backsight checks and why are they important?

What is a Backsight Check's Purpose?

A backsight check allows end users to inspect if the total station still recognizes the control points it measured during set up to be in the same location. If there are errors during a backsight check, restationing is recommended.

Errors in backsight checks are typically tied to total stations being affected by wind, vibrations, tripod stability, or even being slightly bumped. Backsight checks are quick to do and should be done regularly throughout work. It is usually recommended that a backsight check occurs every 60 minutes, which is the same occurrence rate for the PLT 400's automatic calibration.

If errors in backsight checks are significantly large, perhaps inspect if the jobsite environment may be causing the tool to slip or slide slightly, or if vibrations are affecting where the tool is anchored. Also inspect if the points you use in the backsight checks are reliable - are the points themselves getting bumped or moving from the jobsite conditions?

This article on tool stability and this article on control point best practices may help troubleshoot consistently poor backsight checks.

How to Perform a Backsight Check

Step 1- Open the Application

The backsight check application will be available to you if you are working on a project that has completed a stationing with control points. You will find it under system applications:

Once opened, it will remind you the purpose of the application, that you are re-measuring control points to inspect deviations after the original stationing:

Step 2 - Choose the control point for measuring

The entire control point list will appear for you to use. It is recommended that you choose a control point that was used during the most recent stationing to compare the backsight check as best as possible to your original set up. However, any control point from the list can technically be chosen. The default control point that first appears will be the first control point used during your most recent stationing process.

Notice, however, that the only control points that are not appearing are those that do not have a lockable prism type associated with it. Therefore, control points that are not prisms, such as reflective targets or points on surfaces measured directly with the laser, will not be able to be used as part of this step.

If there is a control point that is not appearing on this list that you would like to use, make sure to edit the point in the point list from the CAD view and assign a prism type to it that you intend to use.

The reason the software only suggests control points with a prism target association is to maintain measurement integrity, as other targets can be affected more easily by environment conditions (light, dust, water, etc.).

Is there a way to perform a backsight check another way?

Yes, but care would need to be taken to set up a backsight check system that is reliable. A common way to do this is after initially stationing the tool, measure and record your desired backsight points of your choice on the jobsite. When you want to perform a manual backsight check, simply open the measure and record application again and measure the points a second time. You can easily use the tablet's measuring functions for CAD to inspect and visualize any potential deviations between the two points.

Step 3 - Measure

Once the correct control point you desire to measure is selected, press the red measure button.

Step 4 - Observe Results

If your  measurements show deviations that are within the tolerance that are in your settings, HCL will simply suggest that the station is in tolerance. See this article to understand what tolerance settings are and how to set them.

If you do not have any tolerance settings defined, or, if you have a backsight measurement that is out of your set tolerance, you will see the deviation data, as you see below:

This data reveals how the measured point is different from the measured location the total station originally found it. In the case above, each coordinate seems to have a slight millimetric deviation, and the coordinate values that are outside the tolerance settings will be highlighted in red.

Step 5 - Restation and Troubleshoot

If backsight data is poor, restationing the tool on your provided control points is recommended. Following a restationing, it is wise to have the total station guide you to a point you had previously laid out during the same work day to investigate potential deviations between the old stationing and the new stationing you just performed.