What are backsight checks and why are they important for accuracy?

Overview

This article explains what backsight checks are, why they are important, and how to ensure you have sufficient backsight points available throughout the life of a project.

When starting a new job, follow the workflow below to get the most value from backsight checks and maintain confidence in your layout accuracy.

Station the total station accurately using the control points provided by the surveyor, general contractor, or other trusted source.
Create additional control points (backsights) immediately after accurately stationing using Measure and Record. Establish several stable prism locations around the jobsite so you are not only dependent on the original control points.
Re-station using your new expanded control network through Control Point Stationing, ensuring that your newly created backsight points are included in the stationing process.

By following these three steps, you establish a robust control network that can be used throughout the project, that can also be used to verify and maintain layout accuracy.

Benefits of This Workflow

Once your additional backsight points have been established and included in a stationing operation, you have optimized two valuable station accuracy verification methods:

"Remeasure Station Setup" to confirm the quality of your current stationing during your workday.
"Backsight Checks" (explained in this article) to quickly verify or inspect that the total station has not moved since it was originally stationed.

Together, these tools help you identify potential issues early, maintain confidence in your layout work, and reduce the risk of costly rework caused by unnoticed station movement.

Best Practice: Create multiple prism-based backsight points distributed throughout the jobsite as early as possible. The more reliable reference points you have available, the easier it will be to verify station accuracy and recover from lost or inaccessible control points later in the project.

Backsight Check Demo

The video below shows the simple way the software automatically performs a backsight check by checking the mounted prisms and targets you used during the stationing process.

Steps to Perform a Backsight Check

Open the application and select "measure"

Choose one of your previous control points used during your initial stationing from the list. Note: only control points from a mounted prism can be used.

Once the correct control point you desire to measure is selected, press the red measure button. The tool will turn to that point's location and attempt to measure it. Double check that the prism type is indeed correct.

Review the results - you will either see "station is within tolerance," or you will see a window showing the errors associated with the backsight measurement.

Restation and Troubleshoot (if needed) - If a backsight check indicates that your station is outside the acceptable tolerance, the first recommended action is to re-station the total station using your known control points. This can easily be done with the "Remeasure Station Setup" application.

Frequently Asked Questions Regarding Backsight Checks

What is the purpose of a backsight check?

A total station is only as accurate as its stationing. Even when a tool is initially set up correctly, environmental conditions, jobsite activity, and accidental disturbances can cause subtle movement that impacts layout accuracy.

Backsight points provide a simple and reliable way to verify that your total station remains accurately positioned throughout the day. By creating and regularly checking your own reference points, you can quickly identify potential stationing errors before they affect your layout work.

Therefore, 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.

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.

Why did my tool not turn to the correct height location of the backsight control point I selected for the backsight check?

If Heights are disabled in your HCL settings, the total station can still navigate to the correct X and Y location of the selected backsight control point. However, because elevation data is not being used, the instrument has no height (Z) information to guide it to the correct vertical position.

As a result, the total station may appear to be targeting the correct horizontal location while not turning to the expected elevation of the control point.

Recommended Workflow

If your layout work is primarily 2D but you still want accurate navigation to backsight control points during stationing and backsight checks, consider the following workflow:

Enable Heights when creating and measuring control points.
Enable Heights during stationing operations.
After stationing is complete, disable Heights if your layout tasks only require X and Y coordinates (2D).
Enable Heights for backsight checks.

By capturing elevation data during control-point creation and stationing, the total station can accurately navigate to the full X, Y, and Z location of those backsight points whenever they are used for station verification. Just remember to enable "heights" when backsight checks occur. At the same time, you can continue working in a simplified 2D workflow for day-to-day layout tasks by temporarily disabling heights.

Best Practice: Even if your project is primarily 2D, maintaining height information for control points and stationing provides a more reliable control network and improves the quality of station verification throughout the project.

Why don't all control points appear within your list of available points for backsight checks?

Only control points that have an assigned lockable prism type will appear in the backsight check list.

As a result, points measured using other methods, such as:

Reflective targets
Surface measurements taken with the laser
Non-prism reference points

will not be available for selection during a backsight check.

Adding a Missing Control Point

If a control point does not appear in the list but you would like to use it for future backsight checks:

Open the Point List from the CAD view.
Locate and edit the desired control point.
Assign the appropriate prism type that will be used to observe the point.
Save your changes.

Once a valid prism type has been assigned, the point will be available for future backsight checks.

Why are only points with prisms allowed?

Backsight checks are intended to be a highly reliable method of verifying station accuracy. Prism targets provide a consistent and repeatable measurement that is less susceptible to environmental influences.

Other target types can be more easily affected by factors such as:

Changing lighting conditions
Dust or debris
Moisture or water exposure
Surface quality variations
Viewing angle differences

By limiting backsight checks to control points with prism target associations, the software helps ensure the integrity and repeatability of the verification process.

Best Practice: Create several permanent prism-based control points around the jobsite after your initial stationing. This gives you multiple reliable reference points that can be used throughout the project to quickly verify station accuracy and identify potential instrument movement before it impacts layout quality.

What does "station is within tolerance" mean?

If all measured deviations fall within the tolerance limits defined in your settings, HCL will indicate that the station is within tolerance and no action is required. For more information about tolerance settings and how to configure them, see this article.

What do the point error values mean when reviewing the backsight check results?

Once the measurement is complete, HCL compares the current measured position of the control point against the position recorded when the total station was originally stationed.

If no tolerance settings have been defined, or if one or more deviations exceed the configured tolerances within your settings, HCL will display the measured deviations for review.

The displayed values show the difference between the control point's original measured location during stationing and its current measured location. These deviations are reported for each coordinate component (N,E,H), helping you determine whether the total station may have shifted since it was stationed.

Coordinates that exceed the configured tolerance limits are highlighted in red, making it easy to identify potential issues at a glance.

What additional accuracy verification should I do during layout?

After performing a stationing (especially after re-stationing), and periodically during the workday, it is always best practice to validate your set up accuracy by having the total station navigate to a point that was previously laid out earlier in the day and compare its current position to the original layout location.

This comparison can help you determine:

Whether the original stationing was accurate
Whether the total station may have shifted during the workday
Whether any discrepancies exist between the previous and current station setup
Whether additional investigation or rework is required before continuing

If significant differences are observed, inspect the jobsite control network and station setup for potential causes, such as disturbed control points, tripod movement, environmental changes, or setup errors.

Best Practice: After re-stationing, verify multiple known points whenever possible. Consistent agreement across several points provides greater confidence that the total station is accurately positioned and ready for continued layout work.