How to use the Reflective Target Georeferencing workflow

New user guide for successful introduction to the Reflective Target Georeferencing workflow.

With the appropriate reflective targets, and their X, Y, Z coordinate measurements, it is possible to georeference (Rigid transformation) a point cloud using FARO connect.

The rigid transformation applies a single transform to every point in the point cloud, so that the XYZ position of every point is altered. However, the relative position to other points in the point cloud will remain the same. Comparing the shape of the point cloud before and after the rigid transformation should show identical results.

Data capture:

The following is required for this process:

• A minimum of 4 retroreflective targets, of the following specs:

  • Material: Survey grade or 3M Diamond grade retroreflective targets.

  • Size: 300mm x 300mm (recommended), 200x200mm (minimum)

  • Shape: Circular or square.

  • Crosshairs: Required for surveying the target

  • Number of targets: Six targets recommended, minimum four targets required. Using more targets provides a higher level of redundancy if/when control point/s cannot be matched to the target/s.

• A properly formatted, tab delimited coordinate file.

  • Measurements should be X, Y, Z, and in Meters. Follow example:

When capturing data, standard mapping best practices should be employed. Start and finish the scan in the same location/orientation, thus ‘Closing the Loop’ to achieve the best results. When scanning reflective targets, keep the sensor 1 to 3 meters from the target/s and ensure that the LiDAR is facing the target and capturing the target for at least 3 seconds. Add a little more time when using the REV 6 LiDAR scanner. For more information: GeoSLAM for Elios 3 Online Training (light version)

Processing data:

With the data exported, and coordinate file formatted, there are two ways to georeference the project.

One way is to process using the Flyability Reflective Target Georeferencing workflow. A problem with this workflow is if any aspect of the data (targets, measurements, scanned environment, etc) fails, the entire process fails with no results regardless where failure occurs.

An alternative approach is to process the project using the Flyability SLAM Processing workflow (How to process a point cloud) and then follow up with the georeferencing. In this way, there is the ability to view the point cloud, targets, and employ any filtration as needed before georeferencing, and/or if georeferencing fails. Setting the Color Mode to Intensity will provide the best vision for viewing the targets.

This is the better workflow when georeferencing data collected with a standard REV 6 LiDAR scanner.

How to georeference the processed project:

Once the project has finished processing you can georeference it using the Registration tab:

1. Click the Registration tab.

2. Select the Reflective Target Georeferencing (Cloud) workflow.

1. Click Pick from Project and select the processed .laz. If the point cloud was filtered before, ensure to select the filtered point cloud.

2. Click the dropdown arrow and select Browse System. Navigate to, and select the coordinate .txt file. After this selection, subsequent processes can use Pick from Project for selection.

3. Click the dropdown arrow and select Elios 3.

4. Click Pick from Project and select the trajectory.

5. Click Run.

On completion of processing the selected point cloud will be georeferenced with the following files:

• <filename>_registered.laz: Registered output point cloud file.

• <filename>_registered.gs-traj: Registered output trajectory file.

• alignment_dashboard.gs-chart: Alignment dashboard displaying the accuracy of the registration.

• Survey_error.gs-accuracy: Accuracy table of each control point and overall RMSE.

• .gs-control: Name of the imported surveyed control point file.

For more information: GeoSLAM for Elios 3 Online Training (light version)

If aspects are failing please see our FARO / FAQ Troubleshooting article for more information.