Georeferencing point clouds using survey points

[c_blenkinsopp]

I have a registered point cloud which includes 7 individual scans which were imported with the coordinate system set as OSGB36. I now wish to georeference the cloud using GPS surveyed targets within the cloud. I have returned to the registration page and provided coordinates of multiple targets throughout the scan (I have tried multiple combinations of locations and numbers of targets). When I go back to the project, the coordinates have transformed but are nowhere near any of the survey points I have entered.

Any assistance would be much appreciated. 

[alexlibrelon]

It sounds like you're encountering difficulties georeferencing your registered point cloud using survey points in ReCap Pro. Here are some troubleshooting steps and considerations to help you achieve accurate georeferencing:

1. Coordinate System Verification:

• Match Survey Points System: Double-check that the coordinate system of your survey points (OSGB36 in your case) matches the coordinate system you defined for the point cloud when importing it into ReCap Pro. Any mismatch can lead to incorrect transformations.

2. Target Point Selection:

• Clear Visibility: Ensure the survey targets you're selecting within the point cloud are clearly visible and well-defined. Avoid picking noisy points or points with low confidence values.
• Spatial Distribution: Choose survey points distributed across the entire point cloud, not concentrated in one area. This provides a better reference for the georeferencing process.
• Multiple Points (Recommended): Use at least 3-4 well-distributed survey points for a more robust georeferencing solution. The more points you use, the more accurate the transformation should be.

3. ReCap Pro Registration Settings:

• Transformation Model: Consider adjusting the transformation model used in ReCap Pro's registration settings. The default might be set to "Rigid" (preserving distances). Depending on your data, a "Similarity" (allowing scaling) or "Affine" (allowing shearing) model might be more appropriate. Experiment with different models if necessary.

4. Outlier Detection:

• Review Outliers: After applying the georeferencing transformation, check for significant outliers (points with large discrepancies between their surveyed and transformed positions). Outliers can sometimes affect the overall accuracy. You can use ReCap Pro's outlier detection tools to identify and potentially remove them.

5. Software Limitations (Consider):

• Complex Transformations: While ReCap Pro is a powerful tool, it might not be ideal for highly complex georeferencing scenarios. If your survey points require a 7-parameter transformation (including scaling, shearing, and rotation), consider using specialized georeferencing software designed for such tasks.

Additional Tips:

• Review Documentation: Refer to the ReCap Pro documentation for detailed instructions and explanations on the georeferencing process and available settings.
• Online Resources: Search online forums or communities for ReCap Pro users. You might find discussions or tips related to georeferencing point clouds with survey points.

By following these steps and considering potential limitations, you should be able to achieve more accurate georeferencing of your point cloud using the survey points in ReCap Pro. If the issue persists or your project requires a more complex transformation, consider exploring specialized georeferencing software for optimal results.

[sroyZU85N]

I appreciate your guidance! I'll verify the datum settings to confirm whether the GPS targets were still set to WGS84, while OSGB36. I'll also examine the registration process to ensure that the targets were properly matched. Your suggestion to re-register using fixed control points and checking residuals is a great idea if it enhances the alignment. Thanks for your assistance!