How to: Use GCPs for Elevation Adjustment and Accuracy Assessment
If the current vertical coordinate system of a point cloud is different from the target system for final product delivering, for example, LiAcquire Pro currently generates point cloud in the vertical coordinate system of WGS84 ellipsoid height but the final point cloud needs to be delivered in local geoid height, ground control points (GCPs) can be used for elevation adjustment in LiDAR360 and elevation accuracy assessment following the workflow in this article.
Note: The workflow introduced in this guide will only adjust elevation values of a point cloud. The horizontal coordinates will remain unchanged in the output point cloud. To perform projection transformation of a point cloud, please refer to the Reproject guide, and to perform data transformation, please refer to the Transformation guide.
Step 1: Use Control Point Report to generate elevation differences file
The Control Point Report tool will create a report of elevation differences between point cloud and GCPs, which can be used as input for Elevation Adjustment tool, and for elevation accuracy assessment. At least three GCPs are required to run Control Point Report tool, and it is recommended to place GCPs on flat and concrete surfaces and distribute evenly across the entire survey region.
Please follow instructions on Control Point Report user manual to run the tool.
Step 2: Use Elevation Adjustment to adjust elevation values
The Elevation Adjustment algorithm builds a triangular network model for the differences between GCP elevations and point cloud elevations, and then adjusts the elevations of the point cloud by interpolating correction values from this triangular network model. With the control point report file generated in Step 1, run Elevation Adjustment tool following tool manual to generate a new point cloud in the target elevation model.
Step 3: Use Control Point Report to generate elevation accuracy report
To evaluate the elevation accuracy of the adjusted point cloud, run Control Point Report tool again using a separate set of validation check points as Known Points.
Note: When surveying a small area, the elevation differences between one height model and the other are similar across the area. When homogenous enough, one could apply a constant adjustment value to the entire point cloud using the Transformation tool as dZ.
NOAA provides an online/desktop tool to find out the elevation difference between two height models in a specific location: https://vdatum.noaa.gov/vdatumweb.