Z Bump – Debias Operation Validity Testing

A common practice amongst GeoCue customers (and internally) is to use previously established control points to compare with an sUAS produced point cloud to gather absolute accuracy statistics. The vertical mean error from these statistics is often used to “Debias” a dataset. This is simply shifting the point cloud data to match the measured control points in the vertical component. We recommend performing this Z bump operation, only if specific statistical thresholds are met.

The user basically needs to find out the certainty of the mean value. This can be obtained by examining the Standard Deviation of the Mean (SDOM):

where sr is the standard deviation of the residuals and N is the number of checkpoints

If the absolute value of the Z mean is larger than the SDOM, then you are justified in debiasing. Simply, if the uncertainty in the mean is small relative to the mean itself, the mean can be shifted (debiased) by doing a Z bump. To go a bit further, we recommend that debiasing only be performed if the absolute value of the Z mean is at least 3 times the value of the SDOM. We determine these values by using the Control Report functionality in LP360 / TV Evo. The user will simply subtract the SDOM value from the Mean Error contained within the report:

Vertical accuracy assessment portion of the LP360 control report dialog

For a more in depth discussion regarding this topic, please see Lewis’ LIDAR magazine article from August 2019.

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