I am calculating various gridmetrics using the Lidar processing tools in FUSION. The dataset I am working with contains 154 flight lines of high density data, at a nominal pulse spacing (NPS) of 1.5 meters (full metadata available here). Specifically, I am using the grid metrics command in FUSION to calculate all returns above 2m (column 53). The attached figure shows the problem I have been encountering. You can see there are detectable grid values at the scan line overlap areas. There are even detectable values in the non-overlap scan line areas over lakes (observed as white blob in screenshot).
How should I be interpreting these apparently faulty results--is it a result of the Lidar dataset, or is it some type of processing issue with FUSION?