From a private conversation I had with Karl back in 2014:
Overlap is any area that is covered by 2 or more swaths. Nice, but not the phenomenon or condition we want and need to identify. What we are concerned with is more properly called OVERAGE. Harder to define without a picture, but, imagine a project intended for single coverage at 20% sidelap, with swaths 1km wide. The intent is to use the center 800m of each swath … what I refer to as “the tenderloin”, and to edge-match all of the project’s tenderloins together for a nice seamless, gapless single coverage.
In this scenario (which was and remains a VERY common practice for years), for each swath, the 100m outboard of the tenderloin (on each side) is “excess” data. Still perfectly good data, just not needed to accomplish the single coverage. That 100m on each side of each swath is the OverAGE for that swath. And those are the points that are to be flagged with the (regrettably mis-named) “Overlap” Flag. (On the other hand, if you tagged all of the points in the true “OverLAP”, only 60% of the swath would remain, and without those points, you’d have a 200m gap between swaths).
The genesis of all this was that data producers would classify all of the overage points as Class 12, and then do their DEMs based on the tenderloin-based single coverage. Which made nice DEMs, because you eliminated the stripes of double-density (higher variability) data that were plainly visible in the resulting rasters. The PROBLEM with that is users who were interested in something OTHER than the DEM could not do anything with those points without “damaging” the dataset classification (losing the differentiation of the “overage” points). It’s all a remnant of the days when DEMs were King and nobody regarded the point cloud as anything but a means to that end. The practices became entrenched, but then we realized that Lidar was not just for DEMs anymore. Now we are struggling to break old bad habits.
In the v1.0 Spec (using LAS v1.2/1.3), we required that “overage” point be flagged using some means other than Class 12, that allowed both “normal” classification AND differentiation of the overage condition. Makes the data useful beyond just the DEM. How this was done was defined, because data producers had their own systems and processes and there could not be a one-size-fits-all solution. NOW, however, using LAS v1.4, there IS a one-size-fits-all solution: The “Overlap Flag”.
And from a followup conversation:
For “Overlap/age”, your definition is keyed on modeling, and gives the impression that these points should not be used for analysis, in a general or broad-ranging sense. I think the real key is the spatial location, relative to the swath and its overlapping swaths. I would say: Overlap: bit flag to mark points that are unnecessary to achieve a consistent “depth of coverage” (meaning single, double, whatever). No implications are made regarding point quality. To be certain, that description needs a great deal of refinement before “publication”, but it properly limits the term to spatial location/relationships. I would not want to make any implication as to whether anybody should or should not use the points however they choose. That’s their business. Additionally, SOME people may interpret your definition as extending to not “modeling” these points for classification purposes.