The peak in question is Relay Peak by Lake Tahoe in Nevada, currently ranked with 318 ft. of interpolated prominence. Searching for .las data in The National Map yields an interesting (and quite uncommon) result: two separate surveys are available covering this same spot! The first survey ("CA-NV_LakeTahoe_2010") was flown in August of 2010, and the second survey ("CA NoCAL Wildfires B1 2018") was flown in July-August of 2018.
Let's walk through what happens next...
The few times this has happened before, I have defaulted to using the newer survey, and that's what I do this time too, because newer=better, right? Despite the krumholtz trees around the area, identifying the summit and saddle ground is easy enough since the vegetation is sparse, and results are quickly forthcoming. Using the 2018 data, the peak ends up with 298 ft. of prominence. I mark the peak as demoted and move along.
On a hunch, I think "what if there was snow or something at the saddle?" Maybe I should double-check with the other survey's data since it's so close. So, I download the 2010 data and...whoa! There are a WHOLE LOT MORE POINTS! In fact, we can quantify this using Lasinfo as "point density," or the number of last-returns per unit area. The 2010 LiDAR data has 13.5 points per square meter, and the 2018 LiDAR data has a measly 5.8 points per square meter!
As it turns out, with the 2010 data, the summit location is almost identical (less than 1 ft. from the 2018 summit point), but the elevation is about 2 ft. higher, and the peak ends up with exactly 300 ft. of prominence.
___________________________
Now, for the geeks, it's a little weirder than it sounds. You see, since the selected summit points in the 2010 and 2018 surveys were so close together, it's improbable that there's actually a super narrow 2-ft. tall rock sitting on top. The answer lies in the arcane magic/math of point spread functions and LiDAR footprints. While we sometimes talk about LiDAR "points," those points aren't infinitely small. They actually represent a spatial average due to loss of collimation of the laser beam. As the laser pulse travels from the airplane to the ground and back to the optics, it spreads out and ultimately reflects off a non-infinitesimal area of the ground. In fact, the true footprints of nearby points can often overlap! Not only this, but the pulse doesn't return to the LiDAR optics all at once; rather, the point spread function for different distances to the ground affects how "blurry" the laser looks--this is called the waveform, and waveform analysis is ongoing research. How exactly all of this comes together for various LiDAR systems is a serious academic research topic and is usually ignored for all practical purposes. However, when we start pushing the limits of the data by trying to measure rough topography to the nearest foot, this is a problem we will encounter.
The likely reason that the 2010 survey has higher summit values is because the higher point density tells us that the airplane <probably> flew at a lower altitude (or used a better scanner, or both). In either case, the laser footprint is almost certainly smaller in the 2010 survey, yielding higher accuracy and changing the ranked status of this peak. This is why I am not comfortable declaring East Twin Peaks higher than West Twin Peaks in the Wyoming 13ers list, since the western summit's highest LiDAR point is only 2 ft. lower and that peak has an extremely sharp summit block.
Moral of the story: the determination of summit elevations by airborne LiDAR data has several feet of uncertainty in sharp/rocky areas due to the unknowable effects of the laser footprint on each point's reported Z coordinate. Highly technical aspects of LiDAR data (like the point density and the laser footprint) can make or break a peak's ranked status in some cases. I suggest a new category of LiDAR-soft-ranked peaks with prominence in a range somewhere around 295-299 ft.
FYI if anyone really cares, the answer is to buy one of THESE and use Terrestrial Laser Scanning (TLS).
Pictures: 2010 LiDAR data (first) and 2018 LiDAR data (second) with exactly the same color ramps in the same location, both shown with approximately 3 m lines for scale.
