Not saying you're wrong but personally I'd like to hear the FAA's ideas on that.
The FAA will just tell you the questions are moot since the entire area around the Hoover Dam is a No-Fly-Zone. So don't bother us with any of these annoying drone questions.
Being the surface of water or surrounding terrain is the measurement to base from, I'd guess the measurement of the dam's structure itself would only be what is sticking out of the water or the terrain around it,
That's actually what I would prefer as well, as it's the only measurement that makes any sense. Treating the section of the dam protruding above the water's surface like any other "obstacle" makes perfect sense.
If it's just an island made of natural rock poking above the level of the water's surface, you would be entitled to fly as much as 400' above the rock, since AGL is measured vertically from the water's surface and from the rock's surface. That is the "perfectly simple" and easy to understand concept that everybody keeps referring to.
If the regulation simply said for drones to stay
within 400' of any "obstacle", then everything is perfectly clear. You could fly up vertical cliff-faces, mountains, buildings, towers, trees etc, as long as you always stayed closer than 400' to that obstacle, whatever it is. The regulation for manned aircraft should say the opposite, stay more than 500' away from all obstacles. Then we'd have a clear 100' separation for everyone's safety.
All of the confusion and debates arise because the 400' AGL is supposed to be measured only
vertically above "ground", which can also mean water, but not structures, and not trees, and not treehouses, and not structures disguised as trees, and not...
(By the way, the actual height of any "obstacle" hazardous to aviation will be marked on your aviation sectional charts, but that height is always referenced to MSL, not AGL. That's the only measure that makes any sense at all, because the top is all we care about. Nobody needs to know how many levels of parking garage descend however deep into the ground below that office tower "structure".)
Is there a legal definition of "structure" somewhere in the FAA regs. Does it define how the height of a structure is measured? Is it measured from the height of the abutting earthworks (or water), or is it measured from base of its foundations? If you do a Google search for the height of the Hoover Dam, it's always listed as 726.4 feet. That's a pretty precise figure, and obviously not based on what is exposed above the water line (which is constantly varying).
If I'm flying my plane above a dense Amazon jungle I will do my utmost best to avoid descending below the solid level of the treetop canopy. I really don't care one iota whether those trees are 400' tall, or only 1 foot tall. I'm only seeing the
tops of the trees. From my aerial perspective, that treetop canopy counts every bit as much as solid "ground level" as if it was a solid granite mountain top, or the solid ground of a cow pasture, or the solid concrete top of the Hoover Dam.
As far as I'm concerned, "Above Ground Level" should always be measured from the highest surface of any hazardous "obstacle", rather than trying to debate whether it's actually measured from the bottom of a lake or the water's surface, or the bottom of a structure's foundation, or from where that structure first protrudes through the surrounding face of the earth.
The top of any tree, or the highest peak of any tall microwave tower effectively
is ground level, just as much as any peak of any solid mountain top. Manned aircraft don't want to hit
any of them.
All that really matters to manned aircraft is how
high (MSL) you need to climb to clear the
top of any
obstacle, or how far you need to go
around it to stay safely away from it (500' in every direction). If that space is safely
cleared of manned aircraft, then that empty space should be safely available for use by any drone flyer when staying
within 400' (vertically, horizontally, whatever) of any obstacle. That leaves a comfortable 100' buffer of separation so everybody stays safe.
Why all this song and dance about "structures" needing to be treated differently than "ground", and trees being ignored altogether, and only being allowed to fly along cliff faces as long as they're not actually
vertical cliff faces.
Everything you wouldn't want to hit with an aircraft, including trees, is an "obstacle". Planes stay away from obstacles, drones stay close. That is perfectly simple, no?
