What do we mean by "altitude" ?

[Zbip57]

I don't know if it's in CFR Part 77 but it's in FAA documents. I'll give you a hint... it has to do with "obstacles"

As far as I can tell Part 77 prescribes, "Standards for Determining Obstructions to Air Navigation or Navigational Aids or Facilities". For example, you cannot construct a tall apartment building right at the end of a runway. And you cannot construct a tall building if it hides or obstructs any navigational beacons, etc.

Part 77 makes frequent reference to "obstructions to air navigation", and permanent or temporary "construction or alteration". Part 77.13 lists many different types of potential "obstructions", including but not limited to "structures".

§77.13 Applicability.
This subpart describes the standards used for determining obstructions to air navigation, navigational aids, or navigational facilities. These standards apply to the following:

(a) Any object of natural growth [400' tall trees?], terrain [400' vertical cliffs?], or permanent or temporary construction or alteration, including equipment or materials used and any permanent or temporary apparatus.
(b) The alteration of any permanent or temporary existing structure by a change in its height, including appurtenances, or lateral dimensions, including equipment or material used therein.

[...] the wording is very specific and intentional to state "Structures" and "Structures" are defined by the FAA as to not have any grey area.

You insisted that the word "Structures" was specifically and intentionally used in §107.51 for the extra 400' height and radius allowance for tower inspections. You say "Structures" are defined by the FAA to eliminate any grey area, yet cannot point to that definition.

So my question remains. Why does the FAA provide an extra operating space of 400' height and radius specifically and intentionally around only undefined "Structures", but not equally around other "obstructions to air navigation" like natural growth [400' tall trees], terrain [400" vertical cliffs]?

You wrote, manned aircraft are supposed to avoid proximity to "anything on the ground", "to fly at say 500' they would maintain that over anything protruding from the ground (house, trees, tower, etc.)"

If that airspace is supposedly clear of manned aircraft, what's the point of restricting recreational flyers to 400' AGL absolutely, and not allowing them to fly above a 400' tree, up a 400' vertical cliff, or over a 400' "structure"?

 

[bill1950]

As I feen finding out, altitude especially on say a mini or mini2 can be 10 meters or more plus or minus. And as was pointed to me out in a thread where I asked about the accuracy of the altitude of my mini2. I was actually shown figures where be it uncommon but possible with the right barometric pressure to be ask much as 30 meters off. So in rare cases I could be watching my altitude on my controller and be at 400AGL and actually be around 500 feet. There is no way to calibrate the sensor or altitude on the Mini (not sure on higher DJI drones) so we really need to use common sense and really know the terrain. Topo maps can be of great help. I try to know the height of the buildings or height of trees or hills.
We have power lines that are a known 70 feet AGL. We have a tower ne’er me that I know is 125 feet. I try to fly about 5 mt away from it and point my gamble straight ahead and get a general idea of my height. I usually find that I am 5 to ten mt high or low. Most of the time closer to the 5.

 

[Zbip57]

There is no way to calibrate the sensor or altitude on the Mini.

The "calibration" is done automatically each time at startup and takeoff. It registers that spot as zero height. However, due to changes in atmospheric pressure caused for example by a change in temperature, that same spot may later no longer show up as zero when landing.

Barometric altimeters in real aircraft need to be calibrated to local atmospheric pressure. In my pre-flight checklist flying out of St.Catharines Airport (near Niagara) I'd turn the calibration dial to ensure the altimeter accurately showed 312' elevation while still sitting on the ground, or the tower would always tell you what the correct calibration setting is before departure. They tell you a new setting when returning to land. All navigation is done using sea level as the base reference for all altitudes.

I was actually shown figures where be it uncommon but possible with the right barometric pressure to be ask much as 30 meters off.

A sudden change in temperature could certainly cause that much change in barometric pressure.

The Mini has infrared sensors on its bottom which do accurately measure height above ground, as long as the sensors are within range (~10m). But the DJI Fly app doesn't display that height. You can compare the barometric altimeter vs VPS sensor using the Phantomhelp system to plot your flight logs.

 

[bill1950]

The "calibration" is done automatically each time at startup and takeoff. It registers that spot as zero height. However, due to changes in atmospheric pressure caused for example by a change in temperature, that same spot may later no longer show up as zero when landing.

Barometric altimeters in real aircraft need to be calibrated to local atmospheric pressure. In my pre-flight checklist flying out of St.Catharines Airport (near Niagara) I'd turn the calibration dial to ensure the altimeter accurately showed 312' elevation while still sitting on the ground, or the tower would always tell you what the correct calibration setting is before departure. They tell you a new setting when returning to land. All navigation is done using sea level as the base reference for all altitudes.


A sudden change in temperature could certainly cause that much change in barometric pressure.

The Mini has infrared sensors on its bottom which do accurately measure height above ground, as long as the sensors are within range (~10m). But the DJI Fly app doesn't display that height. You can compare the barometric altimeter vs VPS sensor using the Phantomhelp system to plot your flight logs.

Can you explain further please. I know nothing about the Phantomhelp system to plot my logs.

 

[Paul B]

OP appears to be in France, so FAA regs are totally irrelevant! EASA regs state 120m above ground level, so the dangling bit of string analogy seems good.