If I set a new Home Point during flight - does the MM know the altitude of that home-point? If not, I can see scenarios where it flies into trees, mountain walls or obstacles during a RTH-process, due to the fact that it is missing data for the altitude of the new Home Point. Am i right?
What sort of scenarios are you worried about?
If you set your RTH height to 40m, for instance, that's always referenced back to the height of your original power up and takeoff location. As others have pointed out, that recorded zero height remains the same even if you subsequently record a new home point while flying.
It's important to always survey your surroundings to take note of the highest possible obstacle you might encounter along your flight path, and set your RTH height to something safely exceeding the height of that tallest obstacle.
But it's also important to remember that, if the Mini is already flying at a height
greater than your configured RTH height, when RTH is triggered it will return at that current height. It will
not first
descend to the configured RTH height.
So if you are planning to fly around behind a tall tree or other obstacle, make sure you are either already flying at a height greater than the top of that tree, or be certain you have configured your RTH height higher than that tree. Then, if control signal is lost the Mini will either turn for home already at its current safe height, or it will first climb to its set RTH height before heading home.
My original DJI Phantom-1 actually
did record a new height whenever a new Home Point was recorded while in flight. This way you could trick it into exceeding its max ceiling limit. I made a video demonstrating this back in 2014.
With a RTH height set to 20m, the Phantom would ascend to 20m before turning for Home. That's all normal. Then I recorded a new Home Point while hovering 10m off the ground. Triggering RTH would then again cause it to climb another 20m above that recorded Home Point before heading home. In this case it climbed to 30m above ground level.
At the time, the question was what happens when it then tries to "land" at its new Home Point that's 10m above the ground? Would the motors turn off 10m above ground, or would it continue to descend until landing normally on the ground? That's why I made this video.
DJI changed this in later products. So now the Mini has a hard-coded maximum ceiling of 500m. You cannot fly higher than 500m above the recorded takeoff height. Upon reaching that limit, you
cannot simply record a new Home Point while hovering at that height to reset to zero and then climb an additional 500m.