Does different Launch points affect saved waypoints

[Rebtech]

Here's my scenario. I camp at a lake that has a dam, and at the beginning of the season, the water levels are really low (about 100-150 feet). What I'm trying to do is create a flight path via waypoints and repeat this flight next year when the dam is at its lowest point, and then again when it's at its highest. I want to take the two videos and show the transitions between spring and fall.
Right now, the lake is at its highest level. I launched my drone from my site (which is 30-50 feet above the current water level). I then walked around the whole camp with my Air 3, setting up camera views and waypoints—21 in total.
The next day, I asked a friend if we could use his boat to follow the drone just in case.
I didn't want to launch my Air 3 from my site and then have to walk down to the beach, hop in the boat, and then hit start. I was worried about losing the RC connection, so I decided to take my drone to the beach and launch from there. I got my Air 3 up and started the mission.
All was good until it came to skimming the lake at about 20 feet above the water. The drone just wanted to dive right into the lake, so I had to hit pause. In my mind, I had set a good buffer of at least 20-30 feet to make the flight more cinematic, but it just wanted to dive into the lake.
I did some Google research and apparently, the drone's altitude is based on your initial launch site. So, if I had launched from my campsite, it would have been fine! But because I decided to launch from the beach, which is basically 20 feet lower, that must be the reason it wanted to go into the water.
I thought that once you set a waypoint, it was based on the GPS coordinates of the drone and it shouldn't matter where you launch.

 

[Belzorium]

Here's my scenario. I camp at a lake that has a dam, and at the beginning of the season, the water levels are really low (about 100-150 feet). What I'm trying to do is create a flight path via waypoints and repeat this flight next year when the dam is at its lowest point, and then again when it's at its highest. I want to take the two videos and show the transitions between spring and fall.
Right now, the lake is at its highest level. I launched my drone from my site (which is 30-50 feet above the current water level). I then walked around the whole camp with my Air 3, setting up camera views and waypoints—21 in total.
The next day, I asked a friend if we could use his boat to follow the drone just in case.
I didn't want to launch my Air 3 from my site and then have to walk down to the beach, hop in the boat, and then hit start. I was worried about losing the RC connection, so I decided to take my drone to the beach and launch from there. I got my Air 3 up and started the mission.
All was good until it came to skimming the lake at about 20 feet above the water. The drone just wanted to dive right into the lake, so I had to hit pause. In my mind, I had set a good buffer of at least 20-30 feet to make the flight more cinematic, but it just wanted to dive into the lake.
I did some Google research and apparently, the drone's altitude is based on your initial launch site. So, if I had launched from my campsite, it would have been fine! But because I decided to launch from the beach, which is basically 20 feet lower, that must be the reason it wanted to go into the water.
I thought that once you set a waypoint, it was based on the GPS coordinates of the drone and it shouldn't matter where you launch.

GPS is basically a 2d positionning system, latitude and longitude, so a gps point does not hold the heigh at which this point is, that is the job of some other systems (could be an accelerometter, a barometer or a laser beam). If i remember correctly, the dji drones uses laser and camera sensors to know at which altitude they are standing, so it should not depend on the position you take off at. Dji drones tend to have, however, some difficulties to compute their altitude over water, due to the water surface making the lasers go crazy. sometime they will think they are just above the water where they are 20 feet above it, and sometime (and i'm guessing that was what was happening to you there) they think they are too high as the laser stuggles to bounce back to the detector. in the case of a predefined fligh using waypoints, the altitude is also kept in each waypoint, you said you were on a boat, so maybe the ground detection sensors got confused due to the waves generated by the motion of the boat. A fix to that could be to go a bit higher above water, something like 40 to 50 feet. but I wan't to point out that this is pure speculation, and that it may depend on something else entirely, and if it is really the ground detection sensors causing this issue, then it might be also depending of the environment (like the sunlight or the water purity or idk)

 

[Felix le Chat]

Unpredictably variable water levels and saved waypoints missions? I think you might be setting yourself up for trouble if you don't set your flight altitude considerably higher. Skimming the surface might look cool, but watching your expensive drone turn into a submarine?

 

[GadgetGuy]

I thought that once you set a waypoint, it was based on the GPS coordinates of the drone and it shouldn't matter where you launch.

You thought wrong. It absolutely matters where you launch a Waypoint Mission from. Waypoint Elevations are always relative to the launch point. However, you can still relocate yourself after launching the drone from the set original location, before starting the Waypoint Mission.

 

[tribar]

DJI drones use barometric pressure to determine their altitude.
The downward sensors have a limited range of about 10 meters and will ONLY be used within this range together with the barometer.
(consult the manual)

The barometric pressure at take-off is used as reference, meaning ALL altitudes are relative to the take-off point (zero altitude).

Also with DJI Fly waypoint missions:

The drone WILL descent between 2 waypoints with the same altitude when the next waypoint has a higher altitude (and vice versa).

More info here:

Litchi Missions vs DJI Fly Missions

Wes Barris

www.litchiutilities.com

 

[Yorkshire_Pud]

The drone WILL descent between 2 waypoints with the same altitude when the next waypoint has a higher altitude (and vice versa).

More info here:

and page 42 of the drone's 1.0 & 1.4 manuals,
"The flight route will curve between waypoints, so the aircraft altitude between
waypoints may become lower than the altitudes of the waypoints during the flight.
Make sure to avoid any obstacles below when setting a waypoint."

 

[Jurgen]

I fly about 30 feet over water tracking efoilers and at least once per session I need to do a quick abort because it wants to be a fish. For waypoint missions, fly higher.

 

[bJako]

You thought wrong. It absolutely matters where you launch a Waypoint Mission from. Waypoint Elevations are always relative to the launch point.

Correct. Also, vertical Accuracy Range is way worse than horizontal, so even you start from same point, flight path is slightly different (mostly vertikal). So, it needs little extra work to combine footages.

Also GPS is not 2D but 3D, if you see at least 4 satellites.