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)

 

[Yorkshire_Pud]

I think you will find that all heights set and used in the way point flight are relative to the take off point, so your initial 20ft skim was probably at -10ft.
It is my understanding that with the second flight it would still attempt to fly the skim at -10ft relative to the beach.
As for horizontal positioning, I don't know if it is at set points on the map or relative to takeoff point but I suspect it would be relative to the take off point otherwise, if flight 2's take off point were 100miles from that of flight 1, then ...........

For control purposes the drone primarily measures height via barometer,.
The VPS system measures height via an IR emitter and sensor but that system does not control the drone's height, except in maintaining a ground/object clearance of 50cm or thereabouts and it is unreliable over water, as per the manual. The VPS height measurement is also of limited range.

 

[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?