Altitude reference: Takeoff vs. ground level
I have growth plan.
For a waypoint mission, the "reference altitude" drop down says "takeoff location" or "ground level".
Is "ground level" the same as "terrain follow (AGL)"? Why the different name?
https://support.dronelink.com/hc/en-us/articles/6896503977363-Terrain-Follow-AGL-in-Map-Missions
Furthermore, if it is AGL, what difference does it make where I take off from? It gives a warning that the not using a specific takeoff location will be dangerous: Specifically the warning is:
"Ground elevations used without a takeoff takeoff restriction. Taking off from a different location could result in an unsafe altitude offset."
AGL should mean air to ground altitude , no matter where I take off from.
Is it because it calibrates the barometer to altitude using the takeoff location?
Comments
6 comments
Basically, the answer is yes, and it is very good at doing it, try executing a mission that's set up with a specific takeoff from even a short distance outside that location and 2 things will happen:
As for the subtle difference between AGL and Terrain follow (AGL), I've seen an explanation somewhere but can't find it at the minute, maybe Jim McAndrew can elaborate
The only component that supports terrain follow right now is Maps, which means Dronelink automatically adjusts the altitudes along the path every x meters to start y meters above the ground. In the case of other components, such as waypoints, where you set an altitude at a specific place such as at a waypoint, selecting AGL just means that at that specific point, the drone will be at that height above the ground. If you select AGL on two adjacent waypoints, Dronelink does not currently calculate intermediate "above ground level" altitudes every x meters between the waypoints, so it isn't really terrain follow. This feature request is on our list to do, but if you want it now, your best bet is to add Path Actions along the path with AGL altitudes specified as often as you want it to undulate with the ground beneath.
Dronelink downloads the MSL altitude of the takeoff location at planning time, and then calculates all the other AGL altitudes specified in the mission as MSL altitudes + an offset from the takeoff location taking into account the drone's altitude reading, which is barometric relative to the the takeoff location. If you takeoff from a different location, it is possible/likely that this location has a much different MSL altitude than the planned takeoff location, so all the relative offsets will be incorrect, thus it is safer and more accurate to force a takeoff location if you are using AGL altitudes.
Please let me know of any of this has changed in the last two years, but I'll assume not.
One additional question regarding the "at planning time" part of this response: is that only in the web-based planner, or does this include editing a flight plan in the flight app in the field?
Thanks,
John
It hasn’t changed. The in app planner is just the web app embedded.
OK, cool. I want to make sure I understand what will trigger a "re-download and calculation of altitudes". For example, when I was in the field yesterday I had to change my takeoff location from what I had planned from home. Is all that's necessary is editing the takeoff location? Am I assuming correctly that will trigger a recalculation of the altitudes for all the waypoints set to AGL?
Thanks!
If you make a change that triggers it, you should see a message that pops up telling you that new elevations were updated. Changing the takeoff location is definitely one such change.
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