Waypoint mode - altitude changes

[BoxerJaxman]

I'm slowly getting more adventurous with my Inspire and finally trying our the waypoint mode. I flew it fine a couple of weeks ago and saved a favourite path. This weekend I loaded the same path, the waypoints all looked good and so executed the flight. The waypoints uploaded ok and then the bird flew to the first waypoint but for some reason it seemed to think the waypoint was about 30' lower than it really was, so flew (almost) straight into the ground. I manually overrode and pushed the bird up, however it clearly had the correct lat/long for the points but all way too low. So I recorded a new path to see if it was perhaps something to do with the barometer and weather changes from two weeks previous, however the new path had the same problem.
Questions therefore:
1. Does waypoint mode rely on the barometer or purely GPS?
2. Is it reasonable to fly the same route weeks later?
3. Has anyone else experienced the sudden elevation change when flying waypoint mode?
Lesson reinforced:
When flying in an autonomous mode, always good to have the hand near the function switch to take it out of f-mode, as well reinforced the idea of flying within LOS.

 

[The Editor]

I'm slowly getting more adventurous with my Inspire and finally trying our the waypoint mode. I flew it fine a couple of weeks ago and saved a favourite path. This weekend I loaded the same path, the waypoints all looked good and so executed the flight. The waypoints uploaded ok and then the bird flew to the first waypoint but for some reason it seemed to think the waypoint was about 30' lower than it really was, so flew (almost) straight into the ground. I manually overrode and pushed the bird up, however it clearly had the correct lat/long for the points but all way too low. So I recorded a new path to see if it was perhaps something to do with the barometer and weather changes from two weeks previous, however the new path had the same problem.
Questions therefore:
1. Does waypoint mode rely on the barometer or purely GPS?
2. Is it reasonable to fly the same route weeks later?
3. Has anyone else experienced the sudden elevation change when flying waypoint mode?
Lesson reinforced:
When flying in an autonomous mode, always good to have the hand near the function switch to take it out of f-mode, as well reinforced the idea of flying within LOS.

GPS is not used for altitude (it is way too inaccurate).
The barometer is utilised for altitude hold/calculation and although the barometer will zero at take off point (assuming it was the same as last time) the barometric fluctuations are likely to be different from the day you first flew the mission.

 

[BoxerJaxman]

That's what I was thinking which makes sense for the path recorded two weeks previously, but for the one that I recorded and then ran less than 5 minutes later, the barometer couldn't have fluctuated that much. Also, is there a way to accurately run a waypoint routine over a period of weeks/months? Ideally I'd like to have a matching shot of our farm in winter and summer then blend the two.