Hi again ,
I am still having problem in Auto modes using Pixhack FC
My Question is
What's the different between ( Absolute , Relative , Terrain ) Altitude in Mission planner ?
Whats is the function of the check box ( verify Height ) ?
also in the right side under Home Location there are
Lat : -------------
Long : ------------
Alt (abs ) :-----------
Dose Alt (abs ) effect the flight altitude ?
Why always altitude keep drop in auto mode ?
this is my screen captured for the mission planner , flight plane Tab
dose there any problem in my MP version,?
hope I can figure out any useful information ,,
regards ..
Replies
I never used terrain altitude. But I imagine that when you use the Mission Planner it'll update the surrounding area of the plane (while it has telemetry link).
I just know that Mission planner get's terrain data from google that uses the GDEM data of the whole world, courtesy of the Ministry of Economy, Trade and Industry of Japan.
About the absolute altitudes, for the data to be really from above mid sea level, it must have somewhere at some point some geoid undulation model to calculate that altitude, because the only altitude that GPS gives directly is elipsoid altitude with WGS84 elipsoid as reference.
The Geoid undulation is the difference between the "mid sea level" (or orthometric height) sort of speak against elipsoid height.
https://en.wikipedia.org/wiki/Undulation_of_the_geoid
If you have that difference, having an elipsoidal height (given by the GPS) you obtain the orthometric height.
But if that's the case (Mission Planner using some geoid undulation model), I have no information about that and frankly I never investigated it, because I've always been happy with flying with relative (to my position's) altitude.
It seems the only way to get absolutely correct ASL altitude control would be if you knew the MSL relative barometric pressure, and were able to calibrate your barometer to that. But that changes day to day depending on weather. I guess you could change your parameters to use GPS altitude only, but I know its not as accurate as the baro.
There are plenty gravimetric data available to process and get geoid undulation models with milimiters precision. And gravity is much more stable than ir pressure, I garantee you.
I've worked a lot with gravimetric raw data, especially he GOCE gravimetric satellite of the european space agency and I've crunched that data to create grid files to be used with geodetic quality "GPS" receivers (it's more apropriate nowadays to say GNSS, including all the other constelations available like GLONASS, GALILEO, BEIDOU, etc, but I'll keep calling GPS for all to keep it simple) in surveying, and thus obtain on the fly the ASL from "GPS" observations.
Someone who would work without those grid files (not different from the terrain data from "google", but of course with differente values) would have to have extra work to connect their surveys to local altimetric benchmarks to have the local ASL height "fixed".
If we could get that undulation grid as easy as google does with the terrain data (from aster-gdem) then we would know with much more certainty the real ASL altitude or our planes, rather than knowing just the hight from the ground, like we have now.
I simply don't use much the terrain data, because that data isn't that much precise, e.g. in many places I go to fly with has many hills, google earth presents it simple as a flat, so I don't give it much credibility.
Anyway, the aster-gdem data was made already many years ago, and the GOCE data is almost fresh, and processing a global grid is very easy with today's computers, as long as one doesn't make the grid too tight. But I find it interesting that there was the possibility at least for mission planner that one could be allowed to import local (but much tighter) grid geoid undulation data, as altitudes would be calculated with much higher accuracy.
This is a question I'd love to hear fully explained as well. Thanks for posting.
you are welcome , I am facing problems and try to find it and solve it . also share information with other .
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