# What different between ( Absolute , Relative , Terrain ) Alt in MP ?

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 ..

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### Replies to This Discussion

This is a question I'd love to hear fully explained as well.  Thanks for posting.

Absolute altitude is the altitude given by the GPS above the reference ellipsoid of the GPS constelation, the WGS84.
Relative altitude is the height relative to the plane/copter's initial position.
Terrain altitude I guess it's the altitude above the terrain model as is in the ground station.

I wonder if there is the possibility to import to groundstation the geoid undulation model so one could calculate instantly and accurately the orthometric altitude, i.e. the altitude above mean sea level. That would be very interesting.

So, I've typically used relative altitude, because that's what is the most straight-forward for my brain, but it's surprising that there's not a way to just use good ol' MSL.  As a pilot, I would appreciate using MSL.

So, is Terrain Altitude only usable if the plane is connected via Mavlink?  How would it get the terrain altitude in real time?

Thanks for your help, by the way!

Absolute = above sea level (ASL)

Relative = above home altitude - the APM sets a reference altitude when it is turned on

Terrain = above ground level (AGL) - the ground station references google terrain information

Verify height means that the Mission Planner will use Google Earth topology data to adjust your desired altitude at each waypoint to reflect the height of the ground beneath. So if your waypoint is on a hill, if this option is selected the Mission Planner will increase your ALT setting by the height of the hill. This is a good way to make sure you don’t crash into mountains!

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.

you are welcome , I am facing problems and try to find it and solve it . also share information with other .

Good information James ,

but I want to know if I use Absolute = above sea level (ASL)  should I but my waypoint alt in ASL ex: my ASL is 860 and my waypoint alt is 200 should I set it to  860 + 200 = 1060 ?

home alt (abs ) is ASL on Ground ? or Home point Alt AGL ?

I've only worked in relative mode.  To me, absolute settings are a good way to mess something up.

Home altitude should be the ground altitude ASL.  Your RTL altitude is set above the home point in a separate parameter which sets it AGL.

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.

Another thought - could your barometer be misreading your altitude?  Do you have an OSD?  When you get into the air and flying, does your OSD report your altitude correctly?

I'm wondering if you may have an air vent in the wrong place that is pulling negative pressure in your aircraft fuselage and affecting your APM's ability to read altitude.

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.

My OSD looks reading the altitude correctly from the ground as I also test it in ground with also my Garmin watch ,

dose there any way to test barometer ?

am interesting in know if I have air vent in fuselage , how ? can you provide me with any information or source guide .?

A lot of thanks James .

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