With all the questions that a newbies asks, and in most instances are replied to INCORRECTLY, is the issue of take-off and landing position accuracy. So allow me to enlighten you about this (I`ve used GPS instruments for more than 35 years in my work!!).
General public availability of relatively accurate GPS came only after 2000.  Before that DGPS calculations and very highly sophisticated and expensive equipment used and had to be applied to GPS readings to get reasonable accurate positions.
The following picture (from the internet) shows how the "position" of a specific unit varies with time. This means eg. that if a unit claims to be accurate to 2m the position will be found inside of a circle with a RADIUS of 2m [4m DIAMETER] only 80% of the time.

Coming from lots of "old" experience with GPS (back in Garmin II/III days with dithering), I can say that most people don't realize that it isn't an exact fix.  With the addition of DGPS, it can be very accurate.  I'm not sure what the addition of GLONAS has done for us however.  Can you shed any light on that?  I figure anything within the old 30 foot circle number is pretty good.  My birds seem to come back to within about 10 feet at "all" times.  That said, I rarely test RTH, preferring to manually fly back and land myself.

Also, for our quads, it's important to remember that RTH requires both the GPS AND the compass to be working properly.  If the compass is not calibrated, RTH will fail.

StanfordWebbie Posted at 2018-3-13 09:46
Also, for our quads, it's important to remember that RTH requires both the GPS AND the compass to be working properly.  If the compass is not calibrated, RTH will fail.

So for better accuracy how often should we re-calibrate are compass even though the sensors show no problems.

Thanks
Rudy

sky6105 Posted at 2018-3-13 11:41
So for better accuracy how often should we re-calibrate are compass even though the sensors show no problems.

Thanks

Anytime you take off from a different location it is advised to re calibrate the compass.  So many things can effect it so if you take off in a different location with different amounts of metals around it could be different.  

That makes sense thanks for the quick post.
Does that also go for the IMU Cal.

sky6105 Posted at 2018-3-13 11:41
So for better accuracy how often should we re-calibrate are compass even though the sensors show no problems.

Thanks

You don’t ever need to calibrate the compass unless the app tells you to.

Avoid Compass Crash.

Tip To Help Avoid compass interference and crash.
1/ Never calibrate Compass unless prompted to.

2/ start AC leave until you receive enough gps lock ,

3/ if you get Compass interference turn off AC and move to another location,
without interference.

4/ In bottom left hand corner on your map you will see small red triangle, check to make sure that this triangle is pointing in the same direction (heading) as your AC, this will show good compass on the ground.

Raise AC to height of 8ft
Hover for 20 seconds
Fly forward 2ft
Backward 2ft
Left 2ft
Right 2ft
Up 2ft
Down 2ft
Yaw left
Yaw right
Each time returning to hover position
If you have a good horizontal each time, you will then know you have good gps good IMU and good Compass, and your ready to fly.

This exercise takes less than 1 minutes and is worthwhile doing before every flight,

Very good information thank you.

Hiya,

When to calibrate the compass differs from the spark and the MA.
see this for the MA:

To be honest, i never did a calibration of my Spark (only once before my first flight), guess i do the same with the MA.But always perform a good pre flight check and a check after take-off. When all looks good i go flying!

cheers JJB

hallmark007 Posted at 2018-3-13 11:50
You don’t ever need to calibrate the compass unless the app tells you to.

Avoid Compass Crash.

Good note from hallmark007.  One other thing I do is to check the strength of the compass signal in settings.  If I'm not seeing two green bars, then I recalibrate the compass.  But as he said, it's pretty rare unless there's a major change in location from prior flights.

JJBspark Posted at 2018-3-13 12:05
Hiya,

The when to calibrate the compass differs from the spark and the MA.

There is much confusion with calibration with dji, Mavic when first introduced instructions were to only calibrate when prompted by app ,now there is a change in the manual tells you to calibrate every (10km) of movement, in spark phantom and inspire don’t calibrate unless prompted by app, so dji is not really sure why or when you need to calibrate, as it says in the MavAir manual it is recommended if you move 50km , I for one don’t really buy this I’m just back from a 2000 mile trip to Sweden where I was not prompted to calibrate so I didn’t but checked compass levels just to make sure and everything was fine.
I’m thinking dji are trying to figure out what is the best practice for users to avoid getting a bad compass and I believe they still haven’t found the answer yet. One good thing they have included in MavAir is it won’t let you take off if it thinks there is any problem with compass, but I have checked the green bars when getting this warning and it reads perfect, so this is a bit of a conflict and this is why I think dji are still unsure what to do themselves.

This is just my opinion on compass in all of Aircraft I mentioned above for what it’s worth.

Calibration gives the compass the information it needs to determine which magnetic influences are part of the MavAir and what is the earth's normal magnetic field.
The compass doesn't lose calibration and doesn't ever need re-calibration unless you add or remove equipment to/from the MavAir.

hallmark007 Posted at 2018-3-13 11:50
You don’t ever need to calibrate the compass unless the app tells you to.

Avoid Compass Crash.

Absolutely correct! In seven months I had to calibrate compass only one time (with my Spark) while I have been in three different countries with quite some distance in between. Most of the time I have used RTH for convenience and because I each time admire the accuracy. Again, most of the time these RTHs have been precision landings on the spot, in some cases the maximum deviation was about 50 cm which I then have corrected manually. At the bottom line no complaints at all, I am rather more than impressed.

hallmark007 Posted at 2018-3-13 12:54
There is much confusion with calibration with dji, Mavic when first introduced instructions were to only calibrate when prompted by app ,now there is a change in the manual tells you to calibrate every (10km) of movement, in spark phantom and inspire don’t calibrate unless prompted by app, so dji is not really sure why or when you need to calibrate, as it says in the MavAir manual it is recommended if you move 50km , I for one don’t really buy this I’m just back from a 2000 mile trip to Sweden where I was not prompted to calibrate so I didn’t but checked compass levels just to make sure and everything was fine.
I’m thinking dji are trying to figure out what is the best practice for users to avoid getting a bad compass and I believe they still haven’t found the answer yet. One good thing they have included in MavAir is it won’t let you take off if it thinks there is any problem with compass, but I have checked the green bars when getting this warning and it reads perfect, so this is a bit of a conflict and this is why I think dji are still unsure what to do themselves.

And once again I can only agree. DJI seem to be more cautious now because of unclear compass errors that users have had in the past. But I continue to see two sides of the Medal. Each new calibration bears the risk that it might be a bad calibration and lead to problems thereafter. Thus my precautionary approach will remain to calibrate only, if my now MA tells me to do it. And even then I shall first check if I am maybe in a location with strong interference and verify if the request to calibrate remains after changing to a potentially better location.

hallmark007 Posted at 2018-3-13 11:50
You don’t ever need to calibrate the compass unless the app tells you to.

Avoid Compass Crash.

I do what you do as well as checking the sensors....like you said it only takes a minute to check that all is well !!

DINGO DOWN UNDA Posted at 2018-3-13 13:36
I do what you do as well as checking the sensors....like you said it only takes a minute to check that all is well !!

It has served me well for a long time.....

AG0N-Gary Posted at 2018-3-13 09:43
Coming from lots of "old" experience with GPS (back in Garmin II/III days with dithering), I can say that most people don't realize that it isn't an exact fix.  With the addition of DGPS, it can be very accurate.  I'm not sure what the addition of GLONAS has done for us however.  Can you shed any light on that?  I figure anything within the old 30 foot circle number is pretty good.  My birds seem to come back to within about 10 feet at "all" times.  That said, I rarely test RTH, preferring to manually fly back and land myself.

Thanks for your comment.

The removal of DGPS immediately made consumer GPS devices more accurate.  Taking into account that not only the new systems like GLONASS and Biedouw implements newer technology so does the GPS receivers in our devices as well.  Thus all in all we are getting more and more accurate GPS in general.

However consumer GPS as used in handheld and consumer UAV`s will never rival commercial applications mainly due to cost.

Finally a lot depends on atmospheric conditions as well as obstacles such as trees and buildings.

Allow me a "novice" note.  I agree that experienced operators should not rely on RTH, however it does give some peace of mind to newbies