Hi Guys,

Yesterday I took the Mavic out for a flight, while my dad took his sail boat out for a ride. It was hard for me to see the mavic since the lake was a bit further away, but there were no obstacles like power lines e.g. I thought, if I stay above 10-15 meters altitude I won't hit any sails. At a given moment my dad called and said he saw the mavic and that there were no other sails close to him, so I dropped altitude to 5 meters, just to be sure and went in for a circulair motion.

But when I reviewed the footage I noticed that I was way lower than 5 meters, I was flying hardly a meter above the water! The water is approximately 4 meters deep, so could it be that it the Mavic took the bottom of the lake as altitude display? Am I a rookie for not knowing this? But if that's the case, how would you know which height is safe enough to fly?

Your Mavic takes its altitude height from the ground it took off from, so if you were standing 3m above the lake when you took off raised 3 m giving you 3m above ground and 6m above lake. So you fly over lake reduce your height by 2 m you are now 4 m above the lake but you altitude is reading height 1 m , because it is taking altitude from where it took off. Hope you can make sense out of this.

Just remember your AC always measures its altitude from the ground it took off from.

hallmark007 Posted at 2017-5-29 04:12
Your Mavic takes its altitude height from the ground it took off from, so if you were standing 3m above the lake when you took off raised 3 m giving you 3m above ground and 6m above lake. So you fly over lake reduce your height by 2 m you are now 4 m above the lake but you altitude is reading height 1 m , because it is taking altitude from where it took off. Hope you can make sense out of this.

Just remember your AC always measures its altitude from the ground it took off from.

Thanks! That's clear. I was standing behind a dyke (typically dutch, like a sand wall to prevent the water from flooding) and below the water level. So yeah, that makes sense. Good to know. Thanks a lot!

hallmark007 Posted at 2017-5-29 04:12
Your Mavic takes its altitude height from the ground it took off from, so if you were standing 3m above the lake when you took off raised 3 m giving you 3m above ground and 6m above lake. So you fly over lake reduce your height by 2 m you are now 4 m above the lake but you altitude is reading height 1 m , because it is taking altitude from where it took off. Hope you can make sense out of this.

Just remember your AC always measures its altitude from the ground it took off from.

Im not positive that the elevation measurement it takes is entirely  accurate.  I was reviewing video today that was taken at 12' altitude and the controller says its at 29 feet. All in a flat cow pasture in Florida abot 20 feet from the launch point. So even on flat ground I would not rely entirely on what the display says. just use it as a rough guide

hallmark007 Posted at 2017-5-29 04:12
Your Mavic takes its altitude height from the ground it took off from, so if you were standing 3m above the lake when you took off raised 3 m giving you 3m above ground and 6m above lake. So you fly over lake reduce your height by 2 m you are now 4 m above the lake but you altitude is reading height 1 m , because it is taking altitude from where it took off. Hope you can make sense out of this.

Just remember your AC always measures its altitude from the ground it took off from.

But doesn't it tell you a negative number or 0? But I would agree with you if you are still in the positives.

Panaceabeachbum Posted at 2017-5-29 05:12
Im not positive that the elevation measurement it takes is entirely  accurate.  I was reviewing video today that was taken at 12' altitude and the controller says its at 29 feet. All in a flat cow pasture in Florida abot 20 feet from the launch point. So even on flat ground I would not rely entirely on what the display says. just use it as a rough guide

If you go outside 15m you should get a more accurate reading from barometer, under you might be getting reading VPS height and you correct its only a guide.

KevDrones Posted at 2017-5-29 05:33
But doesn't it tell you a negative number or 0? But I would agree with you if you are still in the positives.

Yes so if you fly over a cliff it will show barometer reading minus what ever is underneath.

hallmark007 Posted at 2017-5-29 05:53
Yes so if you fly over a cliff it will show barometer reading minus what ever is underneath.

So I think he went bellow his launch place and it stated a lower reading that we he is actually.

KevDrones Posted at 2017-5-29 05:52
So I think he went bellow his launch place and it stated a lower reading that we he is actually.

No he launched from below he's in holland lots of water above ground level and plenty of dykes.

yes, I was behind the dyke below water level. I made a rough sketch to explain it. But it's a good thing to consider, especially since a lot of land is below water/sea level here.

Love the sailboat pic.  Hate the ugly wind turbines behind it.  Destroys the landscape portion.

Wottt Posted at 2017-5-29 07:09
yes, I was behind the dyke below water level. I made a rough sketch to explain it. But it's a good thing to consider, especially since a lot of land is below water/sea level here.

Hi Wottt,

For the future please remember one thing - GPS is notorious for misreporting elevation. It was built to provide a position fix but the elevation is a whole different ball game - you should assume that when you read 5m on the height this will be something between -5 and +15 - sorry, that's the accuracy. There is a good reason for that - WGS84 model is not a geoid but and elipsoid - that's why. But that's a lot of theory that most people are not interested in ;-)

Happy flying! Cheers!

Things look closer to the camera than they really are and to me it looks like the camera is about top of the cabin height on the boat . How far is it from the top of the cabin of the boat to the waterline ?

rydfree41 Posted at 2017-5-29 09:14
Things look closer to the camera than they really are and to me it looks like the camera is about top of the cabin height on the boat . How far is it from the top of the cabin of the boat to the waterline ?

Just the opposite!  It is a wide angle lens.  If it looks close to you in the camera, you are VERY close.

GPS accuracy is about 1/3 of horizontal for vertical.  If you are currently getting accuracy of location of +/- 10 feet, your vertical accuracy is roughly +/- 30 feet.  Never depend on telemetry alone.  Always fly with visual line of sight.  Eyes on!

rydfree41 Posted at 2017-5-29 09:14
Things look closer to the camera than they really are and to me it looks like the camera is about top of the cabin height on the boat . How far is it from the top of the cabin of the boat to the waterline ?

Just the opposite!  It is a wide angle lens.  If it looks close to you in the camera, you are VERY close.

GPS accuracy is about 1/3 of horizontal for vertical.  If you are currently getting accuracy of location of +/- 10 feet, your vertical accuracy is roughly +/- 30 feet.  Never depend on telemetry alone.  Always fly with visual line of sight.  Eyes on!

AG0N-Gary Posted at 2017-5-29 12:04
Just the opposite!  It is a wide angle lens.  If it looks close to you in the camera, you are VERY close.

GPS accuracy is about 1/3 of horizontal for vertical.  If you are currently getting accuracy of location of +/- 10 feet, your vertical accuracy is roughly +/- 30 feet.  Never depend on telemetry alone.  Always fly with visual line of sight.  Eyes on!

I was talking about the effect of the water appearing to be closer to the Mavic than it probably was . I took this video of a old herring nest and never went below  the tallest limb . At 1:20 in the video it looks like the drone is down in between them .

rnrnrn Posted at 2017-5-29 08:48
Hi Wottt,

For the future please remember one thing - GPS is notorious for misreporting elevation. It was built to provide a position fix but the elevation is a whole different ball game - you should assume that when you read 5m on the height this will be something between -5 and +15 - sorry, that's the accuracy. There is a good reason for that - WGS84 model is not a geoid but and elipsoid - that's why. But that's a lot of theory that most people are not interested in ;-)

haha thanks, the message is clear, there is a wide range between -5 and
+15

rydfree41 Posted at 2017-5-29 09:14
Things look closer to the camera than they really are and to me it looks like the camera is about top of the cabin height on the boat . How far is it from the top of the cabin of the boat to the waterline ?

about a meter!

Wottt Posted at 2017-5-30 04:56
about a meter!

Wow then that's a short fellow at the stern !