altitude changed?

[engravosaurus]

Hi
Launch appeared Ok and altitude at that location then 0.
Checked altitude at another 2nd location and found -10.
Later in flight returned to that 2nd location and found altitude +5.
Why would altitude of that 2nd location change during the flight?
or
Did I inadvertently hit a button?
Thank you.

 

[mdaemon]

there is a change due to temperature drift of the barometric sensor.

 

[engravosaurus]

Ouch. +5 is wrong as the 2nd location is clearly below the launch location. Will have to try it again and see what happens. 15' off could be a problem I would think.

 

[msinger]

The altitude is estimated by the Phantom. It has no way of measuring it with absolute precision.

 

[Fly Dawg]

The Phantom's barometer, is not that accurate. +/- 10 feet would be considered normal, depending on weather conditions.

 

[John Locke]

I've seen errors over 30', but that's happened only once.

 

[engravosaurus]

thanx
good to know

 

[FlashBuddy]

I found myself rising up over 400 feet when I would swear I was at 200 feet. I'm blaming that on the barometer ;-P

 

[engravosaurus]

so I did this little “test” today
launched at house, so altitude 0
then flew to cottage (downhill), then to house, repeat

and found for altitude
house cottage
0 -8
8 2
14 6
19 11
23 15
27 16
31 21
34 21

so after several passes seemed to stabilize

but 0 to 34 and -8 to 21 is lotsa difference

could be trouble over water

is there something I should calibrate?
or
maybe use VPS under 33'

 

[John Locke]

so I did this little “test” today, launched at house.........
so after several passes seemed to stabilize but 0 to 34 and -8 to 21 is lotsa difference could be trouble over water, is there something I should calibrate? or maybe use VPS under 33'

There's nothing to calibrate. The barometer sensor that helps calculate these approximate numbers is not accurate, but that's OK IMO. It's really not that important, as these numbers shouldn't affect your flight much, there's no a safety issue if you keep VLOS. As you approach landing, if you have VPS on, the craft should use VPS to provide an accurate altitude when under 30' AGL, but hopefully you can see the craft to land and not need an altitude number anyway. I've never used the altitude numbers for landing, only for curiosity to estimate my AGL altitude mid flight. The calculated altitude is on ONLY relevant to the launch location, so the numbers really don't provide much useful info, except for curiosity purposes, and know approximately how high you're flying, to help stay below 400' AGL. However, if the land around your launch site isn't flat, the altitude numbers aren't relevant for AGL calculation anyway. Bottom line, it's not important that the numbers aren't accurate. In reality, it makes no difference in your flight safety regardless if they were accurate within 5' or within 40', it doesn't help or hurt, if you fly normal. As for water, nothing but VLOS will be accurate enough, forget those numbers for water flying. I use FPV to determine altitude above water, together with VLOS if possible.

 

[engravosaurus]

Got it, thanks. However...
I use the drone to scan the beach. Beach is about 150' below the house and is not in VLOS. And when scanning the beach the drone is over water as the landward side is covered with tall Douglas fir and alder. So I am nervous about the displayed height.
I did once launch at the house and scramble down the bank and at the bulkhead the height was recorded as -145. If the thing can be 30' off then it could have been anything between -115 and -175.
Could watch VPS but as I understand it is finicky over water.
Not sure what to do. Thing is kinda expensive to lose it in Saratoga Passage.

 

[With The Birds]

What you should do, based on your own data, is allow for a 35ft error margin.

 

[engravosaurus]

Thanks. Or maybe 50' since the thing is so expensive. Go no lower than -100'.

 

[With The Birds]

Thanks. Or maybe 50' since the thing is so expensive. Go no lower than -100'.

What are you scanning the beach for? Do you need to be closer than 50ft to the surface of the water? You might be overthinking this a little. The barometer can be a good 10m out occasionally, the same as the GPS can be 5m off planned course on an automated flight. Leave healthy margins for the known errors and enjoy....

 

[John Locke]

Got it, thanks. However...
I use the drone to scan the beach. Beach is about 150' below the house and is not in VLOS. And when scanning the beach the drone is over water as the landward side is covered with tall Douglas fir and alder. So I am nervous about the displayed height.
I did once launch at the house and scramble down the bank and at the bulkhead the height was recorded as -145. If the thing can be 30' off then it could have been anything between -115 and -175.
Could watch VPS but as I understand it is finicky over water.
Not sure what to do. Thing is kinda expensive to lose it in Saratoga Passage.

I'm not sure what good the numbers are for you in that scenario. Maybe I'm missing something.

I've flown off beach cliffs from 100 to 300' below the launch site. I had no problem navigating via FPV, but I have my rules for safety. You shouldn't try to get too close to the ground or water in that scenario. I try to stay at least 15' AGL via FPV estimate. If you get below 5' you will certainly know it, as it looks like 1' to me when flying that low. The main thing I worry about when I'm less than 15' of ground/water ANYWHERE, is some kid throwing a rock at it to see if he can hit it. Kids will be kids, I know I was. That's the primary reason I generally don't fly lower than about 15' anywhere I fly if I can't see it VLOS.

 

[With The Birds]

I'm not sure what good the numbers are for you in that scenario. Maybe I'm missing something.

I've flown off beach cliffs from 100 to 300' below the launch site. I had no problem navigating via FPV, but I have my rules for safety. You shouldn't try to get too close to the ground or water in that scenario. I try to stay at least 15' AGL via FPV estimate. If you get below 5' you will certainly know it, as it looks like 1' to me when flying that low. The main thing I worry about when I'm less than 15' of ground/water ANYWHERE, is some kid throwing a rock at it to see if he can hit it. Kids will be kids, I know I was. That's the primary reason I generally don't fly lower than about 15' anywhere I fly if I can't see it VLOS.

15ft is brave.... I stick to 100ft when I’m not close enough to clearly see the AC, don’t want to inadvertently buzz someone. 100ft seems to be a magic number where you don’t inflame people (even when out over the water).

 

[John Locke]

15ft is brave.... I stick to 100ft when I’m not close enough to clearly see the AC, don’t want to inadvertently buzz someone. 100ft seems to be a magic number where you don’t inflame people (even when out over the water).

The 15ft altitude is safe IMO, but going slow. I wouldn't fly over people that low, only when it's vacant. Flying FPV is pretty easy as long as you realize you have about a quarter second "delay to real time" in the display. Flying level is quite easy to cruise flat ground that low, but slow. The biggest danger flying low is a wire strung across the airspace. I find flying less than 10mph is needed to provide enough time to STOP when I see a wire. Going any faster and you really don't have enough time, given the 1/4 second video delay. I will sometimes check the flight area with binoculars to see what I can prepare for, and use for POI's before I fly. The 15' flight height is for more dramatic video shots, but that's pretty much my limit for low altitude when nobody is around. Animals are different, I'll get closer if they'll allow me too, but I have to use the Mavic for that, since it has less noise. Since I have refresh on my Mavic I get more brave with it, capturing stuff like the geese in this video below. Tripod mode works great for these type of flights.

 

[engravosaurus]

Thanks all

Question #1 was how to monitor beach
I think I will stay at -100' from launch, i.e 50' above water. Should be good enough. No reason for more risk. Can see logs, erosion, animals, etc at 50'
Question #2 was how do things work
Exploring altitude variance was fun. Seems 30' is not uncommon as was mentioned.

 

[andy_k]

simple solution to barometer errors - VLOS

 

[John Locke]

simple solution to barometer errors - VLOS

That's impossible on 90% of my flights! That suggestion isn't much help.

Here's quick clip I shot today, my last flight of the year. I couldn't see the drone because of the weeds. I'm down to about 2' above the water capturing these birds in the riverbed. VPS goes a bit bonkers with moving water. It occasionally wants to ascend when I get too close to the water. It's best to leave VPS off and fly ultra slow when that low.

Note: clip is still uploading. View it in 3hrs for 4K support.