Avoiding the rebar effect

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I would like to make my takeoff and home point on a concrete portion of my driveway. Thinking of using a wooden table to distance my takeoff point from the rebar underneath. How tall should the table be? .... or is this just a wish that should not be pursued at all .... and why?
 
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I would like to make my takeoff and home point on a concrete portion of my driveway. Thinking of using a wooden table to distance my takeoff point from the rebar underneath. How tall should the table be? .... or is this just a wish that should not be pursued at all .... and why?
Even a big plastic or wooden box to raise the compass a foot or two would probably be sufficient.
Just confirm that the drone icon in the radar display is pointing the same way as the real drone (if the drone is facing south, the icon should also face south).
 

REL

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Even a big plastic or wooden box to raise the compass a foot or two would probably be sufficient.
Just confirm that the drone icon in the radar display is pointing the same way as the real drone (if the drone is facing south, the icon should also face south).
Thanks for that tip. I'll make sure to check that :)
 
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On the the P3 4K the compass is in one of the legs very close to the ground. Getting it up just a foot or 2 most times solves the problem.
 
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Consider buying a “good,” fluid-dampened compass to check the relative effects of the metal on a compass. A compass, such as the Suunto M-3 NH (Northern Hemisphere), or Silva Explorer 2.0 may provide some insight.

The electronic “compass” in the drone may be more sensitive than the compass that you buy, but we don’t know.

Also, most driveways and sidewalks are reinforced with welded wire fabric (WWF) rather than rebar. I’m speculating, but I would not be surprised if WWF is less of an issue than rebar since well-defined, concentrated magnetic lines of force shouldn’t form as readily in the WWF.

Usually, I use a “table” that’s about chest high to setup and launch from. I find this more convenient for preflight checks, etc. I made my table from a used tripod that I got from a thrift store and a scrap of 1/2” plywood salvaged from a prior project. I installed a 1/4” x 20 TPI T-Nut in the center of the board so it attaches easily to the tripod (see photos). I don’t have to get down on the wet ground when setting up.

The adjustable legs and head of the tripod. and the bubble level I installed on the board allow me to level the table on any surface. I’m not sure it’s needed, but I assume the level surface is better for the drone to calibrate on.
 

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REL

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Consider buying a “good,” fluid-dampened compass to check the relative effects of the metal on a compass. A compass, such as the Suunto M-3 NH (Northern Hemisphere), or Silva Explorer 2.0 may provide some insight.

The electronic “compass” in the drone may be more sensitive than the compass that you buy, but we don’t know.

Also, most driveways and sidewalks are reinforced with welded wire fabric (WWF) rather than rebar. I’m speculating, but I would not be surprised if WWF is less of an issue than rebar since well-defined, concentrated magnetic lines of force shouldn’t form as readily in the WWF.

Usually, I use a “table” that’s about chest high to setup and launch from. I find this more convenient for preflight checks, etc. I made my table from a used tripod that I got from a thrift store and a scrap of 1/2” plywood salvaged from a prior project. I installed a 1/4” x 20 TPI T-Nut in the center of the board so it attaches easily to the tripod (see photos). I don’t have to get down on the wet ground when setting up.

The adjustable legs and head of the tripod. and the bubble level I installed on the board allow me to level the table on any surface. I’m not sure it’s needed, but I assume the level surface is better for the drone to calibrate on.

Thank you Earthman for the additional details, suggestions and photos of your tripod setup. I'm assuming that the tripod legs must be of aluminum ..... so would not affect the compass. I believe you are correct about it being WWF and not rebar in my driveway cement, but it definitely is an issue as my P3P won't even allow me to start the motors while resting directly on the driveway. I will attempt to build something similar to what you have developed for my own use. Thank you again for your input.
 
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Worth noting; Any compass errors that may occur due to rebar in the concrete are temporary and will disappear as soon as you take off. Of course, it helps to have enough confidence in your compass and your senses to KNOW that the error is indeed due to rebar in the concrete. In that instance, I would recommend an aggressive take off to create distance between said rebar and drone ASAP. That said...

Many years of flying under my belt, and many launch sites from concrete, I've never experienced compass error due to rebar. I HAVE been able to induce compass error on purpose by parking my drone near a manhole cover, but never on concrete or asphalt away from visible metal objects.

D
 

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Worth noting; Any compass errors that may occur due to rebar in the concrete are temporary and will disappear as soon as you take off. Of course, it helps to have enough confidence in your compass and your senses to KNOW that the error is indeed due to rebar in the concrete. In that instance, I would recommend an aggressive take off to create distance between said rebar and drone ASAP. That said...

Many years of flying under my belt, and many launch sites from concrete, I've never experienced compass error due to rebar. I HAVE been able to induce compass error on purpose by parking my drone near a manhole cover, but never on concrete or asphalt away from visible metal objects.

D
Thanks for your input Harleydude. Based on what you said about your experiences taking off from concrete, I'll try the same spot again on my concrete driveway. I was sure that rebar (or WWF) in the concrete was the cause my P3P's refusal to allow me to start the motors. Perhaps it was another issue that caused the no-start, and I was just thorough enough to identify corrrectly. Since then I have been launching from my neighbor's driveway (about 50 feet way) ....but it is asphalt.
I'll report back next week.
 
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Thanks for your input Harleydude. Based on what you said about your experiences taking off from concrete, I'll try the same spot again on my concrete driveway.

Keep an eye out for manhole covers. Those WILL effect your drone's compass.



I was sure that rebar (or WWF) in the concrete was the cause my P3P's refusal to allow me to start the motors. Perhaps it was another issue that caused the no-start, and I was just thorough enough to identify corrrectly.

Next time use your iPad's "Screen Record" option. I find that examining iPad footage post-flight can often reveal details you may have missed while flying in real time. Plus you can post the video to YouTube which allows other more-experienced pilots scrutinize your flight.



Since then I have been launching from my neighbor's driveway (about 50 feet way) ....but it is asphalt.

Also look for water meter covers or any other metal in the area.




I'll report back next week.

Good deal.

D
 
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Thank you Earthman for the additional details, suggestions and photos of your tripod setup. I'm assuming that the tripod legs must be of aluminum ..... so would not affect the compass. I believe you are correct about it being WWF and not rebar in my driveway cement, but it definitely is an issue as my P3P won't even allow me to start the motors while resting directly on the driveway. I will attempt to build something similar to what you have developed for my own use. Thank you again for your input.

Thanks for the feedback about your P3P’z start issues on your driveway. It’s good to know.

My tripod is aluminum.

I have only started from a concrete sidewalk once with my Spark. It was a confined site behind a house at the top of a landslide, so not many choices for a home point. There wasn’t room for my tripod. The Spark didn’t give me any trouble, so maybe the sidewalk didn’t have any reinforcing, or...

I always calibrate the compasses in my DJI drones in an area at each site that is clear of metal before each flight. So, maybe compass calibration will address your problem too. If you have time, try recalibrating your compass away from the driveway and any metal before trying to take off from the drive. Let us know if that works.
 

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[QUOTE="Earthman,
I always calibrate the compasses in my DJI drones in an area at each site that is clear of metal before each flight. So, maybe compass calibration will address your problem too. If you have time, try recalibrating your compass away from the driveway and any metal before trying to take off from the drive. Let us know if that works.
[/QUOTE]
Thanks for your suggestions.
I'm going to try it again without compass recalibration (since I haven't done that since the initial
no-start issue). If it fails to start in the same spot again ..... then I will recalibrate and try a second time.
 
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Thanks for your suggestions.
I'm going to try it again without compass recalibration (since I haven't done that since the initial
no-start issue). If it fails to start in the same spot again ..... then I will recalibrate and try a second time.

That much calibration is completely unnecessary. I shoot all over my state going weeks even months without compass calibration. If I travel out of state, I calibrate. This goes for my P4P's, my Inspires 1's or my Mavic Pro. But even then I get the feeling I don't have to. Rest assured, if ever there are compass errors, the app will tell you.

D
 
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Meta4

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Worth noting; Any compass errors that may occur due to rebar in the concrete are temporary and will disappear as soon as you take off.
This is quite misleading and could get your drone into trouble.
You are confusing a simple compass error with a yaw error.
A compass error is confusingly named.
It would be more accurately called a compass warning.
That's when the compass detects a magnetic field much stronger than the earth's normal magnetic field and warns you of a problem, as you've found out with a manhole cover.

But a yaw error is much more dangerous as the magnetic field that causes it is no stronger than the compass warning threshold.
You get no warning but the magnetic field is not aligned with the earth's magnetic field.
Your compass is affected by it and the IMU initialises the the gyro sensors based on the incorrect compass reading.
If you launch and get out of the magnetic influence, your compass is now reading correctly but the gyro sensors are giving conflicting information.
The result is usually a fast, out of control drone making a curved flight and crashing.
This happens because the drone detects that it has moved from it's position and tries to correct, but the correction puts it further off course and it corrects and corrects etc etc.

We see yaw error incidents frequently in the forums and the most common cause is launching from reinforced concrete surfaces.
It can be caused by launching a few feet from a large steel object or somewhere that puts the compass sensor just an inch or two from a small steel bolt etc.
Many years of flying under my belt, and many launch sites from concrete, I've never experienced compass error due to rebar.
Here's what's inside a typical sidewalk:
i-chzV8fn-L.jpg

Launching from reinforced concrete is like buying a lottery ticket.
The mesh might be deep enough that it doesn't cause a problem.
You might place the compass in your landing gear far enough from the steel to avoid issues.
Or you might put your drone where the mesh is closer to the surface and/or overlapped (centre of pic above) etc and get your compass sensor in just the right spot to end up with a yaw error.
The result of that is usually very bad.
I always calibrate the compasses in my DJI drones in an area at each site that is clear of metal before each flight.
So, maybe compass calibration will address your problem too. If you have time, try recalibrating your compass away from the driveway and any metal before trying to take off from the drive.
Calibrating the compass cannot do anything to solve magnetic issues caused by something that's not part of the drone.
If your Phantom is hovering and flying properly, recalibrating the compass is completely unnecessary.
(I still haven't calibrated anything on my P4 pro and it flies perfectly after 4.5 years including international travel).
Compass calibration is probably the most misunderstood aspect of drone flying.
To get a good understanding of what it really does, read the start of this thread:
 

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Rest assured, if ever there are compass errors, the app will tell you.
But rest assured that your compass won't warn you of a yaw error.
It's good practice to avoid launching from anywhere that could put your compass in a magnetically affected area.
It's the small magnetic fields that can cause the biggest issues if you are unlucky enough to experience a yaw error incident first hand.
 
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This is quite misleading and could get your drone into trouble.
You are confusing a simple compass error with a yaw error.
A compass error is confusingly named.
It would be more accurately called a compass warning.
That's when the compass detects a magnetic field much stronger than the earth's normal magnetic field and warns you of a problem, as you've found out with a manhole cover.

But a yaw error is much more dangerous as the magnetic field that causes it is no stronger than the compass warning threshold.
You get no warning but the magnetic field is not aligned with the earth's magnetic field.
Your compass is affected by it and the IMU initialises the the gyro sensors based on the incorrect compass reading.
If you launch and get out of the magnetic influence, your compass is now reading correctly but the gyro sensors are giving conflicting information.
The result is usually a fast, out of control drone making a curved flight and crashing.
This happens because the drone detects that it has moved from it's position and tries to correct, but the correction puts it further off course and it corrects and corrects etc etc.

We see yaw error incidents frequently in the forums and the most common cause is launching from reinforced concrete surfaces.
It can be caused by launching a few feet from a large steel object or somewhere that puts the compass sensor just an inch or two from a small steel bolt etc.

Here's what's inside a typical sidewalk:
i-chzV8fn-L.jpg

Launching from reinforced concrete is like buying a lottery ticket.
The mesh might be deep enough that it doesn't cause a problem.
You might place the compass in your landing gear far enough from the steel to avoid issues.
Or you might put your drone where the mesh is closer to the surface and/or overlapped (centre of pic above) etc and get your compass sensor in just the right spot to end up with a yaw error.
The result of that is usually very bad.

Calibrating the compass cannot do anything to solve magnetic issues caused by something that's not part of the drone.
If your Phantom is hovering and flying properly, recalibrating the compass is completely unnecessary.
(I still haven't calibrated anything on my P4 pro and it flies perfectly after 4.5 years including international travel).
Compass calibration is probably the most misunderstood aspect of drone flying.
To get a good understanding of what it really does, read the start of this thread:

I don't have time to address this fully, but here's an easy test.

Put a magnet near your compass. This will cause the "magnetic error" you spoke of earlier. This is persistent until fixed with another magnet "pulling" the electronic compass in the opposite direction. I've used magnets to fix compass error DUE to magnetic interference. It's tedious, but can be done.

Conversely, the manhole cover is NOT "magnetic." It creates "interference," which goes away as soon as you pull the drone away from the manhole cover.

So, to recap:

Ferrous Metal = temporary error that goes away.
Magnet = persistent error that must be fixed.

D
 
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Meta4

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Conversely, the manhole cover is NOT "magnetic." It creates "interference," which goes away as soon as you pull the drone away from the manhole cover.
Put your hand compass close to the manhole cover and you'll see that the magnet in the compass needle is deflected towards it.
Compasses and magnets are attracted to steel.
 
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Put your hand compass close to the manhole cover and you'll see that the magnet in the compass needle is deflected towards it.
Compasses and magnets are attracted to steel.

A manhole cover is ferromagnetic because it's made of ferrous metals. But it's not magnetic because it hasn't been magnetized.

Here's an experiment that will settle our difference once and for all.

Park you drone on a manhole cover. You will get compass error. Now remove the drone from the manhole cover. The compass returns to normal. This is ferromagnetism at work. Now park your drone on the roof of your car. You will get compass error. Remove the drone from your car. The compass returns to normal. This is ferromagnetism at work.

Now get an Earth magnet and run it all over the body of the drone. You will get compass error. Remove the magnet. Compass error will remain.

This is the difference between magnetic interference vs. ferromagnetic interference.

Here's a blurb from dronesense.com, which jibes with everything I've been saying:

================ Start quote =================

So when should you calibrate, and more importantly not re-calibrate your UAS’ magnetometer? You should always calibrate before an aircraft’s first flight to feed accurate positioning data into the flight system. Additionally, you should calibrate if you were flying in Texas and now want to fly in California as there will be much declination. However, if you’re in your hometown or roughly less than 250 miles away from it, you should not need to calibrate. If you are getting calibration errors in this instance, first try moving your aircraft to a soft patch of grass and away from ferromagnetic or electromagnetic sources such as rebar, drainage pipes, underground power lines, irrigation systems, and rocks.

=============== End quote ================

D
 

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