How to establish a magnetic north line on the ground

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JohnWF

Pre-takeoff checklist
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John at Salida
In order to calibrate my magnetometer I need either an accurate true north or a magnetic north line on the ground at the airport...there is no compass rose here or anywhere around here.

If I had a theodolite, which I don't, I could 'shoot' Polaris, but that isn't something I can do either.

I have several compass units but they are too small to be able to use to get the very great accuracy I seek.

Your suggestions?
 
The runways at Salida, CO, if that's where you are, are 61 and 241 degrees according to AirNav. Also, you could use a GPS to get a north/south line.
Just a couple of thoughts.
Denis
 
A simple means of pointing the airplane toward magnetic north is to taxi the airplane slowly and use the GPS ground track to determine when you are taxiing in a magnetic north direction. Make small corrections to the direction of travel of the airplane, and continue to taxi for several seconds for the GPS to accurately determine your ground track. The GPS cannot determine your track unless you are moving. (Note: Be sure your GPS is displaying MAGNETIC track, not TRUE track if using it to align the airplane with magnetic north.) If you don't have the proper GPS, ask someone to get properly aligned, then mark under spinner(engine shut down)/tail with tape. Now pull your plane over the same tape marks.
 
There is a magnetic field sensor inside iPads and iPhones (and I think about every other vendor's tablet computers and smart phones) that has been used by about a million different apps to point to magnetic north. Some also have the current declination values, so given the tablet's GPS location they can immediately point to true north, too. As far as I can tell with my iPad 2 and iPhone 4 compass apps, they are probably accurate within 1 degree. Or at least more accurate than you can read the compass in most airplanes.
 
Jim Logajan said:
There is a magnetic field sensor inside iPads and iPhones (and I think about every other vendor's tablet computers and smart phones) that has been used by about a million different apps to point to magnetic north. Some also have the current declination values, so given the tablet's GPS location they can immediately point to true north, too. As far as I can tell with my iPad 2 and iPhone 4 compass apps, they are probably accurate within 1 degree. Or at least more accurate than you can read the compass in most airplanes.
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Yeah but you know better than most that those Apps are using a combination of the mag field detector, accelerometers, and GPS combined (including Assisted GPS from the cell network on gadgets so-equipped) to give a composite direction.

AFAIK, Apple (for one) doesn't expose the raw unfiltered magnetic data to even developers, and for good reason. The devices inside the gadget are constantly "bothered" by the environment around them.

The raw unfiltered data would look like dog poo inside a typical GA cockpit full of electronics, and even outdoors would swing wildly in response to various things, just like a real mag compass.

Just hold your iPhone up near your mag compass in the airplane and watch it freak out. Same crap happens to the iPhone from other E/M sources.

Analog to digital is and always will be, hard. My thermometer in the bedroom says it's 59.4F in here. Of course the low battery light has also illuminated since last night. Voltage sensitive, much? :)

The composite data when walking outdoors away from other stuff, is great. But mag compasses are mag compasses (including mag field sensors) and suffer various problems (which are different for the mechanical and electronic varieties but still just as messy).

:)
 
I'd abandon the idea of finding magnetic north because of local anomalies and the additional concern of the calibration of the instrument you are using to determine it. There are ways to find true north as simple as watching the shadow of a stick on the ground. Try Google.
 
Wayne, I think your suggestion will work. I used a Q-Starz GPS and HP4700 in a Jeep this morning and could maintain a selected course well. Will do same in the plane Tuesday (this weekend the airport is jam packed with gliders so there's little room to taxi around.

That stick and shadow would work but I have a limited amount of real estate to play with at the airport.

Thanks to all for suggestions.
 
I would think that finding true north and applying isogonic variation, would be about as accurate as anything. And I would think you could get within a minute or three.
You can rent a theodolite, from most any tool rental store.
I guess your term very accurate, must be within a few milliseconds.;) :popcorn:

Oh, and if you did have a compass rose, how accurate would it be, if it was, say, 5 years old? Remember Isogons don't stay put.
 
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I use a simple accurate method to calibrate mine. First for an accurate calibration your plane needs to be oriented to the cardinal magnetic points NSEW. You cannot use intermediate directions. In-flight calibration is more accurate than on the ground. There is no ground magnetic influence and your plane own generated field is stabilized

1. Go to Google Earth and find roads that are line up true North-South and East West. Most farm land roads are line up with true North

2. Find the magnetic variation for your area

3. Align the airplane along these roads and adjust the compass reading to what the magnetic reading would be when the mag variation is accounted for.

4. Do the same for the cardinal points NSEW and split the error for 180 directions.

Fly the plane as slow as practical to avoid running out of road.

Unlike the ground swing, turning the plane in-flight is much easier.

I found this method to be more accurate and quicker than ground compass swings.

José
 
Piloto said:
3. Align the airplane along these roads and adjust the compass reading to what the magnetic reading would be when the mag variation is accounted for...
Click to expand...

What method do you use to ensure that your track and heading coincide?
 
Silvaire said:
What method do you use to ensure that your track and heading coincide?
Click to expand...

I am glad you ask, I thought it was obvious. I have a Mooney and the bottom window edge is parallel to the fuselage. So I line up the window edge with the road. Do not try to crab into the wind but to maintain parallel angle with the road. You can also line up with nose but do not crab into the wind. Once you have stablish a true north set your DG to stablish the remaining cardinal points and verify with road alignment.

On most planes the front seat windows are parallel to the fuselage center line. Verify this by measuring the distance between pilot/copilot window back and front window sides. If they are the same distance then the windows are paralell to the fuselage center line.

José
 
Silvaire said:
What method do you use to ensure that your track and heading coincide?
Click to expand...

If he can find his way across the Atlantic, I am sure piloto can fly a parallel line. And, an airborne compass swing will always be more accurate than one done on the ground.
 
kgruber said:
If he can find his way across the Atlantic...
Click to expand...

I was just asking, didn't know if he was doing it mathematically by flying both directions or by flying in a slip. Apparently the latter.
 
I have several compass units but they are too small to be able to use to get the very great accuracy I seek.
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Borrow or buy a prismatic compass. Like:

http://www.cammenga.com/Military-Compass-s/20.htm

or go nuts:

http://www.stanleylondon.com/CompassFrancisBarkerM73M88.htm

Eliminate any ferrous metals in your pockets, belt buckle, etc. and stay away from steel objects; you should be able to shoot a very accurate line.

The way these work is that you simultaneously see your target (someone holding a pole, maybe) and the compass card, usually magnified. So you can easily get sub-degree accuracy.

Surveyors probably have similar tools. I know about the prismatic compasses because I have an "English Marching Compass" complete with mother-of-pearl compass card, leather case, and leather shoulder carrying strap --- just for the joy of owning such an amazing instrument.
 
JohnWF said:
In order to calibrate my magnetometer I need either an accurate true north or a magnetic north line on the ground at the airport...there is no compass rose here or anywhere around here.

If I had a theodolite, which I don't, I could 'shoot' Polaris, but that isn't something I can do either.

I have several compass units but they are too small to be able to use to get the very great accuracy I seek.

Your suggestions?
Click to expand...
If you truly want or need "accuracy, your only options are to calibrate with something NIST traceable with better accuracy (preferably twice as accurate) than you are shooting for. Anything else is going to be subject to unknown local magnetic variation no matter how accurately you align with the geography.
 
You could always use a handheld GPS and do a "Direct to" the present coordinates of the magnetic north pole.
 
JeffDG said:
You could always use a handheld GPS and do a "Direct to" the present coordinates of the magnetic north pole.
Click to expand...

Obama

The problem with that approach are the winds. If you are flying along a course line with westerly winds the nose of the airplane will be pointing slightly toward the west instead of the magnetic north. The other issue is that magnetic lines are no straight. So even though you have a straight course to the magnetic pole the local magnetic field may not reflect that.

José
 
Piloto said:
Obama

The problem with that approach are the winds. If you are flying along a course line with westerly winds the nose of the airplane will be pointing slightly toward the west instead of the magnetic north. The other issue is that magnetic lines are no straight. So even though you have a straight course to the magnetic pole the local magnetic field may not reflect that.

José
Click to expand...

No, I'm talking about a line on the ground...
 
Clip4 said:
If some parts of tbe US, roads and property boundaries are very accurately positioned east/west, north/south.
Click to expand...

Understood but if you are in flight and flying a course parallel to the section lines the only time your heading would coincide with your track would be if there were absolutely zero wind or the wind was exactly on the nose or tail. The only way to maintain the heading/track coincidence would be to fly in a slip (which I believe is what ipiloto said he was doing) Skywag however says you don't need to do that.

That's the part that needs explaining to me.
 
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