Magnetic compass, disturbance near ground, bad compass fix when arming?

A few other interesting informations about magnetic problems :

Sources of declination anomalies :

- local magnetic anomalies (0-90 degrees; 3-4 degrees frequently) :

Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.

Some areas with local anomalies :

A few areas with magnetic anomalies (there are thousands more):

-North of Kingston, Ontario; 90° of anomalous declination.

-Kingston Harbor, Ontario; 16.3° W to 15.5° E of anomalous declination over two kilometers (1.2 miles); magnetite and ilmenite deposits.

-Near Timmins, Ontario, W of Porcupine.

-Savoff, Ontario (50.0 N, 85.0 W). Over 60° of anomalous declination.

-Michipicoten Island in Lake Superior (47.7 N, 85.8 W); iron deposits.

-Near the summit of Mt. Hale, New Hampshire (one of the 4000-footers, near the Zealand Falls hut on the Appalachian Trail) ; old AMC Guides to the White Mountains used to warn against it.

-Around Georgian Bay of Lake Huron.

-Ramapo Mountains, northeastern New Jersey; iron ore; compass rendered useless in some areas.

-Near Grants, New Mexico north of the Gila Wilderness area; Malpais lava flows; compass rendered useless.

The USGS declination chart of the USA (GP-1002-D) shows over a hundred anomalies. The following table lists the most extreme cases.

Anomalous declination(degrees) Lat.  Long.  Location                                               46.4 W                         40.2  106.2  75 km.(45 mi.) W Boulder, Colorado 24.2 E                         40.7  75.3   20 km. (12 mi.) NE Allentown, Pennsylvania 16.6 E to 12.0 W over 10km(6mi)46.7  95.4   250 km. (150 mi.) NW Minneapolis, Minnesota 14.8 E                         33.9  92.4   85 km. (50 mi.) S Little Rock, Arkansas 14.2 E                         45.5  82.7   In Lake Huron, Ontario 13.8 W                         45.7  87.1   Escanaba, on shore of Lake Michigan 13.7 E                         48.4  86.6   In Lake Superior, Ontario 13.5 E                         48.5  122.5  80 km. (50 mi.) N Seattle, Washington 13.0 W                         42.2  118.4  In Alvord Desert, Oregon 12.2 W                         38.9  104.9  10 km. (6 mi.) W Colorado Springs, Colorado 11.5 E                         47.8  92.3   120 km. (75 mi.) N Duluth, Minnesota

- vertical magnetic fields

There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions.

- Solar Activity

The influence of solar magnetic activity on the compass can best be described as a probability. During severe magnetic storms, compass needles at high latitudes have been observed swinging wildly.

I've never seen this problem at my location, but you could have a ground with metallic or rare earths material causing that.

You could have as well DC or low frequency ground currents caused by industrial power grounding or electrical high voltage lines grounding near your location. I've ever seen such a problem in my city where the earth grounding between two buildings was causing a large low frequency ground current.

If the ground do influence your compass reading, then you'll have a small yaw rotation after take off, but what is important is that your mag offset are right above the ground and that compass is not influenced when rising throttle by battery, cabling ESC and motors current.

The first condition will be fulfill by a correct compass calibration, 5 ft above the ground for you and a correct declination value. The second condition will be met moving away the compass from the battery, power cables and motors as much as possible, and / or using the newer compassmot calibration that will be available inside version 3.0. This version should be available in a few days (latest RC4 tests seems to be fine).

Be aware that in some areas, mag declination can be shifted as much as 90° from the predicted geomagnetic earth model ! You can check that with a map and a compass using a simple bearing method.

Another point when traveling, is that when you are crossing isogonic lines at high latitudes, the declination can change at over a degree per kilometer !

From a previous post i did in the dev list :

I feel that autodeclination cannot be trusted in some areas. Local errors can go up to four degrees and in some extreme cases, the magnetic field can be even more disturbed to the point where a normal compass will not work at all.

For those reasons i think that manual calibration should be preferred.

Another problem is user induced magnetic field during calibration. Screens, computers, speakers, indoor structures, and even a metallic belt or watch can produce strong magnetic fields. I've often seen ten or more degrees error because of those fields.

A warning should be added in the Wiki when the calibration procedure will be ok for Wiki inclusion.

See those informations from

http://earthsci.org/education/fieldsk/declin.htm

Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.

There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions. anomalies. Anomalous declination is the difference between the declination caused by the Earth's outer core and the declination at the surface.

And another post where i did propose a simple method to check for declination :

As i did warn in a precedent message, local declination can be different from the predicted declination because of ground anomalies or local magnetic fields interferences.

Differences can go as high as 90°.

Here is a simple method i imagined to check for large local declination distortions. Can be used as well for compass healthy verification.

I did not test it, i would be interested to know if it does work.

1) Calibrate the compass (compass dance). Be carefull to not wear metallic objects during calibration and do it away from urban or ground possible magnetic sources.

2) Place the copter on the ground in a clear sky view, its nose or an arm oriented toward a far geographic mark that you will be able to find back later on Google Map.

3) With Mission Planner wait for a stable GPS fix and place waypoint 1 at the copter location.

4) Still using Mission Planner place waypoint 2 on the far geographic mark you did use for geograpic mark sighting.

5) Use a waypoint calculator to get the true course between waypoint 1 and 2.

For example use this calculator (set it for km units and WGS 84 earth model)

http://www.waypoint.org/gps1-calc.html

6) Get the copter compass heading from mission planner Quick window, Yaw value.

7) Compute the difference between compass heading and waypoint 1 to 2 true course. This should be the true local declination.

Add the arm angle offset if you did not use the copter nose for geographic mark sighting (usefull for X frames).

8) Eventually repeat the procedure at another location to be sure that very local magnetic sources did not influence the compass reading.

Replies

Developer
Olivier ADLER June 2, 2013 at 9:36am
A few other interesting informations about magnetic problems :

Sources of declination anomalies :

- local magnetic anomalies (0-90 degrees; 3-4 degrees frequently) :

Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.
Some areas with local anomalies :

A few areas with magnetic anomalies (there are thousands more):

    -North of Kingston, Ontario; 90° of anomalous declination.

    -Kingston Harbor, Ontario; 16.3° W to 15.5° E of anomalous declination over two kilometers (1.2 miles); magnetite and ilmenite deposits.

    -Near Timmins, Ontario, W of Porcupine.

    -Savoff, Ontario (50.0 N, 85.0 W). Over 60° of anomalous declination.

    -Michipicoten Island in Lake Superior (47.7 N, 85.8 W); iron deposits.

    -Near the summit of Mt. Hale, New Hampshire (one of the 4000-footers, near the Zealand Falls hut on the Appalachian Trail) ; old AMC Guides to the White Mountains used to warn against it.

    -Around Georgian Bay of Lake Huron.

    -Ramapo Mountains, northeastern New Jersey; iron ore; compass rendered useless in some areas.
    -Near Grants, New Mexico north of the Gila Wilderness area; Malpais lava flows; compass rendered useless.

The USGS declination chart of the USA (GP-1002-D) shows over a hundred anomalies. The following table lists the most extreme cases.

Anomalous declination(degrees) Lat. Long. Location 46.4 W 40.2 106.2 75 km.(45 mi.) W Boulder, Colorado 24.2 E 40.7 75.3 20 km. (12 mi.) NE Allentown, Pennsylvania 16.6 E to 12.0 W over 10km(6mi)46.7 95.4 250 km. (150 mi.) NW Minneapolis, Minnesota 14.8 E 33.9 92.4 85 km. (50 mi.) S Little Rock, Arkansas 14.2 E 45.5 82.7 In Lake Huron, Ontario 13.8 W 45.7 87.1 Escanaba, on shore of Lake Michigan 13.7 E 48.4 86.6 In Lake Superior, Ontario 13.5 E 48.5 122.5 80 km. (50 mi.) N Seattle, Washington 13.0 W 42.2 118.4 In Alvord Desert, Oregon 12.2 W 38.9 104.9 10 km. (6 mi.) W Colorado Springs, Colorado 11.5 E 47.8 92.3 120 km. (75 mi.) N Duluth, Minnesota
- vertical magnetic fields

There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions.

- Solar Activity

The influence of solar magnetic activity on the compass can best be described as a probability. During severe magnetic storms, compass needles at high latitudes have been observed swinging wildly.
Developer

Olivier ADLER June 2, 2013 at 9:31am

I've never seen this problem at my location, but you could have a ground with metallic or rare earths material causing that.

You could have as well DC or low frequency ground currents caused by industrial power grounding or electrical high voltage lines grounding near your location. I've ever seen such a problem in my city where the earth grounding between two buildings was causing a large low frequency ground current.

If the ground do influence your compass reading, then you'll have a small yaw rotation after take off, but what is important is that your mag offset are right above the ground and that compass is not influenced when rising throttle by battery, cabling ESC and motors current.

The first condition will be fulfill by a correct compass calibration, 5 ft above the ground for you and a correct declination value. The second condition will be met moving away the compass from the battery, power cables and motors as much as possible, and / or using the newer compassmot calibration that will be available inside version 3.0. This version should be available in a few days (latest RC4 tests seems to be fine).

Be aware that in some areas, mag declination can be shifted as much as 90° from the predicted geomagnetic earth model ! You can check that with a map and a compass using a simple bearing method.

Another point when traveling, is that when you are crossing isogonic lines at high latitudes, the declination can change at over a degree per kilometer !

From a previous post i did in the dev list :

I feel that autodeclination cannot be trusted in some areas. Local errors can go up to four degrees and in some extreme cases, the magnetic field can be even more disturbed to the point where a normal compass will not work at all.

For those reasons i think that manual calibration should be preferred.

Another problem is user induced magnetic field during calibration. Screens, computers, speakers, indoor structures, and even a metallic belt or watch can produce strong magnetic fields. I've often seen ten or more degrees error because of those fields.

A warning should be added in the Wiki when the calibration procedure will be ok for Wiki inclusion.

See those informations from

http://earthsci.org/education/fieldsk/declin.htm

Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even your belt buckle, frequently induce an error of three to four degrees.

There exist places on Earth, where the field is completely vertical; where a compass attempts to point straight up or down. This is the case, by definition, at the magnetic dip poles, but there are other locations where extreme anomalies create the same effect. Around such a place, the needle on a standard compass will drag so badly on the top or the bottom of the capsule, that it can never be steadied; it will drift slowly and stop on inconsistent bearings. While traveling though a severely anomalous region, the needle will swing to various directions. anomalies. Anomalous declination is the difference between the declination caused by the Earth's outer core and the declination at the surface.

And another post where i did propose a simple method to check for declination :

As i did warn in a precedent message, local declination can be different from the predicted declination because of ground anomalies or local magnetic fields interferences.

Differences can go as high as 90°.

Here is a simple method i imagined to check for large local declination distortions. Can be used as well for compass healthy verification.

I did not test it, i would be interested to know if it does work.

1) Calibrate the compass (compass dance). Be carefull to not wear metallic objects during calibration and do it away from urban or ground possible magnetic sources.

2) Place the copter on the ground in a clear sky view, its nose or an arm oriented toward a far geographic mark that you will be able to find back later on Google Map.

3) With Mission Planner wait for a stable GPS fix and place waypoint 1 at the copter location.

4) Still using Mission Planner place waypoint 2 on the far geographic mark you did use for geograpic mark sighting.

5) Use a waypoint calculator to get the true course between waypoint 1 and 2.

For example use this calculator (set it for km units and WGS 84 earth model)

http://www.waypoint.org/gps1-calc.html

6) Get the copter compass heading from mission planner Quick window, Yaw value.

7) Compute the difference between compass heading and waypoint 1 to 2 true course. This should be the true local declination.

Add the arm angle offset if you did not use the copter nose for geographic mark sighting (usefull for X frames).

8) Eventually repeat the procedure at another location to be sure that very local magnetic sources did not influence the compass reading.

This reply was deleted.