Magnetometer

W3C Working Draft, 18 October 2017 Candidate Recommendation,

This version:
https://www.w3.org/TR/2017/WD-magnetometer-20171018/ https://www.w3.org/TR/2018/CR-magnetometer-20180320/
Latest published version:
https://www.w3.org/TR/magnetometer/
Editor's Draft:
https://w3c.github.io/magnetometer/
Previous Versions:
https://www.w3.org/TR/2017/WD-magnetometer-20170814/
Version History:
https://github.com/w3c/magnetometer/commits/gh-pages/index.bs
Feedback:
public-device-apis@w3.org with subject line “ [magnetometer] … message topic … ” ( archives )
Issue Tracking:
GitHub
Editors:
Anssi Kostiainen ( Intel Corporation )
Rijubrata Bhaumik ( Intel Corporation )
Bug Reports:
via the w3c/magnetometer repository on GitHub
Test Suite:
web-platform-tests on GitHub

Abstract

This specification defines a concrete sensor interface to measure magnetic field in the X, Y and Z axis.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the Device and Sensors Working Group as a Working Draft. Candidate Recommendation. This document is intended to become a W3C Recommendation. This document will remain a Candidate Recommendation at least until in order to ensure the opportunity for wide review.

If you wish to make comments regarding this document, please send them to public-device-apis@w3.org ( subscribe , archives ). When sending e-mail, please put the text “magnetometer” in the subject, preferably like this: “[magnetometer] …summary of comment… ”. All comments are welcome.

Publication as a Working Draft Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

The entrance criteria for this document to enter the Proposed Recommendation stage is to have a minimum of two independent and interoperable user agents that implementation all the features of this specification, which will be determined by passing the user agent tests defined in the test suite developed by the Working Group. The Working Group will prepare an implementation report to track progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy . W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy .

This document is governed by the 1 March 2017 February 2018 W3C Process Document .

The CR exit criterion is two interoperable deployed implementations of each feature.

For changes since the previous version, see the online HTML diff tool .

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

1. Introduction

Magnetometer extends the Generic Sensor API [GENERIC-SENSOR] to provide information about the magnetic field as detected by the device’s primary magnetometer sensor. The magnetometer sensor measures the magnetic field for all three physical axes (x, y, z) in μT (micro Tesla).

This specification defines two new interfaces:

The magnetic field is a field that exerts magnetic force on magnetometer sensor due to the magnetic effect generated by electric currents, magnetic materials or Earth’s magnetic force that is attributed to the combined effects of the planetary rotation and the movement of molten iron in the Earth’s core.

Hard iron distortion is created by objects that produce a magnetic field , such as magnetized iron.

Soft iron distortion stretches or distorts the magnetic field and is caused by metals such as nickel and iron.

The calibrated magnetic field is a magnetic field with hard iron distortion and soft iron distortion correction applied.

The uncalibrated magnetic field is the magnetic field without hard iron distortion correction and with soft iron distortion correction applied, and as such reports changes in the magnetic field caused by magnetized objects moving near the magnetometer.

2. Examples

let sensor = new Magnetometer();
sensor.start();
sensor.onreading = () => {
    console.log("Magnetic field along the X-axis " + sensor.x);
    console.log("Magnetic field along the Y-axis " + sensor.y);
    console.log("Magnetic field along the Z-axis " + sensor.z);
};
sensor.onerror = event => console.log(event.error.name, event.error.message);

3. Security and Privacy Considerations

Magnetometer provides information about magnetic field, and in theory, can expose location of a user. For example, attack vector could be pre-magnetized surface in a particular location, or mapping between location and constant magnetic field disturbances caused by the building. Due to non-uniform strength of the Earth’s magnetic field, another attack vector could be exposure or validation of the user’s location. For example, if the end user is connected through VPN, magnetic field associated with geo IP information can be compared with magnetometer readings at real location, therefore, tell whether user is using VPN or not.

Uncalibrated magnetometer readings could be affected by magnetized objects nearby, such as jewelry, thereby exposing information that might be used for keystroke monitoring .

To mitigate these Magnetometer specific threats, user agents should use one or both of the following mitigation strategies:

The generic These mitigation strategies complement the generic mitigations are described defined in the Generic Sensor API [GENERIC-SENSOR] .

4. Model

The Magnetometer sensor type has two associated Sensor subclasses, Magnetometer and UncalibratedMagnetometer .

The Magnetometer and UncalibratedMagnetometer have an associated PermissionName sensor permission name which is "magnetometer" .

The latest reading for a Sensor of Magnetometer sensor type includes three entries whose keys are "x", "y", "z" and whose values contain magnetic field about the corresponding axes. Values can contain also device’s uncalibrated magnetic field and hard iron distortion depending on which object was instantiated.

For uncalibrated magnetometer, the latest reading includes three entries whose keys are "x", "y", "z" and whose values contain uncalibrated magnetic field around the 3 different axes, and three additional entries whose keys are "xBias", "yBias", "zBias" and whose values contain the hard iron distortion correction around the 3 different axes.

The sign of the magnetic field values must be according to the right-hand convention in a local coordinate system defined by the device. (see figure below).

Note: Magnetometer coordinate system.

4.1. Reference Frame

The local coordinate system of a mobile device is usually defined relative to represents the device’s screen when reference frame for the device in its default orientation (see figure below). Magnetometer and the UncalibratedMagnetometer readings . It can be either the device coordinate system or the screen coordinate system .

5. API

5.1. The Magnetometer Interface

]
[Constructor(optional MagnetometerSensorOptions sensorOptions), SecureContext,
  Exposed=Window]
interface Magnetometer : Sensor {
  readonly attribute double? x;
  readonly attribute double? y;
  readonly attribute double? z;
};
5.2.
The
UncalibratedMagnetometer
Interface

enum MagnetometerLocalCoordinateSystem { "device", "screen" };
]
 {
  ;
  ;
  ;
  ;
  ;
  ;

dictionary MagnetometerSensorOptions : SensorOptions {
  MagnetometerLocalCoordinateSystem referenceFrame = "device";
};

To Construct construct a Magnetometer Object , or to Construct an UncalibratedMagnetometer Object Magnetometer object the user agent must invoke the construct a Sensor magnetometer object abstract operation. operation for the Magnetometer interface.

Supported sensor options for Magnetometer are "frequency" and "referenceFrame".

5.2.1. 5.1.1. Magnetometer.x

The x attribute of the Magnetometer interface represents the magnetic field around X-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "x" as arguments.

5.2.2. 5.1.2. Magnetometer.y

The y attribute of the Magnetometer interface represents the magnetic field around Y-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "y" as arguments.

5.2.3. 5.1.3. Magnetometer.z

The z attribute of the Magnetometer interface represents the magnetic field around Z-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "z" as arguments.

5.2.4. 5.2. The UncalibratedMagnetometer Interface

[Constructor(optional MagnetometerSensorOptions sensorOptions), SecureContext,  Exposed=Window]interface UncalibratedMagnetometer : Sensor {  readonly attribute double? x;  readonly attribute double? y;  readonly attribute double? z;  readonly attribute double? xBias;  readonly attribute double? yBias;  readonly attribute double? zBias;
};

To construct an UncalibratedMagnetometer object the user agent must invoke the construct a magnetometer object abstract operation for the UncalibratedMagnetometer interface.

Supported sensor options for UncalibratedMagnetometer are "frequency" and "referenceFrame".

5.2.1. UncalibratedMagnetometer.x

The x attribute of the UncalibratedMagnetometer interface represents the uncalibrated magnetic field around X-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "x" as arguments.

5.2.5. 5.2.2. UncalibratedMagnetometer.y

The y attribute of the UncalibratedMagnetometer interface represents the uncalibrated magnetic field around Y-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "y" as arguments.

5.2.6. 5.2.3. UncalibratedMagnetometer.z

The z attribute of the UncalibratedMagnetometer interface represents the uncalibrated magnetic field around Z-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "z" as arguments.

5.2.7. 5.2.4. UncalibratedMagnetometer.xBias

The xBias attribute of the UncalibratedMagnetometer interface represents the hard iron distortion correction around X-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "xBias" as arguments.

5.2.8. 5.2.5. UncalibratedMagnetometer.yBias

The yBias attribute of the UncalibratedMagnetometer interface represents the hard iron distortion correction around Y-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "yBias" as arguments.

5.2.9. 5.2.6. UncalibratedMagnetometer.zBias

The zBias attribute of the UncalibratedMagnetometer interface represents the hard iron distortion correction around Z-axis. In other words, this attribute returns the result of invoking get value from latest reading with this and "zBias" as arguments.

6. Abstract Operations

6.1. Construct a magnetometer object

input

magnetometer_interface , a Magnetometer interface identifier or an UncalibratedMagnetometer interface identifier .

options , a MagnetometerSensorOptions object.

output

A Magnetometer or UncalibratedMagnetometer object.

  1. Let allowed be the result of invoking check sensor policy-controlled features with Magnetometer .

  2. If allowed is false, then:

    1. Throw a SecurityError DOMException .

  3. Let magnetometer be a new instance of the interface identified by magnetometer_interface .

  4. Invoke initialize a sensor object with magnetometer and options .

  5. If options . referenceFrame is "screen", then:

    1. Define local coordinate system for magnetometer as the screen coordinate system .

  6. Otherwise, define local coordinate system for magnetometer as the device coordinate system .

  7. Return magnetometer .

7. Limitations of Magnetometer Sensors

This section is non-normative .

The direction and magnitude of the Earth’s field changes with location, latitude in particular. For example, the magnitude is lowest near the equator and highest near the poles. Some hard-iron interference, meaning presence of permanent magnets (e.g. magnets in the speaker of a phone) in the vicinity of the sensor also affects the accuracy of the reading. Presence of electronic items, laptops, batteries, etc also contribute to the soft iron interference. Flight Mode option in mobile phones might help in decreasing the electro magnetic interference.

In addition to the above spatial variations of the magnetic field , time based variations, like solar winds or magnetic storms, also distort the magnetosphere or external magnetic field of the earth. Earth.

7. 8. Use Cases and Requirements

This section is non-normative .

Magnetometers can be used for a variety of use-cases, such as: for example:

Metal detection
Requirements with respect to data coarseness and sampling frequency can vary depending on the use case at hand. For example, metal detection or input using magnetic button may require only coarse can likely be implemented with coarser data and low using lower sampling rates suffice. Gesture recognition or frequency compared to gesture recognition, indoor navigation are more real time use cases where there might be a need for more accurate sensor readings navigation, or VR and higher sampling rates. Head mount tracking in Virtual Reality may require AR use cases. In sensor fusion from accelerometer, gyroscope and magnetometer. Magnetometer data may be used to help in calibration of gyroscope readings and align yaw readings with use cases, the magnetic north. The required sampling rate might depend frequency that yeilds optimal results is dependent on the e.g. sensor fusion algorithm and characteristics of other motion sensors involved.

8. 9. Compass Heading Using Magnetometers

This section is non-normative .

Compasses, instruments that align themselves with the magnetic poles of the Earth, have been used in navigation for centuries. The earth’s Earth’s rotational axis defines the geographic north and south poles that we use for map references. It turns out that there is a discrepancy of around 11.5 degrees (around 1000 miles) between the geographic poles and the magnetic poles. Declination angle is applied to the magnetic direction to correct for this situation.

If the device is always level to the earth’s Earth’s surface, compass heading [COMPASSHEADING] can be determined by using just the x and y component of the earth’s Earth’s magnetic field, that is, the directions planar with the earth’s Earth’s surface. To determine geographic north (or true north) heading, add the appropriate declination angle .

Magnetic declination or declination angle is the angle on the horizontal plane between magnetic north and the true north and depends on the position on the Earth’s surface, and changes over time. By convention, declination is positive when magnetic north is east of true north, and negative when it is to the west. You can get real time value for magnetic declination e.g. using the Magnetic declination calculator provided by the National Oceanic and Atmospheric Administration (NOAA).

The magnetic north is calculated as follows:

let sensor = new Magnetometer();
sensor.start();
let heading = Math.atan2(sensor.y, sensor.x) * (180 / Math.PI);
console.log('Heading in degrees: ' + heading);

The geographic north at a given latitude and longitude is can be calculated as follows:

// Get the latitude and longitude, omitted for brevity here.
let latitude = 0, longitude = 0;
// Get the magnetic declination at the given latitude and longitude.
fetch('https://www.ngdc.noaa.gov/geomag-web/calculators/calculateDeclination' +
      '?lat1=' + latitude + '&lon1=' + longitude + '&resultFormat=csv')
  .then(response => response.text()).then(text => {
    let declination = parseFloat(text.replace(/^#.*$/gm, '').trim().split(',')[4]);
    // Compensate for the magnetic declination to get the geographic north.
    console.log('True heading in degrees: ' + (heading + declination));
});

Note: If the device is not level to the Earth’s surface, a developer needs to apply various tilt compensation techniques for which she needs a 3-axis accelerometer. Data from the orientation sensor, which is a fusion of the accelerometer and magnetometer sensors, is required to implement this particular use case.

9. 10. Acknowledgements

Tobie Langel for the work on Generic Sensor API.

10. 11. Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

A conformant user agent must implement all the requirements listed in this specification that are applicable to user agents.

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[ACCELEROMETER]
Anssi Kostiainen; Alexander Shalamov. Accelerometer . 18 October 2017. WD. URL: https://www.w3.org/TR/accelerometer/
[GENERIC-SENSOR]
Rick Waldron; et al. Generic Sensor API . 2 18 October 2017. WD. URL: https://www.w3.org/TR/generic-sensor/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard . Living Standard. URL: https://infra.spec.whatwg.org/
[PERMISSIONS]
Mounir Lamouri; Marcos Caceres; Jeffrey Yasskin. Permissions . 25 September 2017. WD. URL: https://www.w3.org/TR/permissions/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels . March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WEBIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL . 15 December 2016. ED. URL: https://heycam.github.io/webidl/

Informative References

[COMPASSHEADING] Compass Heading using Magnetometer . Informational. URL: http://www51.honeywell.com/aero/common/documents/myaerospacecatalog-documents/Defense_Brochures-documents/Magnetic__Literature_Application_notes-documents/AN203_Compass_Heading_Using_Magnetometers.pdf
[MAGINDOORPOS]
IndoorAtlas. Indoor positioning using magnetometer Janne Haverinen, Anssi Kemppainen. Global indoor self-localization based on the ambient magnetic field . Informational. URL: http://www.indooratlas.com/wp-content/uploads/2016/03/magnetic_positioning_opus_jun2014.pdf https://doi.org/10.1016%2Fj.robot.2009.07.018
[MAGITACT]
Hamed Ketabdar, Kamer Ali Yüksel, Mehran Roshandel. Magitact . Informational. URL: http://dl.acm.org/citation.cfm?id=1720048
[MOTION-SENSORS]
Kenneth Christiansen; Alexander Shalamov. Motion Sensors Explainer . 30 August 2017. NOTE. URL: https://www.w3.org/TR/motion-sensors/
[VRBUTTON]
Boris Smus. Magnetic input for Google cardboard . Informational. URL: https://bugs.chromium.org/p/chromium/issues/detail?id=445926

IDL Index

]
[Constructor(optional MagnetometerSensorOptions sensorOptions), SecureContext,
  Exposed=Window]
interface Magnetometer : Sensor {
  readonly attribute double? x;
  readonly attribute double? y;
  readonly attribute double? z;
};
[]

enum MagnetometerLocalCoordinateSystem { "device", "screen" };
dictionary MagnetometerSensorOptions : SensorOptions {
  MagnetometerLocalCoordinateSystem referenceFrame = "device";
};
[Constructor(optional MagnetometerSensorOptions sensorOptions), SecureContext,
  Exposed=Window]
interface UncalibratedMagnetometer : Sensor {
  readonly attribute double? x;
  readonly attribute double? y;
  readonly attribute double? z;
  readonly attribute double? xBias;
  readonly attribute double? yBias;
  readonly attribute double? zBias;
};
#local-coordinate-system Referenced in: 4. Model