Add "Local Coordinate System" concept

This is required for fixing w3c#257

index.bs

@@ -495,6 +495,28 @@ through a process called [=sensor fusion=].
 [=calibration|Calibrated=] [=raw sensor readings=] are referred to as <dfn>sensor readings</dfn>,
 whether or not they have undergone [=sensor fusion=].
 
+If the [=sensor readings=] represent spatial measurements (e.g.
+acceleration, angular velocity), they must be resolved in
+a coordinate system which is called <dfn export>local
+coordinate system</dfn>.
+
+Scalar physical quantities (i.e. temperature) do not require
+a [=local coordinate system=] for resolution.
+
+The [=local coordinate system=] normally used in a mobile device is
+a Cartesian coordinate system which is defined relatively to the
+device’s screen, so that X and Y axes are parallel to the screen
+dimentions and Z axis is perpendicular to the screen surface.
+
+[=extension specification|Extensions to this specification=]
+may explicitly define the [=local coordinate system=] for the associated
+[=sensor type=] or make it configurable per {{Sensor}} object.
+
+Depending on the number of orthogonal axes in which the measurements
+can be simultaneously performed by a [=device sensor|sensor=], this
+[=device sensor|sensor=] can be <dfn>uniaxial</dfn>, <dfn>biaxial</dfn>,
+or <dfn>triaxial</dfn>.
+
 The term <dfn id="concept-platform-sensor">platform sensor</dfn> refers to platform interfaces,
 with which the user agent ineracts to obtain [=sensor readings=] for a single [=sensor type=]
 originated from one or several [=device sensors=].
@@ -565,7 +587,7 @@ for the given [=sensor types=] they are targeting.
 through a process called <dfn>sensor fusion</dfn>.
 This process provides [=high-level|higher-level=] or
 more accurate data (often at the cost of increased latency).
-For example, the [=sensor readings|readings=] of a three-axis magnetometer
+For example, the [=sensor readings|readings=] of a [=triaxial=] magnetometer
 needs to be combined with the [=sensor readings|readings=] of an accelerometer
 to provide a correct bearing.
 
@@ -1409,6 +1431,8 @@ Gets the {{DOMException}} object passed to {{SensorErrorEventInit}}.
 
     1.  If |sensor_instance|.{{[[state]]}} is "activated",
         1.  Let |readings| be the [=latest reading=] of |sensor_instance|'s related [=platform sensor=].
+        1.  If the [=extension specification=] defines a [=local coordinate system=] for |sensor_instance|,
+            1. Remap |readings| values to the [=local coordinate system=].
         1.  Return |readings|[|key|].
     1.  Otherwise, return null.
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