W3C home > Mailing lists > Public > public-geolocation@w3.org > May 2011

Re: axes and normalization of orientation events

From: George Percivall <gpercivall@opengeospatial.org>
Date: Mon, 16 May 2011 11:42:13 -0400
Cc: public-geolocation@w3.org
Message-Id: <481CE1A0-0930-482F-996B-846AA5C18C4F@opengeospatial.org>
To: Steve Block <steveblock@google.com>, Dean Jackson <dino@apple.com>
Steve, Dean,

Typical practice is to look down at a map.
Typical practice is to measure heading clockwise from north while looking down at the map.
So, the coordinate system using right hand rule for this approach is North - East - Down. 

The OGC Moving Object Snapshot uses NED in this way:

- Local Tangent Plane (Section 5.3)

"The velocity in MovingObjectSnapshot is defined relative to Euclidean plane corresponding to a Local Tangent Plane (LTP) at the location of the object.  The LTP is defined with its origin at the point where the object is located on (or possibly near) the Earth’s surface. LTP is oriented in three dimensions with the vertical axis taken to be straight down, parallel to the gravitational gradient, with the plumb line. The other two axes are perpendicular to the vertical axis aligned with local geographic north and east."

- Heading" (Section 7)

"...value must be in degrees measured clockwise from due North"


On May 16, 2011, at 11:16 AM, Steve Block wrote:

> Hi Dean,
>> However, in researching it I notice that the W3C spec lists the axes in a different order than the convention used by
>> the rest of the world.
> I'm aware that in vehicle dynamics the x axis usually points in the
> direction of motion, but do you have a reference for a standard for
> non-vehicle applications, like a UA? We chose the Earth frame to be
> East-North-Up and it made sense to align the body frame such that it
> matches the Earth frame when viewing a map on the device, though I
> don't feel strongly about this.
>> Also, I notice that the spec says "Rotations use the right-hand convention: positive rotation around an axis is
>> clockwise when viewed along the positive direction of the axis". It's highly likely that I'm misinterpreting this, but a
>> rotation around Z using the right hand rule (from X axis to Y axis) is counter-clockwise. In fact, the little arrow on the
>> diagram shows this.
> Yes, I think you're misinterpreting this. Talking about clockwise vs
> counter-clockwise only makes sense when you specify the direction the
> observer is facing. +ve rotation about an axis appears clockwise when
> the observer is facing the direction of the +ve axis. Conversely, if
> the observer is facing the -ve axis direction (as is the case for the
> z axis in the second diagram), the rotation appears counter-clockwise.

George Percivall
@Percivall on Twitter
OGC -- Making Location Count

Received on Monday, 16 May 2011 15:42:37 UTC

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