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Re: Orientation event draft

From: Steve Block <steveblock@google.com>
Date: Wed, 21 Apr 2010 17:42:09 -0700
Message-ID: <j2nb39f53b21004211742odb8dd8a4z6d89fcf338354801@mail.gmail.com>
To: public-geolocation <public-geolocation@w3.org>, Martin.Thomson@andrew.com
Thanks for the comments Martin.

> I have a recommendation on x, y and z though.  In this case, we do have
> names.  These should instead be labelled, East, North and Up.
OK, I'll add these names. Though I think we should keep the labels x,y,z too.

> I also have a problem with the use of magnetic north.  I understand the
> limitations of the measurement devices, but there are other ways of defining
> cardinal directions that are more directly compatible with geolocation.
Yes, you're right, we should use True North (the direction towards the
North Pole, which lies on the line of 90 latitude for the datum in
use) for the WGS84 datum (the datum used by Geolocation). Geolocation
uses True North as the reference for its heading property.

> Rather than defining "Up" to be the tangent to the gravipotential surface, you can define up in terms of location (as a vector in
> WGS84 Cartesian coordinates):
I think that rather than defining up to be the direction of the vector
from the WGS84 origin as you suggest, I think it's more common to use
the direction perpendicular to the ground for these local coordinates.
Though we should clarify the spec to say that the ground should be
taken to be tangent to the local surface of the WGS84 spheroid.

> The drawback is that in order to measure this you need to know the relationship between magnetic north and these.  Note also that
> magnetic north moves.  You also need to know the relationship of the gravipotential surface with East and North - the EGM (Earth
> Gravipotential Model) provides that.
Yes, it's true that these definitions require significant computation
to obtain the required values from the sensor data. Though we don't
require a particular level of accuracy and for many applications, the
obvious approximations (magnetic north == true north, gravity is
parallel to the Up axis) will suffice.

> Though I suspect that implementations will be forced to substitute zero.
Why do you think this?

>> > Would I be right in assuming that 0, 0, 0 orientation equates to a device with the screen facing
>> > up and the top of the screen toward north?
>> Yes
> But you still have to assume this.  An explicit statement might seem a little redundant, but that small effort will pay you back.
The spec states that 'The transformation from the Earth frame to the
device frame is expressed in terms of 3 rotations', so at (0, 0, 0)
the frames are aligned by definition. It also says 'Starting with the
two frames aligned, the rotations are applied in the following
order:'. Can you suggest how you'd like to add further clarification?

> Right hand rotation is well understood.  Using that terminology would be helpful.
OK, I'll add that terminology.

Steve

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Received on Thursday, 22 April 2010 00:42:40 GMT

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