> There is not enough detail in the CF document describing this. Can’t argue with that. ;-)
I don’t find the Euler angle stuff particularly illuminating, so in case anyone else is in the same boat... As I understand it, the “grid_north_pole_latitude” and “grid_north_pole_longitude” attributes dictate the location of the new, grid north pole in normal lat/lon coordinates. So if they are set to 51.5 and 0 respectively then the grid’s north pole is sitting somewhere in London. With the location of the pole sorted out, the “north_pole_grid_longitude” attribute pins down(*) the rotation by specifying the *grid* longitude of the *real* north pole. In other words, “where on my rotated grid does the real north pole lie?”. *) Except where grid_north_pole_latitude is +/-90, in which case it doesn’t help at all. But then the pragmatists in the audience will question why you are bothering specifying a rotated pole coordinate system where the pole hasn’t been rotated. ;-) I hope that matches your current understanding! Regards, Richard Hattersley From: CF-metadata [mailto:[email protected]] On Behalf Of DeTracey, Brendan Sent: 20 October 2014 12:54 To: [email protected] Subject: Re: [CF-metadata] Rotated pole definition Thanks for the replies. Shouldn’t the second rotation angle be pi/2-grid_north_pole_latitude? To visualize I was holding a water bottle and imagining the rotations to get the north pole at 0N,-90E. First rotation 90 about the north pole(z). Second rotation has to be 90 degrees(not 0) about the new east-west(y’) axis (perpendicular to the new prime meridian). The euler angle convention matches the fourth case(intrinsic) on the euler angle Wikipedia page: (http://en.wikipedia.org/wiki/Euler_angles#Conventions) i.e. 4. z-y’-z″ (intrinsic rotations) with the angles ordered pi + grid_north_pole_longitude, pi/2 - grid_north_pole_latitude, north_pole_grid_longitude If so, this really needs to be more clearly stated in the reference. I came here because I could not get a clear answer for these same parameters for the Climate Data Operators, and then assumed they were taken from CF. Thanks again, Brendan ------------------------------------------ Brendan DeTracey [email protected]<mailto:[email protected]> (902)426-9727 3-30 VS Marine Ecosystem Section / Section de l'écosystème marin From: Hedley, Mark [mailto:[email protected]] Sent: October-17-14 8:14 AM To: DeTracey, Brendan; [email protected]<mailto:[email protected]> Subject: RE: Rotated pole definition Hello Brendan I believe the intent of the rotated pole grid mapping definition is to describe a polar coordinate reference system, based on the earth, but with a different axis of rotation. To do this, a new north pole location is specified in the earth polar coordinate reference system by - grid_north_pole_latitude - grid_north_pole_longitude This defines a point on the surface of the non-rotated system. A new rotational axis is defined through this location and the centre of the body. A further rotation is then applied about this new axis, as defined by: - north_pole_grid_longitude (optional, default 0). For a spherical geometry, this is equivalent to 3 rotation transforms of the basis (theta,phi) (ordered operation): - rotate in the theta-hat direction by: - Pi + grid_north_pole_longitude - rotate in the phi-hat direction by: - grid_north_pole_latitude - rotate in the theta-hat direction by: - north_pole_grid_longitude | 0 This provides a new basis which coordinates may be defined with respect to. I keep an inflatable globe on my desk to help me with this. I have not found a clear diagram on line which illustrates this unambiguously, or managed to draw one for myself; I would like to. I hope this information is correct and useful. mark ________________________________ From: CF-metadata [[email protected]] on behalf of DeTracey, Brendan [[email protected]] Sent: 16 October 2014 12:49 To: [email protected]<mailto:[email protected]> Subject: [CF-metadata] Rotated pole definition Hi, I am looking for clarification on the rotated pole definition. Is the globe rotated such that the north pole traces a great circle from its original to its new position? And then rotated counter clockwise about the new pole by north_pole_grid_longitude degrees? There is not enough detail in the CF document describing this. Brendan
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