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 [cf-metadata-bounces at cgd.ucar.edu] on behalf of DeTracey, Brendan [Brendan.DeTracey at dfo-mpo.gc.ca]
Sent: 16 October 2014 12:49
To: cf-metadata at cgd.ucar.edu
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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