On Apr 19, 2018, at 10:06 AM, Daniel Lee <[email protected]
<mailto:[email protected]>> wrote:
Hi Jim,
I for one find this more confusing than Ethan's definition, but maybe
it's because I'm too far gone in my discipline to see the scope for
misunderstanding.
That being said, if we're being that general my feeling says to me
that we may risk converging on a standard which isn't really
applicable to any more specific application. Currently there is
aproposal for CF-2 devoted specifically to swath data
<https://github.com/Unidata/EC-netCDF-CF/blob/master/swath/swath.adoc>,
and this has the potential to cover the need for a specific
geostationary projection as well. Maybe that would also be a good
path to take for other coordinate systems with non-linear
relationships between projection coordinates and coordinates of other
CRS - kind of general, but not overly abstract.
Cheers,
Daniel
*From:*CF-metadata [mailto:[email protected]]*On
Behalf Of*Jim Biard
*Sent:*19 April 2018 15:32
*To:*[email protected] <mailto:[email protected]>
*Subject:*Re: [CF-metadata] Fix Geostationary projection, including
proposal for two new standard names
Hi.
Here's a couple of thoughts.
The definition that Ethan has proposed fails to note that the angles
are with respect to a normal to the projection surface at a point
along the normal. I guess the phrase "angular distance" implies this,
but my first read had me feeling confused about what was being
described. On checking, I see that this is a minimalist variation on
the projection_x/y_coordinate definitions. Do folks think that this
is clear enough as is?
I know we tend not to follow this course, but I am wondering if we
might not be better served overall by taking a more generic approach
and defining parametric coordinates u and v (projection_u_coordinate
and projection_v_coordinate). The canonical units would be '1'
(unitless). The definition for parametric_u_coordinate would be
something like
"u" indicates an independent variable, or parameter, associated
with an axis of a coordinate grid where this parameter is not a
linear distance in a projection coordinate system, a Cartesian
coordinate element, or a geographic latitude or longitude. The
geographic latitude and longitude of each point in the coordinate
grid are functions of the parameters associated with the grid
axes. The relationship between the parametric coordinates and
latitude and longitude is described by the grid_mapping.
The geostationary projection is one use case covered by parametric
coordinates, and there are others. The native coordinates for most
all satellite swath data are parametric - mirror angle and time, for
example.
Grace and peace,
Jim
On 4/19/18 5:06 AM, Daniel Lee wrote:
Hi Ethan,
At first blush this looks pretty good. If we can agree on this in
a short-ish time frame, it might be possible for EUMETSAT to
publish data exclusively using these standard names - the planned
launch date for MTG I1 is late 2021. This sounds like it's very
far away, but in the space sector our planning horizons are a lot
longer, so there's already a lot of work being done on it right
now and at some point in the near future the specs will freeze.
Best regards,
Daniel
*From:*CF-metadata [mailto:[email protected]]*On
Behalf Of*Ethan Davis
*Sent:*19 April 2018 05:40
*To:*CF metadata<[email protected]>
<mailto:[email protected]>
*Subject:*[CF-metadata] Fix Geostationary projection, including
proposal for two new standard names
Hi all,
Here's an initial proposal for fixing the geostationary
projection as we've been discussing.
Two new standard names:
*Name:*projection_x_angular_coordinate
*Canonical units:*radian
*Definition:*"x" indicates a vector component along the grid
x-axis, when this is not true longitude, positive with
increasing x. Angular projection coordinates are angular
distances in the x- and y-directions on a plane onto which
the surface of the Earth has been projected according to a
map projection. The relationship between the angular
projection coordinates and latitude and longitude is
described by the grid_mapping.
*Name:* projection_y_angular_coordinate
*Canonical units:* radian
*Definition:* "y" indicates a vector component along the grid
y-axis, when this is not true latitude, positive with
increasing y. Angular projection coordinates are angular
distances in the x- and y-directions on a plane onto which
the surface of the Earth has been projected according to a
map projection. The relationship between the angular
projection coordinates and latitude and longitude is
described by the grid_mapping.
Replace the text of the current "Map coordinates:" section with
The x (abscissa) and y (ordinate) projection coordinates are
identified by the `standard_name` attribute values
`projection_x_angular_coordinate` and
`projection_y_angular_coordinate` respectively. In the case
of this projection, the projection coordinates are directly
related to the scanning angle of the satellite instrument.
Add a deprecation note below the current "Notes:"
*Deprecation Note:*
The use of `projection_x_coordinate` and
`projection_y_coordinate` for this projection has been
deprecated.
The initial definition of this projection used these standard
names to identify the projection coordinates even though
their canonical units (meters) do not mach those required for
this projection (radians).
Perhaps we should include information on when the deprecated
feature was in effect:
The initial definition for this projection was agreed on in
May 2012 though it was not in the CF document until 1.7 was
released in Sept 2017. It was corrected in ??? 2018.
And do we also want to include information about large datasets
that use this deprecated technique:
In that time, several satellite missions were developed and
launched that generate data that use this now deprecated
method including GOES-R (operational in Dec 2017), EUMETSAT
???? ...
That could alert people to the likelihood they (or any software
they develop) might run into data using this deprecated feature.
I'll move this to a Trac ticket (with an accompanying GitHub PR)
once we discuss a bit.
Cheers,
Ethan
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