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:cf-metadata-bounces at cgd.ucar.edu] *On
> Behalf Of *Ethan Davis
> *Sent:* 19 April 2018 05:40
> *To:* CF metadata <cf-metadata at cgd.ucar.edu>
> *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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