Hi all,
The angles are viewing angles from the perspective of the satellite. It?s the line connecting the satellite?s view (of the Earth) and the instrument. So as the satellite scans along its scan track, for example, the X-axis viewing angle changes. Knowing the satellite?s position and that its X axis is positive easting and Y axis is positive northing, you can compute the intersection of the viewing angle with the Earth?s surface.
In the specific case of geostationary satellites, these geometries are fairly consistent over time and indeed data is often corrected so that the views are resampled onto a fixed grid before dissemination to users. This means that what you?re actually dealing with is the theoretical viewing angle of the satellite at its ideal position.
The swath proposal deals with the more general case of having views across and along tracks which are aligned with a satellite?s flight path as it orbits about the Earth.
Best regards,
Daniel
From: Martin Juckes - UKRI STFC<mailto:martin.juckes at stfc.ac.uk>
Sent: 20 April 2018 10:06
To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] Fix Geostationary projection, including proposal for two new standard names
Dear All,
I agree with Jim that a little more basic information is needed about what the angles are. I may be misinterpreting the discussion, but I had imagined that the angles as components of a spherical coordinate system centred on the satellite, with the nadir at (0,0) ... is that correct?
If this is true, we could have a fairly simple definition in terms of what the angles mean relative to the satellite. What determines the orientation of the coordinates? Is it assumed that the satellite has a well defined axis acting as the coordinate pole?
regards,
Martin
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jim Biard <jbiard at cicsnc.org>
Sent: 19 April 2018 16:35
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Fix Geostationary projection, including proposal for two new standard names
Daniel,
My two thoughts were independent. I agree that parametric coordinates are more abstract, and thus possibly more confusing, than linear coordinates. But, as Randy pointed out in his reply, the relationship between the angles and longitude and latitude are quite complex. The abstraction of parametric functions and parameters avoids the problem of making definitions that are so specific that they aren't good for anything other than the one geostationary projection. I also am happy to admit to having made a less-than-perfect first pass at a definition.
Grace and peace,
Jim
On 4/19/18 10:18 AM, Randy Horne wrote:
Folks:
RE: ? 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
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.?
specifically,
"are angular distances in the x- and y-directions on a plane onto which the surface of the Earth has been projected?
In the case of both the GOES-R and EUMETSAT, the angular distances are projected onto an Earth ellipsoid, whose definition is captured in the grid mapping.
v/r
randy
On Apr 19, 2018, at 10:06 AM, Daniel Lee <Daniel.Lee at eumetsat.int<mailto:Daniel.Lee at eumetsat.int>> 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 a proposal 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:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Jim Biard
Sent: 19 April 2018 15:32
To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
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: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><mailto: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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