**Title:** Fix geostationary projection **Moderator:**
**Requirement Summary:** We should describe the geolocation data from geostationary satellites correctly. This is not currently the case. **Technical Proposal Summary:** Change the error in the Conventions, deprecate the usage of the previous standard names associated with that in the context where they are incorrect, and add 2 standard names to be able to describe coordinates in such data correctly. **Benefits:** Users who want to have a correct understanding of coordinates in geostationary products without building special cases into their software. **Status Quo:** The current geostationary projection uses `projection_x_coordinate` and `projection_y_coordinate` to encode coordinates. Their units in the projection are described [in Appendix F](http://cfconventions.org/cf-conventions/cf-conventions.html#_geostationary_projection) as radians. However, the `standard_name` attributes have meters as their canonical units. [According to Section 3.3](http://cfconventions.org/cf-conventions/cf-conventions.html#standard-name) if a `standard_name` uses different units than the canonical units, these must be physically equivalent. This is not the case for coordinates as used in geostationary products. This can lead to confusion. **Detailed Proposal:** This had been discussed previously [on the mailing list](http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2018/020014.html) but seems to have fizzled out. The current iteration of the proposal had support from @erget @ethanrd and @JonathanGregory but should be reviewed again after laying dormant since April 2019. In order to fix the aforementioned issue @ethanrd proposed the following, which I would be happy to clean up and put into a PR if we agree on the rough content: ## Add 2 new standard names | Name | Canonical units | Definition | | ---- | --------------- | -----------| | projection_x_angular_coordinate | radian | "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. | | projection_y_angular_coordinate | radian | "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. | ## Update "Map coordinates" section of Conventions 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. ## Deprecate old practice 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). *Potentially to be noted only in commit:* > 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. > > 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 ???? ... -- You are receiving this because you are subscribed to this thread. Reply to this email directly or view it on GitHub: https://github.com/cf-convention/cf-conventions/issues/230 This list forwards relevant notifications from Github. It is distinct from [email protected], although if you do nothing, a subscription to the UCAR list will result in a subscription to this list. To unsubscribe from this list only, send a message to [email protected].
