Add new grid mappings

[edavis]

1. Title

Add new grid mappings to the existing list.

2. Moderator

John Caron

3. Requirement

A number of grid mappings are being used that are not listed in CF. Make additions to the existing list.

This is a follow-up proposal to the proposal in CF Trac ticket #18.

4. Initial Statement of Technical Proposal

Add the following projections:

• Lambert Cylindrical Equal Area (EPSG 9834 for spherical earth, EPSG 9835)
• Mercator (EPSG 9804 for one standard parallel, EPSG 9805 for two standard parallels)
• Orthographic (EPSG 9840)

5. Benefits

Allow existing data using non-CF defined grid mappings to be CF compliant.

6. Status Quo

Data that is otherwise CF compliant using grid mappings that are not supported by CF.

7. Detailed Proposal

CF Wiki page: GridMapNames - currently at version 11

[jonathan]

Dear Ethan

I support adding these projections.

I don't understand the details about them, but I have a couple of questions about Mercator. First, shouldn't there be false_easting and false_northing attributes, as there are for all other projections? (I think these are just offsets for the map coordinates, aren't they?) Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

Cheers

Jonathan

[edavis]

Hi Jonathan,

Replying to jonathan:

I don't understand the details about them, but I have a couple of questions about Mercator. First, shouldn't there be false_easting and false_northing attributes, as there are for all other projections? (I think these are just offsets for the map coordinates, aren't they?)

Yes, I agree. I will add these.

Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

Looks like a mistake on my part since, as you say, Mercator by definition has a latitude of projection at the equator. I will remove the latitude_of_projection_origin attribute.

Made changes to the GridMapNames page - now at version 12.

Thanks,

Ethan

[pbentley]

Replying to edavis:

Second, what is the role of the latitude of projection origin for the Mercator projection? I thought the Mercator projection had the projection cylinder touching the Equator by construction, so perhaps 0 is implicitly the "special latitude" if that's what this attribute means.

Looks like a mistake on my part since, as you say, Mercator by definition has a latitude of projection at the equator. I will remove the latitude_of_projection_origin attribute.

You weren't mistaken, Ethan. The Mercator projection can be used with a standard parallel that is not the Equator. In such cases the projection cylinder is secant to the globe rather than tangent to it at the Equator. Of course, one also gets a second standard parallel of equal latitude but opposite sign in the southern hemisphere.

Any map based on such usage would look the same as a standard Mercator map but would have a different scale, with true scale being preserved at all points along the standard parallel rather than the Equator. Which is probably only relevant for navigation charts and the like.

So I think your second Mercator grid mapping parameter does just need to be 'standard_parallel'.

Thanks for submitting this follow-up proposal. It gets my support.

Cheers, Phil

[edavis]

Hi Phil,

Yup, that's my understanding too. My mistake was having both a standard_parallel and a latitude_of_projection_origin. I think the Mercator attributes are now correct, here's what we have:

• longitude_of_projection_origin
• Either standard_parallel (EPSG 9805) or scale_factor_at_projection_origin (EPSG 9804)
• false_easting
• false_northing

Thanks,

Ethan

[caron]

Dear Velimir and all

There have been no further comments on this ticket, so I will close it, and ask Velimir to implement. The final wording is ​here.

Thanks to Ethan, along with comments by Jonathan and Phil that clarified things.

John

[edavis]

A bit of version confusion:

The link John gave was for version 11. The latest (12) is at:

​http://cf-pcmdi.llnl.gov/trac/wiki/GridMapNames

Ethan