latitude_longitude grid mapping

[taylor13]

Question

The description of latitude_longitude grid mapping here mentions that "It is included so that the figure of the Earth can be described", but it doesn't list any "Map Parameters". Shouldn't it list at least as options semi_major_axis, inverse_flattening, and longitude_of_prime_meridian as in examples 5.9, 5.10, 5.11, 5.19, 7.15, and 7.16? These all appear in Table F.1 and are described as grid mapping attributes.

[davidhassell]

The top of Appendix F: Grid Mappings says:

The attributes which describe the ellipsoid and prime meridian may be included, when applicable, with any grid mapping. These are:
earth_radius
inverse_flattening
longitude_of_prime_meridian
prime_meridian_name
reference_ellipsoid_name
semi_major_axis
semi_minor_axis

and none of these are included in the definition of the individual mappings.

However, I agree with Karl, that in the special case of latitude_longitude, it wouldn't be a bad thing to re-list these parameters for that particular grid mapping.

[JonathanGregory]

Perhaps it would be sufficient to say that the attributes for describing the figure of the Earth are the ones listed at the start of Appendix F? If we list them again, there's a small risk of inconsistency developing in future. We could also consider providing an example which uses grid mapping for just that purpose in the grid mapping section.

[davidhassell]

That works for me, Jonathan. Thanks.

[taylor13]

O.K., I missed the key sentence just above the list of 7 mapping parameters that can apply to any mapping. Perhaps for each of the mapping options (not just latitude_longitude) we should include a line under "Map Parameters:" we should include the sentence:
"Any of the parameters describing the figure of Earth listed at the beginning of this Appendix."

I think there are already several examples in sections 5 and 7 that show for the latitude_longitude mapping that these parameters can be attached.

[larsbarring]

Well I both agree and disagree. Reshuffling/summarising comments a bit (hopefully without obfuscating the originally intended meaning):

1. Initially Karl asks: Shouldn't the description of latitude_longitude grid mapping here list at least as options semi_major_axis, inverse_flattening, and longitude_of_prime_meridian ?
2. And David responds: The top of Appendix F: Grid Mappings gives this information.
3. ... and then in the same comment David continues: " in the special case of latitude_longitude, it wouldn't be a bad thing to re-list these parameters for that particular grid mapping."
4. Then Jonathan suggests: it would be sufficient to say that the attributes for describing the figure of the Earth are the ones listed at the start of Appendix F.

I agree with 1, 2, and 4, but strongly disagree with 3 for the following reasons:

• The latitude_longitude grid mapping is one of two listed grid mappings that actually allow at least a reasonable geo-location of the data without knowledge of the size/shape of the earth. The other one is rotated_latitude_longitude. Of course, with information on the size/shape of the earth the data can be more precisely geo-located.
• All other grid mappings involve a map projection from a grid coordinate system to latitude/longitude. For all these the following situations arises if the grid mapping is specified without size/shape information:
  A) Grid coordinates and latitude/longitude are available. Then it is in principle possible to infer the size/shape parameters, but in practice it is awkward. And without the size/shape information the situation is similar to that of latitude_longitude that Karl pointed at in the initial post.
  B) Only latitude/longitudes are available. Then it is not possible to infer the grid coordinates, and we are back to the situation that Karl describes.
  C). Only grid coordinates are given. Then it is not even possible to calculate latitude and longitude.

That is, for all grid mappings listed in Appendix F it is always an advantage to specify the size/shape of the earth (or whatever planet/body in focus).

Moreover, in situation C, which is allowed since CF-1.9 it is necessary to specify size/shape to allow calculation of latitude/longitude.

[davidhassell]

Hi Lars, Having originally suggested it, I also disagree with 3, as noted above :) I see 3. is just a less redundant version 4.

Are you suggesting with your further observations that some/all of the shape parameters should be mandatory on latitude_longitude and rotated_latitude_longitude grid mapping? (that's not a leading question - I'm just curious!)

[larsbarring]

I was a bit imprecise, what I oppose to regarding 3 is that the the lat/lon grid mapping is special in the sense implied, i.e. that it should be more relevant to include size/shape for this grid mapping than for any other. I argue that it is exact the other way around: It is less relevant.

If you include just the size/shape parameters and no other map projection information for the lat/long grid mapping, what can you use it for?
-- you cannot infer the original model grid coordinates (probably not of any interest/relevance anyway).
-- however you can use the information together with a map projection (etc.) of your liking to e.g. provide a more exact geo-positioning of the data, or to plot a map according to some specified/required projection. Without size/shape information you have to instead make assumptions regarding this if you want to use you data for that purpose.

So, for lat/lon (and rotated lat/lon) grid mapping it is useful to have size/shape information but not required.

For all other grid mappings listed in Appendix F, either it is useful, or it is necessary to have size/shape information.

Hence, I suggest that what should be done is

• Add a sentence along what Jonathan suggests to all grid mappings.
• In the introductory text to Appendix F clearly specify that shape/size parameters are required for all map projections if lat/lon coordinates are not provided. Exceptions are of course the lat/lon and rotated lat/lon map projections.

[taylor13]

I added a thumbs up for Lars' suggestions above, but on further reflection, I'm not sure I understand whether the second bulleted item is a good idea. The first item falls along the lines of the suggestion I made here.

[larsbarring]

@taylor13 I am happy that we agree (all agree as it seems from comments so far) regarding the first point. Could you be more specific in your concerns regarding the second bullet point?