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Ticket Resolution Summary Owner Reporter
#100 fixed Clarifications to the preamble of sections 4 and 5 davidhassell jonathan
Description

Dear all

I have the honour of opening ticket number 100. In this ticket, Steve Hankin and I have a number of changes to propose to the text of section 5, with the intention of clarifying it, not changing what it means. Therefore this is a defect ticket, but please object if you think it is changing the meaning, or making it any less clear!

Cheers

Jonathan

Replace this text in the first paragraph

A variable's spatiotemporal dimensions are used to locate data values in time and space. This is accomplished by associating these dimensions with the relevant set of latitude, longitude, vertical, and time coordinates.

with

A data variable's dimensions are used to locate data values in time and space or as a function of other independent variables. This is accomplished by associating these dimensions with the relevant set of latitude, longitude, vertical, time and any non-spatiotemporal coordinates.

These changes clarify that coordinate systems belong to data variables (rather than any other type of variable in a CF-netCDF file) and recognise the use of non-spatiotemporal dimensions.

Replace this text in the second paragraph

All of a variable's dimensions that are latitude, longitude, vertical, or time dimensions (see Section 1.2, "Terminology") must have corresponding coordinate variables, i.e., one-dimensional variables with the same name as the dimension

with

Any of a variable's dimensions that is an independently varying latitude, longitude, vertical, or time dimension (see Section 1.2, "Terminology") and that has a size greater than one must have a corresponding coordinate variable, i.e., a one-dimensional variable with the same name as the dimension

These changes remove the implication that spatiotemporal scalar coordinates might be prohibited, and allow for discrete axes (section 4.5, used extensively in section 9).

Replace this text in the third paragraph

All of a variable's spatiotemporal dimensions that are not latitude, longitude, vertical, or time dimensions are required to be associated with the relevant latitude, longitude, vertical, or time coordinates via the new coordinates attribute of the variable

with

Any longitude, latitude, vertical or time coordinate which depends on more than one spatiotemporal dimension must be identified by the coordinates attribute of the data variable.

This change is for clarity.

Replace this text in the fifth paragraph

The use of coordinate variables is required whenever they are applicable. That is, auxiliary coordinate variables may not be used as the only way to identify latitude and longitude coordinates that could be identified using coordinate variables.

with

If the longitude, latitude, vertical or time coordinate is multi-valued and varies in only one dimension, it is not permitted to store it as an auxiliary coordinate variable.

This change is for simplicity and clarity. The use of coordinate variables where applicable is already required by the second paragraph.

Append to the fifth paragraph

If the longitude, latitude, vertical or time coordinate is single-valued, it may be stored either as a coordinate variable with a dimension of size one, or as a scalar coordinate variable (Section 5.7).

This change recognises the use of spatiotemporal scalar coordinates.

#93 fixed Two new dimensionless vertical coordinate specifications for s coordinate ocean models cf-conventions@… rsignell
Description

1. Title

Two new dimensionless vertical coordinates to support ocean models

2. Moderator

Rich Signell

3. Requirement

The ocean modeling community needs two additional vertical coordinate specifications to allow modern s-coordinate model output to be CF-compliant and allow more general specification of s-coordinate model output for future development.

4. Initial Statement of Technical Proposal

The existing ocean_s_coordinate dimensionless vertical coordinate specification in CF is limited to a specific vertical stretching function and set of control parameters, while modern versions of ROMS allow for more flexible specification. Additional of these two new generalized vertical coordinate specifications would allow output from existing ROMS-derived models (and other s coordinate models) to be CF-compliant, as well as allowing more flexibility for future s_coordinate model developers and users to write and read CF-compliant model output.

Here are the proposed additions to Appendix D. Dimensionless Vertical Coordinates

Ocean s-coordinate, generic form 1

   standard_name = "ocean_s_coordinate_g1"

Definition:

   z(n,k,j,i) = S(k,j,i) + eta(n,j,i) * (1 + S(k,j,i) / depth(j,i))

    where S(k,j,i) = depth_c * s(k) + (depth(j,i) - depth_c) * C(k)

and where z(n,k,j,i) is the height, positive upwards, relative to ocean datum (e.g. mean sea level) at gridpoint (n,k,j,i); eta(n,j,i) is the height of the ocean surface, positive upwards, relative to ocean datum at gridpoint (n,j,i); s(k) is the dimensionless coordinate at vertical gridpoint (k) with a range of -1 <= s(k) <= 0, s(0) corresponds to eta(n,j,i) whereas s(-1) corresponds to depth(j,i); C(k) is the dimensionless vertical coordinate stretching function at gridpoint (k) with a range of -1 <= C(k) <= 0, C(0) corresponds to eta(n,j,i) whereas C(-1) corresponds to depth(j,i); and the constant depth_c, (positive value), is a critical depth controlling the stretching.

The format for the formula_terms attribute is

formula_terms = "s: var1 C: var2 eta: var3 depth: var4 depth_c: var5"

Ocean s-coordinate, generic form 2

   standard_name = "ocean_s_coordinate_g2"

Definition:

   z(n,k,j,i) = eta(n,j,i) + (eta(n,j,i) + depth(j,i)) * S(k,j,i)

    where S(k,j,i) = (depth_c * s(k) + depth(j,i) * C(k)) / (depth_c + depth(j,i))

and where z(n,k,j,i) is the height, positive upwards, relative to ocean datum (e.g. mean sea level) at gridpoint (n,k,j,i); eta(n,j,i) is the height of the ocean surface, positive upwards, relative to ocean datum at gridpoint (n,j,i); s(k) is the dimensionless coordinate at vertical gridpoint (k) with a range of -1 <= s(k) <= 0, s(0) corresponds to eta(n,j,i) whereas s(-1) corresponds to depth(j,i); C(k) is the dimensionless vertical coordinate stretching function at gridpoint (k) with a range of -1 <= C(k) <= 0, C(0) corresponds to eta(n,j,i) whereas C(-1) correspond to depth(j,i); and the constant depth_c, (positive value) is a critical depth controlling the stretching.

The format for the formula_terms attribute is

formula_terms = "s: var1 C: var2 eta: var3 depth: var4 depth_c: var5"

5. Benefits

The oceanographic community, especially those producing or consuming products from s coordinate models (e.g. variants and derivatives of the Regional Ocean Modeling System (ROMS)) would benefit from the addition of these two new dimensionless vertical coordinate specifications. These two new coordinates have been exercised in the Unidata Common Data Model and the Unidata NetCDF-Java Library for the last two years. It's time to add them officially to the CF Conventions.

6. Status Quo

The only option currently to store modern ROMS results as CF compliant would be to write the entire Z field as a 4D array.

#92 fixed Add oblique mercator projection davidhassell mcginnis
Description

The Oblique Mercator projection is used by at least one regional climate model, RegCM3, which is part of the NARCCAP climate modeling program. Currently we record its map projection information using the transverse_mercator projection, which I have learned is very similar but not quite the same. I propose to add this map projection so we can get it right.

Proposed text:


Oblique Mercator

grid_mapping_name = oblique_mercator

Map parameters:

  • azimuth
  • latitude_of_projection_origin
  • longitude_of_projection_origin
  • scale_factor_at_projection_origin
  • false_easting
  • false_northing

Map coordinates:

The x (abscissa) and y (ordinate) rectangular coordinates are identified by the standard_name attribute value projection_x_coordinate and projection_y_coordinate respectively.

Notes:

Notes on using the PROJ.4 software package for computing the mapping may be found at http://www.remotesensing.org/geotiff/proj_list/hotine_oblique_mercator.html . The Rotated Mercator projection is an Oblique Mercator projection with azimuth = +90.


If adding a new attribute for azimuth is problematic, this proposal could be modified to add the rotated_mercator projection instead, which is a special case of Oblique Mercator with azimuth = 90.

Note that apparently there is a subtle technical difference between an Oblique Mercator projection and a Hotine Oblique Mercator projection that depends on when the rectification from skew grid to map grid is applied. Since most mapping packages don't support a rectified grid angle parameter at all (effectively giving it a default value of 90 degrees, such that it has no effect), to avoid unnecessary proliferation of attributes I propose to omit this parameter and elide this distinction until such time as it proves necessary.

My knowledge of this topic is quite limited; I have made this proposal based on what understanding I have gleaned from the geotiff website and communications with colleagues working with our RegCM3 output in GIS. Commentary from experts would be very welcome.

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