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#92 fixed Add oblique mercator projection davidhassell mcginnis

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 on using the PROJ.4 software package for computing the mapping may be found at . 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.

#25 fixed Correction in Conventions document, table 3.1 cf-conventions@… mlaker1

The Conventions document, version 1.1 at has an error in an entry in the right-hand sub-table of Table 3.1, in section "3.1 Units":

The entry:

1e-2 deci c

should read:

1e-2 centi c

#87 fixed Allow comments in coordinate variables davidhassell ngalbraith

Appendix A, Attributes, indicates that the 'comment' attribute may be used as a global attribute or for non-coordinate data variables. Its description states that it is for 'Miscellaneous information about the data or methods used to produce it.'

For in situ ocean data, certain information about the coordinate variables seems to belong in a comment; coordinate values have characteristics that should be described, but there are no standards for these descriptions yet.

For example, for a surface mooring, this seems like the best place to indicate whether the latitude and longitude are the surveyed anchor position or a GPS fix from the buoy itself, and whether the depth is the combined length of line/chain/instrument cases between the surface and the sensor, or a calculated depth based on measured pressure.

So, I propose that in the table in Appendix A we change the 'Use' of the comment attribute from G,D to G,C,D, to allow comments for variables containing coordinate data.


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