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Ticket Resolution Summary Owner Reporter
#70 fixed Connecting coordinates to Grid Mapping variables davidhassell markh
Description

Title

Connecting coordinates to Grid Mapping variables

Moderator

David Hassell

Requirement

To allow each Coordinate or Auxiliary Coordinate Variable to be explicitly attached to a particular Grid Mapping Variable.

Proposal

To define a coordinate reference system, a grid_mapping variable is defined, with variable attributes to define the coordinate reference system attributes. http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.5/ch05s06.html

To define coordinates, be they coordinate variable or auxiliary coordinates, with respect to a coordinate system an enhanced grid_mapping syntax is proposed.

A data variable's grid_mapping attribute may use a syntax, derived from the cell_methods and cell_measures approach. Using this model, the Grid Mapping variable name is stated, followed by a ':' followed by a space separated list of Coordinate names; a name finishing with a ':' indicates a new Grid Mapping variable name, to be followed by the coordinates which are linked to it.

GRID_MAPPING_VAR: COORD_VAR [COORD_VAR ...] [GRID_MAPPING_VAR: ...]

The current grid_mapping attribute syntax is a regressive case of this, so the proposal is backwards compatible. In the case where the grid_mapping attribute contains no ':' then the grid_mapping is defined by the current methodology.

The grid_mapping attribute is interpreted differently if it uses the enhanced syntax, in which case different coordinates may be linked to different Grid Mappings.

Section 5.6 should include the text:

A data variable may link Coordinates and Auxiliary Coordinate Variables to 
Grid Mapping Variables by supplying the name of the Grid Mapping variable, 
a ':' and a space separated list of Coordinate names:
  
GRID_MAPPING_VAR: COORD_VAR [COORD_VAR ...] [GRID_MAPPING_VAR: ...]

within the data variable's grid_mapping attribute.

Use Case

Consider a file containing a data variable referencing coordinate variables: Easting, Northing, Height (above mean sea level). These are all defined with respect to the Ordnance Survey of Great Britain coordinate reference system, a parametric transverse Mercator with a defined vertical datum.

To be CF compliant I need to include two auxiliary coordinates, latitude and longitude. I can calculate these for a specifically defined coordinate reference system, using the definition of the transverse Mercator projection. I will also include the height with respect to this new datum as an auxiliary_coordinate.

In the example use case then the data variable's grid_mapping attribute would read: "OSGB: Easting Northing Height GeogCS: Lat Lon geogHeight"

dimensions:
  y = 100000 ;
  x = 100000 ;
  z = 100;
variables:
  double x(x) ;
    x:standard_name = "projection_x_coordinate" ;
    x:long_name = "Easting"
    x:units = "m" ;
  double y(y) ;
    y:standard_name = "projection_y_coordinate" ;
    y:long_name = "Northing"
    y:units = "m" ;
  double z(z) ;
    y:standard_name = "height_above_reference_ellipsoid" ;
    y:long_name = "height_above_osgb_msl" ;
    y:units = "m" ;
  double lat(y, x) ;
    lat:standard_name = "latitude" ;
    lat:units = "degrees_north" ;
  double lon(y, x) ;
    lon:standard_name = "longitude" ;
    lon:units = "degrees_east" ;
  float temp(y, x) ;
    temp:standard_name = "air_temperature" ;
    temp:units = "K" ;
    temp:coordinates = "lat lon" ;
    temp:grid_mapping = "crsOSGB: x y z GeogCS: lat lon" ;
  float pres(y, x) ;
    temp:standard_name = "air_pressure" ;
    temp:units = "Pa" ;
    temp:coordinates = "lat lon" ;
    temp:grid_mapping = "crsOSGB: x y z GeogCS: lat lon" ;
  int crsOSGB ;
    crs:grid_mapping_name = "transverse_mercator";
    crs:semi_major_axis = 6377563.396 ;
    crs:inverse_flattening = 299.3249646 ;
    crs:latitude_of_projection_origin = 49.0 ;
    crs:longitude_of_projection_origin = -2.0 ;
    crs:false_easting = 400000.0 ;
    crs:false_northing = -100000.0 ;
    crs:scale_factor_at_projection_origin = 0.9996012717 ;
    crs:vert_CS = "Newlyn" ;
    crs:vert_datum = "Ordnance Datum Newlyn, 2005" ;
    crs:unit = "metre" ;
  int crsWGS84 ;
    crs:grid_mapping_name = "latitude_longitude";
    crs:longitude_of_prime_meridian = 0.0 ;
    crs:semi_major_axis = 6378137.0 ;
    crs:inverse_flattening = 298.257223563 ;

#71 fixed Correction of Vertical perspective projection davidhassell martin.raspaud
Description

Hi,

There is a confusion under the notes of the Vertical Perspective text in the conventions between geos and nsper. Snyder is describing near-sided perspective... Here is the corrected text:

Notes:

A general description of vertical perspective projection is given in [Snyder], pages 169-181.

The corresponding projection is proj.4 is nsper. This should not be confused with the proj.4 geos projection.

#72 fixed Adding the geostationary projection. davidhassell martin.raspaud
Description

There seems to be a need from the weather satellite people to add the "geos" projection to the list of projections in the CF metadata.

"geos" stands for geostationary. The projection is described in detail in the "CGMS 03, LRIT/HRIT Global specification" (Eumetsat) document. (see here also: http://remotesensing.org/geotiff/proj_list/geos.html )

A possible text for this projection is:

Geostationary projection


grid_mapping_name = geos

Map parameters:

latitude_of_projection_origin

longitude_of_projection_origin

perspective_point_height

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 packages for computing the

mapping may be found at http://www.remotesensing.org/geotiff/proj_list/geos.html . These notes assume the point of observation is directly over the equator. The projection coordinates in this projection are directly related to the scanning angle of the satellite instrument.

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