Appendix F. Grid Mappings
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Appendix F. Grid Mappings

Each recognized grid mapping is described in one of the sections below. Each section contains: the valid name that is used with the grid_mapping_name attribute; a list of the specific attributes that may be used to assign values to the mapping's parameters; the standard names used to identify the coordinate variables that contain the mapping's independent variables; and references to the mapping's definition or other information that may help in using the mapping. Since the attributes used to set a mapping's parameters may be shared among several mappings, their definitions are contained in a table in the final section.

We have used the FGDC "Content Standard for Digital Geospatial Metadata" [FGDC] as a guide in choosing the values for grid_mapping_name and the attribute names for the parameters describing map projections.

Example F.1.  Albers Equal Area

grid_mapping_name = albers_conical_equal_area

Map parameters:

  • standard_parallel - There may be 1 or 2 values.

  • longitude_of_central_meridian

  • latitude_of_projection_origin

  • false_easting

  • false_northing

Map coordinates:

The x (abscissa) and y (ordinate) rectangular coordinates are identified by the standard_name attribute values 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/albers_equal_area_conic.html.


Example F.2. Azimuthal equidistant

grid_mapping_name = azimuthal_equidistant

Map parameters:

  • longitude_of_projection_origin

  • latitude_of_projection_origin

  • false_easting

  • false_northing

Map coordinates:

The x (abscissa) and y (ordinate) rectangular coordinates are identified by the standard_name attribute values 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/azimuthal_equidistant.html.


Example F.3. Lambert azimuthal equal area

grid_mapping_name = lambert_azimuthal_equal_area

Map parameters:

  • longitude_of_projection_origin

  • latitude_of_projection_origin

  • false_easting

  • false_northing

Map coordinates:

The x (abscissa) and y (ordinate) rectangular coordinates are identified by the standard_name attribute values 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/lambert_azimuthal_equal_area.html.


Example F.4. Lambert conformal

grid_mapping_name = lambert_conformal_conic

Map parameters:

  • standard_parallel - There may be 1 or 2 values.

  • longitude_of_central_meridian

  • latitude_of_projection_origin

  • false_easting

  • false_northing

Map coordinates:

The x (abscissa) and y (ordinate) rectangular coordinates are identified by the standard_name attribute values 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/lambert_conic_conformal_2sp.html.


Example F.5. Polar stereographic

grid_mapping_name = polar_stereographic

Map parameters:

  • straight_vertical_longitude_from_pole

  • latitude_of_projection_origin - Either +90. or -90.

  • Either standard_parallel or 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 values 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/polar_stereographic.html.


Example F.6. Rotated pole

grid_mapping_name = rotated_latitude_longitude

Map parameters:

  • grid_north_pole_latitude

  • grid_north_pole_longitude

  • north_pole_grid_longitude - This parameter is option (default is 0).

Map coordinates:

The rotated latitude and longitude coordinates are identified by the standard_name attribute values grid_latitude and grid_longitude respectively.

Notes:


Example F.7. Stereographic

grid_mapping_name = stereographic

Map parameters:

  • longitude_of_projection_origin

  • latitude_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 values projection_x_coordinate and projection_y_coordinate respectively.

Notes:

Formulas for the mapping and its inverse along with notes on using the PROJ.4 software package for doing the calcuations may be found at http://www.remotesensing.org/geotiff/proj_list/stereographic.html. See the section "Polar stereographic" for the special case when the projection origin is one of the poles.


Example F.8. Transverse Mercator

grid_mapping_name = transverse_mercator

Map parameters:

  • scale_factor_at_central_meridian

  • longitude_of_central_meridian

  • latitude_of_projection_origin

  • false_easting

  • false_northing

Map coordinates:

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

Notes:

Formulas for the mapping and its inverse along with notes on using the PROJ.4 software package for doing the calcuations may be found at http://www.remotesensing.org/geotiff/proj_list/transverse_mercator.html.


Table F.1. Grid Mapping Attributes

AttributeDescription
false_easting The value added to all abscissa values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_x_coordinate.
false_northing The value added to all ordinate values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of the coordinate variable identified by the standard name projection_y_coordinate.
grid_mapping_name The name used to identify the grid mapping.
grid_north_pole_latitude True latitude (degrees_north) of the north pole of the rotated grid.
grid_north_pole_longitude True longitude (degrees_east) of the north pole of the rotated grid.
latitude_of_projection_origin The latitude chosen as the origin of rectangular coordinates for a map projection. Domain: -90.0 <= latitude_of_projection_origin <= 90.0
longitude_of_central_meridian The line of longitude at the center of a map projection generally used as the basis for constructing the projection. Domain: -180.0 <= longitude_of_central_meridian < 180.0
longitude_of_projection_origin The longitude chosen as the origin of rectangular coordinates for a map projection. Domain: -180.0 <= longitude_of_projection_origin < 180.0
north_pole_grid_longitude Longitude (degrees) of the true north pole in the rotated grid.
scale_factor_at_central_meridian A multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance along the central meridian. Domain: scale_factor_at_central_meridian > 0.0
scale_factor_at_projection_origin A multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance at the projection origin. Domain: scale_factor_at_projection_origin > 0.0
standard_parallel Line of constant latitude at which the surface of the Earth and plane or developable surface intersect. This attribute may be vector valued if two standard parallels are specified. Domain: -90.0 <= standard_parallel <= 90.0
straight_vertical_longitude_from_pole The longitude to be oriented straight up from the North or South Pole. Domain: -180.0 <= straight_vertical_longitude_from_pole < 180.0


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