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.
The attributes which describe the
ellipsoid and prime meridian may be included, when applicable, with
any grid mapping.
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.
grid_mapping_name = albers_conical_equal_area
standard_parallel - There may be 1 or 2 values.
longitude_of_central_meridian
latitude_of_projection_origin
false_easting
false_northing
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 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.
grid_mapping_name = azimuthal_equidistant
longitude_of_projection_origin
latitude_of_projection_origin
false_easting
false_northing
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 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.
grid_mapping_name = lambert_azimuthal_equal_area
longitude_of_projection_origin
latitude_of_projection_origin
false_easting
false_northing
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 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.
grid_mapping_name = lambert_conformal_conic
standard_parallel - There may be 1 or 2 values.
longitude_of_central_meridian
latitude_of_projection_origin
false_easting
false_northing
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 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.
grid_mapping_name = latitude_longitude
This grid mapping defines the canonical 2D geographical coordinate system based upon latitude and longitude coordinates on a spherical Earth. It is included so that the figure of the Earth can be described.
None.
The rectangular coordinates are longitude and latitude identified by the usual conventions (Section 4.1, “Latitude Coordinate” and Section 4.2, “Longitude Coordinate”).
grid_mapping_name = polar_stereographic
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
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 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.
grid_mapping_name = rotated_latitude_longitude
grid_north_pole_latitude
grid_north_pole_longitude
north_pole_grid_longitude - This parameter is option (default is 0).
The rotated latitude and longitude coordinates
are identified by the standard_name attribute values
grid_latitude
and grid_longitude respectively.
grid_mapping_name = stereographic
longitude_of_projection_origin
latitude_of_projection_origin
scale_factor_at_projection_origin
false_easting
false_northing
The x (abscissa) and y (ordinate) rectangular coordinates
are identified by the standard_name
attribute values projection_x_coordinate
and projection_y_coordinate respectively.
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.
grid_mapping_name = transverse_mercator
scale_factor_at_central_meridian
longitude_of_central_meridian
latitude_of_projection_origin
false_easting
false_northing
The x (abscissa) and y (ordinate) rectangular coordinates
are identified by the standard_name
attribute values projection_x_coordinate
and projection_y_coordinate respectively.
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.
grid_mapping_name = vertical_perspective
latitude_of_projection_origin
longitude_of_projection_origin
perspective_point_height
false_easting
false_northing
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 packages
for computing the mapping may be found at
http://www.remotesensing.org/geotiff/proj_list/geos.html
. These notes assume the point of perspective
is directly over the equator. A more general description
of vertical perspective projection is given in
[Snyder], pages 169-181.
In the following table the "Type" values are S for string and N for numeric.
Table F.1. Grid Mapping Attributes
| Attribute | Type | Description |
|---|---|---|
earth_radius | N | Used to specify the radius, in metres, of the spherical figure used to approximate the shape of the Earth. This attribute should be specified for those projected coordinate reference systems in which the X-Y cartesian coordinates have been derived using a spherical Earth approximation. If the cartesian coordinates were derived using an ellipsoid, this attribute should not be defined. Example: "6371007", which is the radius of the GRS 1980 Authalic Sphere. |
false_easting | N |
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 | N |
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 | N | The name used to identify the grid mapping. |
grid_north_pole_latitude | N | True latitude (degrees_north) of the north pole of the rotated grid. |
grid_north_pole_longitude | N | True longitude (degrees_east) of the north pole of the rotated grid. |
inverse_flattening | N | Used to specify the inverse flattening (1/f) of the ellipsoidal figure associated with the geodetic datum and used to approximate the shape of the Earth. The flattening (f) of the ellipsoid is related to the semi-major and semi-minor axes by the formula f = (a-b)/a. In the case of a spherical Earth this attribute should be omitted or set to zero. Example: 298.257222101 for the GRS 1980 ellipsoid. (Note: By convention the dimensions of an ellipsoid are specified using either the semi-major and semi-minor axis lengths, or the semi-major axis length and the inverse flattening. If all three attributes are specified then the supplied values must be consistent with the aforementioned formula.) |
latitude_of_projection_origin | N |
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 | N |
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_prime_meridian | N |
Specifies the longitude, with respect to Greenwich, of the prime
meridian associated with the geodetic datum. The prime meridian defines
the origin from which longitude values are determined. Not to be
confused with the projection origin longitude
(cf. longitude_of_projection_origin, a.k.a. central
meridian) which defines the longitude of the map projection origin.
Domain: -180.0 <= longitude_of_prime_meridian < 180.0 decimal degrees.
Default = 0.0
|
longitude_of_projection_origin | N |
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 | N | Longitude (degrees) of the true north pole in the rotated grid. |
perspective_point_height | N | Records the height, in metres, of the map projection perspective point above the ellipsoid (or sphere). Used by perspective-type map projections, for example the Vertical Perspective Projection, which may be used to simulate the view from a Meteosat satellite. |
scale_factor_at_central_meridian | N |
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 | N |
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
|
semi_major_axis | N |
Specifies the length, in metres, of the semi-major
axis of the ellipsoidal figure associated with the geodetic datum and
used to approximate the shape of the Earth. Commonly denoted using the
symbol a. In the case of a spherical Earth
approximation this attribute defines the radius of the Earth. See
also the inverse_flattening attribute.
|
semi_minor_axis | N | Specifies the length, in metres, of the semi-minor axis of the ellipsoidal figure associated with the geodetic datum and used to approximate the shape of the Earth. Commonly denoted using the symbol b. In the case of a spherical Earth approximation this attribute should be omitted (the preferred option) or else set equal to the value of the semi_major_axis attribute. See also the inverse_flattening attribute. |
standard_parallel | N |
Specifies the line, or lines, of latitude at which the developable map
projection surface (plane, cone, or cylinder) touches the reference
sphere or ellipsoid used to represent the Earth. Since there is zero
scale distortion along a standard parallel it is also referred to as
a "latitude of true scale". In the situation where a conical
developable surface intersects the reference ellipsoid there are two
standard parallels, in which case this attribute can be used as a
vector to record both latitude values, with the additional convention
that the standard parallel nearest the pole (N or S) is provided first.
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 | N |
The longitude to be oriented straight up from the North or South Pole.
Domain: -180.0 <= straight_vertical_longitude_from_pole < 180.0
|