Ticket #80: issue80.txt

File issue80.txt, 5.2 KB (added by etourigny, 7 years ago)

final draft

Line 
1Title
2
3Add missing CF parameters to translate Coordinate Reference System properties to/from OGC Well-Known Text format.
4
5Changes to section 5.6
6
7<at the end of 1st paragraph>
8
9The translation of CF coordinate variables to/from OGC Well-Known Text (WKT) format is summarized in a few examples below and described in detail in <link1>.
10
11The following attributes should be included when there is a CRS (Coordinate Reference System) definition: longitude_of_prime_meridian (if different than 0), semi_major_axis and inverse_flattening or semi-minor-axis.
12
13
14Example 5.9. Latitude and longitude on the WGS 1984 datum
15
16<CF definition>
17
18OGC WKT :
19
20GEOGCS["WGS 84",
21        DATUM["WGS_1984",
22        SPHEROID["WGS 84",6378137,298.257223563]],
23    PRIMEM["Greenwich",0],
24    UNIT["degree",0.0174532925199433]]
25Example 5.10. British National Grid
26<...>
27        char crs ;
28                crs:grid_mapping_name = "transverse_mercator" ;
29                crs:longitude_of_central_meridian = -2. ;
30                crs:false_easting = 400000. ;
31                crs:false_northing = -100000. ;
32                crs:latitude_of_projection_origin = 49. ;
33                crs:scale_factor_at_central_meridian = 0.9996012717 ;
34                crs:longitude_of_prime_meridian = 0. ;
35                crs:semi_major_axis = 6377563.396 ;
36                crs:inverse_flattening = 299.324964600004 ;
37                crs:projected_coordinate_system_name = "OSGB 1936 / British National Grid" ;
38                crs:geographic_coordinate_system_name = "OSGB 1936" ;
39                crs:horizontal_datum_name = "OSGB_1936" ;
40                crs:reference_ellipsoid_name = "Airy 1830" ;
41                crs:prime_meridian_name = "Greenwich" ;
42                crs:towgs84 = 375., -111., 431., 0., 0., 0., 0. ;
43
44OGC WKT :
45
46PROJCS["OSGB 1936 / British National Grid",
47    GEOGCS["OSGB 1936",
48        DATUM["OSGB_1936",
49            SPHEROID["Airy 1830",6377563.396,299.3249646000044],
50            TOWGS84[375,-111,431,0,0,0,0]],
51        PRIMEM["Greenwich",0],
52        UNIT["degree",0.0174532925199433]],
53    PROJECTION["Transverse_Mercator"],
54    PARAMETER["latitude_of_origin",49],
55    PARAMETER["central_meridian",-2],
56    PARAMETER["scale_factor",0.9996012717],
57    PARAMETER["false_easting",400000],
58    PARAMETER["false_northing",-100000],
59    UNIT["metre",1]]
60
61
62Changes to Table F1
63
64Attribute       Type    Description
65
66horizontal_datum_name   S       The name of the geodetic (horizontal) datum, which corresponds to the procedure used to measure positions on the surface of the Earth. Valid datum names and their associated parameters are given in <link1> (horiz_datum.csv, OGC_DATUM_NAME column) and are obtained by transforming the EPSG name using the following rules (used by OGR and Cadcorp): convert all non alphanumeric characters (including +) to underscores, then strip any leading, trailing or repeating underscores. This is to ensure that named datums can be correctly identified for precise datum transformations (see <link2> for more details). Corresponds to a OGC WKT DATUM node name.
67
68towgs84 N       This indicates a list of up to 7 Bursa Wolf transformation parameters., which can be used to approximate a transformation from the horizontal datum to the WGS84 datum. More precise datum transformations can be done with datum shift grids. Represented as a double-precision array, with 3, 6 or 7 values (if there are less than 7 values the remaining are considered to be zero). Corresponds to a OGC WKT TOWGS84 node.
69
70prime_meridian_name     S       The name of the prime meridian associated with the geodetic datum. Valid names are given in <link1> (prime_meridian.csv). Corresponds to a OGC WKT PRIMEM node name.
71
72reference_ellipsoid_name        S       The name of the reference ellipsoid. Valid names are given in <link1> (ellipsoid.csv). Corresponds to a OGC WKT SPHEROID node name.
73
74geographic_crs_name     S       The name of the geographic coordinate reference system. Corresponds to a OGC WKT GEOGCS node name.
75
76projected_crs_name      S       The name of the projected coordinate reference system. Corresponds to a OGC WKT PROJCS node name.
77
78latitude_of_projection_origin   N        The latitude (degrees_north) chosen as the origin of rectangular coordinates for a map projection. Domain: -90.0 <= latitude_of_projection_origin <= 90.0
79
80longitude_of_central_meridian   N        The line of longitude (degrees_east) 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
81
82longitude_of_prime_meridian     N        Specifies the longitude (degrees_east) [...]
83
84longitude_of_projection_origin  N        The longitude (degrees_east) chosen as the origin of rectangular coordinates for a map projection. Domain: -180.0 <= longitude_of_projection_origin < 180.0
85
86straight_vertical_longitude_from_pole   N        The longitude (degrees_east) to be oriented straight up from the North or South Pole. Domain: -180.0 <= straight_vertical_longitude_from_pole < 180.0
87
88Note: the various *_name attributes are optional but recommended when known as they allow for a better description and interoperability with WKT definitions.  Also: reference_ellipsoid_name, prime_meridian_name, horizontal_datum_name and geographic_crs_name must be all defined if any one is defined, and if projected_crs_name is defined then geographic_crs_name must be also.
89
90Changes to References
91
92<link1> https://cf-pcmdi.llnl.gov/trac/wiki/Cf2CrsWkt
93<link2> http://home.gdal.org/projects/opengis/wktproblems.html
94<CT> OpenGIS Coordinate Transformation Service Implementation Specification 1.0 http://www.opengeospatial.org/standards/ct
95