# Custom Query (124 matches)

## Results (61 - 63 of 124)

Ticket | Resolution | Summary | Owner | Reporter | |||
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#69 | fixed | Specification of Coordinate Reference System properties in Well-Known Text format | cf-conventions@… | pbentley | |||

Description |
## 1. Title
## 2. Moderator
## 3. Requirement
## 4. Initial Statement of Technical Proposal## 4.1 Background (Informative)
dimensions: x = 800 ; y = 600 ; time = 30 ; variables: double x(x) ; x:standard_name = "projection_x_coordinate" ; x:long_name = "British National Grid eastings" ; x:units = "m" ; double y(y) ; y:standard_name = "projection_y_coordinate" ; y:long_name = "British National Grid northings" ; y:units = "m" ; double time(time) ; ... double lat(y, x) ; ... double lon(y, x) ; ... // a data variable whose CRS definition is provided by the 'bng_crs' grid mapping variable float precip(time, y, x) ; precip:standard_name = "rainfall_amount" ; precip:coordinates = "lat lon" ; precip:grid_mapping = "bng_crs" ; ... // grid mapping variable containing a WKT definition of the British National Grid int bng_crs ; bng_crs:grid_mapping_name = "transverse_mercator" ; bng_crs:crs_wkt = "PROJCS ["OSGB 1936 / British National Grid", GEOGCS ["OSGB 1936", DATUM ["OSGB 1936", SPHEROID ["Airy 1830", 6377563.396, 299.3249646]], PRIMEM ["Greenwich", 0], UNIT ["degree", 0.0174532925199433]], PROJECTION ["Transverse Mercator"], PARAMETER ["False easting", 400000], PARAMETER ["False northing", -100000], PARAMETER ["Longitude of natural origin", -2.0], PARAMETER ["Latitude of natural origin", 49.0], PARAMETER ["Scale factor at natural origin", 0.9996012717], UNIT ["metre", 1.0]]" ; ...
## 4.2 Proposed Changes to the CF Conventions Document (Normative)This proposal would require the following changes to the CF conventions document. ## Addition of a new subsection 5.6.1 after Example 5.10
- use of the TOWGS84 element to specify horizontal datum transformation parameters (to WGS 1984 datum)
- use of the VERT_DATUM element to specify vertical datum information
- use of additional PARAMETER elements (albeit not essential ones in this example) to define the location of the false origin of the projection
- use of AUTHORITY elements to specify object identifier codes assigned by an external authority, OGP/EPSG in this instance
... int crs ; crs:grid_mapping_name = "transverse_mercator" ; crs:crs_wkt = "COMPD_CS ["OSGB 1936 / British National Grid + ODN", PROJCS ["OSGB 1936 / British National Grid", GEOGCS ["OSGB 1936", DATUM ["OSGB 1936", SPHEROID ["Airy 1830", 6377563.396, 299.3249646], TOWGS84[375, -111, 431, 0, 0, 0, 0] ], PRIMEM ["Greenwich", 0], UNIT ["degree", 0.0174532925199433] ], PROJECTION ["Transverse Mercator"], PARAMETER ["False easting", 400000], PARAMETER ["False northing", -100000], PARAMETER ["Longitude of natural origin", -2.0], PARAMETER ["Latitude of natural origin", 49.0], PARAMETER ["Longitude of false origin", -7.556], PARAMETER ["Latitude of false origin", 49.766], PARAMETER ["Scale factor at natural origin", 0.9996012717], UNIT ["metre", 1.0], AUTHORITY ["EPSG", "27700"] ], VERT_CS ["Newlyn", VERT_DATUM ["Ordnance Datum Newlyn", 2005], UNIT ["metre", 1.0]", AXIS ["Gravity-related height", UP], AUTHORITY ["EPSG", "5701"] ]]" ; ...
## Insertion of an additional row into Table F.1
## Additions to Bibliography
## Errata- In Example 5.10, the final attribute should more accurately be
`scale_factor_at_central_meridian`rather than`scale_factor_at_projection_origin`, since the scale factor applies to the entirety of the central meridian employed in the projection, not just the point of origin.
- The entry for
`grid_mapping_name`in Table F.1 needs to be amended. The data type column should read 'S' not 'N'.
## Changes to Conformance Document
## 5. Benefits
## 6. Status Quo
## 7. References![1] OpenGIS Coordinate Transformation Service Implementation Specification. OGC document 01-009 (link) ![2] OpenGIS Implementation Standard for Geographic Information - Simple feature access - Part 1: Common architecture. OGC document 06-103r4 (link) ![3] Well-known Text Wikipedia entry (link) ![4] GeoAPI description of WKT format (link) |
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#70 | fixed | Connecting coordinates to Grid Mapping variables | davidhassell | markh | |||

Description |
## TitleConnecting coordinates to Grid Mapping variables ## ModeratorDavid Hassell ## RequirementTo allow each Coordinate or Auxiliary Coordinate Variable to be explicitly attached to a particular Grid Mapping Variable. ## ProposalTo 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 CaseConsider 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 ; |
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#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:
The corresponding projection is proj.4 is nsper. This should not be confused with the proj.4 geos projection. |

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