- An orthogonal lat-lon grid: climate data products often appear in this topology. There is a separate lat vector (e.g. -180:1:180 deg) and a separate lon vector (e.g. -90:1:90 deg) that span a part of the globe. Support for these grids is always available. Example:
http://opendap.deltares.nl/thredds/catalog/opendap/deltares/FEWS-IPCC/catalog.html
The minimal netCDF dump of this grid type looks likedimensions: lon = 3 ; lat = 5 ; variables: single VARIABLE(lon,lat), shape = [3 5] VARIABLE:grid_mapping = "wgs84" \\ lat-lon vectors with mapping info single lon(lon), shape = [3] single lat(lat), shape = [5] int32 wgs84([]), shape = [1]
- An orthogonal x-y grid: national data products often appear in this topology. There is a separate x vector (e.g. 0:100:280000 m)
and a separate y vector (e.g. 624800:-100:300000) that span a part in a local projection. Support to plot these grids in lat-lon space is less often available, as it assumes that the client can deal with the two methods to prescribe lat-lon coordinates. The first, implicit option is assuming that the client can do the conversion from the local projection to the a WGS84 lat-lon projection. The CF convention describes how to add projection meta-data to the netCDF file (grid_mapping attribute). In addition, the data producer can transform the x and y vectors to complete lat and lon grids, prescribing the WGS84 position of each grid cell separately. The CF convention describes how to attach these global coordinates to the dataset (coordinates attribute). OpenEarth recommend to supply both options in netCDF files. This option is in fact the same as considering the orthogonal x-y grid as a curvi-linear lat-lon grid, as discussed next. Example:
http://opendap.deltares.nl/thredds/catalog/opendap/tno/ahn100m/catalog.html with clearly defined local projection (Dutch RD) and with full lat-lon matrices included.
http://opendap.deltares.nl/thredds/catalog/opendap/knmi/NOAA/mom/1990_mom/5/catalog.html without clearly defined local projection (a polar stereographic one) but with full lat-lon matrices included.
The minimal netCDF dump of this grid type looks liledimensions: x = 3 ; y = 5 ; variables: single VARIABLE(x,y), shape = [3 5] VARIABLE:coordinates = "lat lon" \\ here we optionally connect the full lat-lon grids VARIABLE:grid_mapping = "epsg" \\ here we choose the x-y sticks \\ x-y vectors with mapping info single x(x), shape = [3] x:grid_mapping = "epsg" single y(y), shape = [5] y:grid_mapping = "epsg" int32 epsg([]), shape = [1] \\ optional: add full lat-lon matrices as if it were a curvi-linear grid single lon(x,y), shape = [3 5] lon:grid_mapping = "wgs84" single lat(x,y), shape = [3 5] lat:grid_mapping = "wgs84" int32 wgs84([]), shape = [1]
individual satellite images that are orthogonal in pixel space, but fully warped in lat-lon space and (ii) curvi-linear coastal circulation models that are orthogonal in grid index space, but that have been warped to fit local coast lines.
- A curvi-linear lat-lon mesh: Example: remote sensing swath
http://data.nodc.noaa.gov/opendap/ghrsst/L2P/AVHRR19_L/NEODAAS/2010/348/20101214-AVHRR19_L-NEODAAS-L2P-14dec100129_wsst.8bit-v01.nc.bz2
The minimal netCDF dump of this grid type looks likedimensions: j = 3 ; i = 5 ; variables: single VARIABLE(col,row), shape = [3 5] VARIABLE:coordinates = "lat lon" \\ here we connect the full lat-lon grids VARIABLE:grid_mapping = "wgs84" \\ mandatory: add full lat-lon matrices as if it were a curvi-linear grid single lon(col,row), shape = [3 5] lon:grid_mapping = "wgs84" single lat(col,row), shape = [3 5] lat:grid_mapping = "wgs84" int32 wgs84([]), shape = [1]
- A curvi-linear x-y mesh: This kind of data is the regular output type of regional ocean circulation models. The CF convention allows to attach only one set of coordinates, either the lat-lon matrices or the x-y matrices. Example:
The minimal netCDF dump of this grid type looks likedimensions: j = 3 ; i = 5 ; variables: single VARIABLE(col,row), shape = [3 5] VARIABLE:coordinates = "lat lon" \\ we can connect only one set of full coordinate matrices \\ VARIABLE:coordinates2 = "x y" \\ the other we can't connect within the CF standards VARIABLE:grid_mapping = "epsg" \\ define full lat-lon matrices single lon(col,row), shape = [3 5] lon:coordinates = "x y" lon:grid_mapping = "wgs84" single lat(col,row), shape = [3 5] lat:coordinates = "x y" lat:grid_mapping = "wgs84" int32 wgs84([]), shape = [1] \\ define full x-y matrices (extra) single x(col,row), shape = [3 5] x:grid_mapping = "epsg" single y(col,row), shape = [3 5] y:grid_mapping = "epsg" int32 epsg([]), shape = [1]