See NetCDF Subset Service for an overview.
Gridded Datasets may be subset by Lat/Lon bounding box, Time, and Variable, and a netCDF file using CF-1.0 Conventions is returned. Example HTML form:
Currently vertical coordinate selection is not supported; all vertical levels are returned.
Gridded Datasets may be thought of as a collection of point data, with location at the center of each grid cell. The Grid as Point service requires selection of a single cell using lat/lon coordinates, then subsetting by Time, Variable, and vertical level is possible. Either an XML, CSV (Comma Seperated Values in ASCII) document or a NetCDF files in Unidata Point Observation format is returned. Example HTML form:
The list of valid variables is available from the Dataset Description.
Variable names with spaces or other illegal characters must be escaped. For Grid as Point, its best not to select multiple variables with different vertical coordinates, see below.
Latitude, longitude values are specified in decimal degrees north and east, respectively.
Specify all of these parameters (order does not matter):
The bounding box has west as its west edge, includes all points going east until the east edge. Units must be degrees east, may be positive or negative, and will be taken modulo 360. Therefore, when crossing the dateline, the west edge may be greater than the east edge. If the grid is on a projection, the 4 corners of the lat/lon bounding box are converted into projection coordinates, then the smallest rectangle including those 4 points is used
The requested point must lie within the dataset spatial range. The grid cell containing the requested point will be used.
3. Specify horizontal stride (Grid Subsetting only)
You can optionally take only every nth point in both the x and y dimensions.
Use one of the following methods for both Grid and Grid as Point Subsetting:
Specify 2 of these 3 parameters (order does not matter):
The intersection of the requested time range with the dataset time range will be returned.
The requested time point must lie within the dataset time range. The time slice closest to the requested time will be returned.
3. Specify time stride (Grid Subsetting only)
You can optionally take only every nth point in time.
You may specify the vertical coordinate. The vertical slice closest to the requested coordinate will be returned. The possible vertical coordinates are shown in the Dataset Descriptor document, and on the HTML form. The following indicates that variables Geopotential_height, Geopotential_height, Temperature, u_wind and v_wind all have a vertical coordinate named isobaric, with possible values 1000.0 850.0 700.0 500.0 400.0 300.0 250.0 200.0 150.0 100.0 hPa:
with Vertical Levels (isobaric) : 1000.0 850.0 700.0 500.0 400.0 300.0 250.0 200.0 150.0 100.0 hPa
If you specify multiple variables with different vertical coordinates, ambiguous results are likely, and you are advised not to use the service in this way. If you do, the following rules are used:
- Variables are examined in order they appear in the query. The first variable with a vertical coordinate with size > 1 is used.
- The service loops through the levels of that vertical coordinate, and finds the level with the closest value to the specified level.
- All 3D variables with a vertical coordinate with size > 1 are sliced with that index.
For Grids as Point:
- Variables are examined in order they appear in the query, and the vertical coordinate for the first variable that has one is used.
- The service loops through the levels of that vertical coordinate, and for each variable looks for the closest value (ignoring units!!)
- If the value is outside the range for that variable, a missing value (NaN) is used.
For example, when choosing these two variables:
with Vertical Levels (isobaric1) : 850.0 700.0 500.0 250.0 hPa
with Vertical Levels (isobaric2) : 1000.0 850.0 700.0 500.0 400.0 300.0 hPa
The vertical coordinate from the first variable is used, giving this result:
Noice that for the second variable, the 1000 and 400 levels are not returned, and the 300 level is misrepresented as being at 250. Restricting the level to a common coordinate value is sensible though:
For Grid Subsetting: If the grid is a lat/lon grid, the lat and lon coordinates will be automatically included (as 1D coordinate variables). When the grid is on a projection, the lat/lon information will not be included unless the query parameter addLatLon is present. In that case, the lat, lon coordinates will be calculated and included into the file (as 2D variables).