Currently vertical coordinate selection is not supported; all vertical levels are returned.
NetCDF Subset Service for Grid as Point Datasets
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:
point="true" indicates the Grid as Point subset service
otherwise, you are using the Grid subset service
B. Specify variables
var=name of variables, separated by ',' (comma).
The list of valid variables is available from the Dataset Description.
Examples:
var=QCSurface
var=QC,LZT,PQ
var=T&var=Pressure&var=RH (allowed, but comma seperated names are preferred)
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.
C. Specify spatial extent
Latitude, longitude values are specified in decimal degrees north and east, respectively.
1. Specify lat/lon bounding box for Grid Subsetting
Specify all of these parameters (order does not matter):
north: latitude north (decimal degrees)
south: latitude south (decimal degrees)
east: longitude east (decimal degrees)
west: longitude west (decimal degrees)
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
Example:
north=17.3&south=12.088&west=140.2&east=160.0
2. Specify lat/lon point for Grid as Point Subsetting:
latitude: latitude of point, decimal degrees north
longitude: longitude of point, decimal degrees east
The requested point must lie within the dataset spatial range. The grid cell containing the requested point will be used.
The
requested time point must lie within the dataset time range. The time
slice closest to the requested time will be returned.
Examples:
time=2007-03-29T12:00:00Z
time=present
3. Specify time stride (Grid Subsetting only)
You can optionally take only every nth point in time.
Example:
timeStride=2 (take every other point in time)
E. Specify the vertical coordinate
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:
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:
For Grids:
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.
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, only accept=netcdf is supported.
For Grid as Point subsetting, accept=xml or accept=csv or accept=netcdf are supported.
G. Adding 2D Lat/Lon arrays to the file
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).
NetCDF File Writing Rules
A valid NetCDF file is returned in CF-1 format.
The specified variables(s) and their attributes are included.
All coordinate variables for the specified variable(s) are included.
If a bounding box or time range is specified, the variable and its coordinates are subsetted.
All global attributes are included.
This document is maintained by John Caron and was last updated on March 2012
NetCDF Subset Service for Gridded Data (version 0.2)
This document is maintained by 
