OPULS: UGrid Subsetting

Overview

We have written a server function that uses Bill Howe's Gridfields library to subset unstructured grids (aka irregular meshes) that conform to the UGrid 0.9 specification that is part of the CF-1.6 specification. Our function has a little latitude in its requirements because it also supports features found in earlier versions of the UGrid specification (e.g., the 'mesh array' can be either the Nx3 or 3xN). However, our function only supports triangular (2D) meshes, not the other mesh schemes outlined in the specification. The function will subset one or more range variables based on the values of the domain variables as expressed in a simple relational expression. The result of the function is an unstructured grid - that is, the subsetting operation will return a a set of variables that could be written to a netcdf file and the result would conform to the UGrid 0.9 specification. Because the specification has evolved over time and older data are still perfectly valid, we often 'tweak' those older datasets using NcML so that the contain the variables and attributes required by the newer versions of the UGrid specification. Our function supports both zero- and ones-based indexing for both the range and domain variables.

Background information on UGrid 0.9

The UGrid specification moved to GitHub in 2013. The 0.9 version of the specification was last edited on 29 Oct 2013. Our function conforms to that specification and to an older version (0.6?), where the major difference is the organization of the mesh array data. For a triangular array, the 0.9 version specifies the array be a Nx3 while the older specification specified a 3xN. Our code supports both.

The ugr5 Function

Name

ugr5 - Restrict the domain of an unstructured 2D Mesh Topology grid

Syntax

ugr5(0, rangeVariable:string, [rangeVariable:string, ... ] condition:string)

• The first parameter indicates that the supplied relational constraint condition is to be applied to coordinate values at the nodes (0) or the faces (2) of the mesh.
• The second, third, ..., parameter is a list of (range) data variables that you wish to subset, they may be associated with nodes or faces of the mesh.
• The last parameter is an expression that defines conditions that the domain variables must meet in order to be included in the result.

Here is an example:

ugr5(0, depth, "28.0<lat & lat<29.0 & -89.0<lon & lon<-88.0")

Used in a url (that returns the DDS for the subset data):

http://54.172.39.65:8080/opendap/hyrax/ugrids/Ike/2D_varied_manning_windstress/test_dir-norename.ncml.dds?ugr5(0,depth,"28.0<lat & lat<29.0 & -89.0<lon & lon<-88.0")

Subsetting the range variables

In the example dataset NECOFS_GOM3_FORECAST.nc (DDS DAS) we can see that there are range variables associated with the nodes of the mesh that have a time dimension and a siglay dimension. Since the time dimension has 145 values, and siglay dimension has 40 values it may be that you wish to only retrieve a single time slice and/or siglay slice. This can accomplished by using an array constraint on the variable as it is passed to the ugrid function. For example, to retrieve the 12th siglay slice of the 5th time slice of the variable salinity you would constrain it like salinity[4][11][*] (note that the indices begin at zero). In the request URL the function call is

ugr5(0,salinity[4][11][*],"38.0<lat & lat<42.0 & -64.0<lon & lon<-60.0")

It is also possible to request, say, every 5th time slice salinity[0:5:*][*][*] or every 3rd siglay slice from 3 to 30 salinity[4][3:3:30][*]. In a request URL this would look like

ugr5(0,salinity[4][3:3:30][*],"38.0<lat & lat<42.0 & -64.0<lon & lon<-60.0")

Sample Data

A server with unstructured grid data and subsetting function on board is here: http://54.172.39.65:8080/opendap/

In the directory ugrids You can find a number of test datasets:

fvcom_1step.nc, hex.nc

These are test data from Bill Howe. The hex file is small but it lacks the special variable the UGrid uses to encode metadata the function depends on to read the unstructured data.

test4.nc, test4-rename-nodedata.ncml, test4-ugrid.ncml

test4 also lacks the special 'metadata variable' but we have written NcML (test4-ugrid) that adds it to the dataset, so you can use ugr5() with a URL that points at the virtual dataset that NcML file defines. The companion NcML file (test4-rename-nodedata.ncml) uses NcML to rename the node data variable and while this works fine for normal DAP accesses, the ugr5() won't work with this - the purpose of this example is to document a known bug where using NcML to rename a range variable breaks the function.

NECOFS_GOM3_FORECAST.nc, NECOFS_WAVE_FORECAST.nc

These files us a 3xN face node connectivity array (corresponding to the older version of the UGrid specification)

Ike/2D_varied_manning_windstress and Ike/2D_varied_manning_windstress_wave

These directories have data from c. 2008 that requires (minimal) NcML to work with our function

RENCI

Data used by the ODSIP EarthCube project. This directory has the same data (or nearly so) where the face node connectivity array is organized a Nx3 and 3xN.

lana

This is a bit of a mystery; it may have been lumped into the ugrid sample data by mistake.

README

A brief recap of this information