Writing your own Java class to identify Coordinate Systems


In order to use a dataset at the scientific datatype layer, the dataset’s coordinate systems must first be identified. This is done by subclassing ucar.nc2.internal.dataset.CoordSystemBuilder which is a super class for implementing Convention-specific parsing of netCDF files. Create a subclass to examine the contents of the dataset and create coordinate system objects that follow this object model:

For more details, see the CDM Object Model.

A CoordSysBuilderIF class must be created for each type of dataset that encodes their coordinate systems differently. This obviously is burdensome, and data providers are encouraged to use existing Conventions for writing their datasets. If those are inadequate, then the next best thing is to define and document a new Convention in collaboration with others with similar needs. If you do so, read Writing NetCDF Files: Best Practices, look at other Convention examples, and get feedback form others before committing to it. Send us a URL to your documentation, and we will add it to the NetCDF Conventions page.

The steps to using your CoordSysBuilderIF class in the Netcdf-Java library:

Write a class that implements ucar.nc2.dataset.CoordSysBuilderIF, such as by subclassing ucar.nc2.dataset.CoordSysBuilder.

  1. Add the class to your classpath.
  2. From your application, call ucar.nc2.dataset.CoordSysBuilder.registerConvention( String conventionName, Class c). This is called “plugging in” your code at runtime.
  3. Open the dataset in enhanced mode, e.g. by calling
NetcdfDatasets.openDataset(locationAsString, enhance, cancelTask);

In this call, an instance of your class will be called to add coordinate system objects to the NetcdfDataset.

The CoordinateSystem objects are then available through the classes in the ucar.nc2.dataset package, for example:

  public List getCoordinateSystems();

  public List getCoordinateAxes();
  public List getCoordinateTransforms();

  public List getAxisType();

  public List getParameters();
  public List getTransformType();

Writing a CoordSysBuilderIF class

These are the steps taken by CoordSystemBuilder to add CoordinateSystems:

  1. Identify which subclass should be used.
  2. Create a new object of that class.
  3. Call augmentDataset( netcdfDataset, cancelTask) to make any changes to the dataset (add attributes, variables, etc).
  4. Call buildCoordinateSystems( netcdfDataset) to add the coordinate system objects.

Your class must implement this interface:

public interface CoordSysBuilderIF {
  public void setConventionUsed( String convName);
  public void augmentDataset( NetcdfDataset ncDataset, CancelTask cancelTask) throws IOException;
  public void buildCoordinateSystems( NetcdfDataset ncDataset);
  public void addUserAdvice( String advice);

You can override the buildCoordinateSystems() method and completely build the coordinate system objects yourself. However, it’s usually easier to take advantage of the code in the CoordSystemBuilder superclass, which translates standard _Coordinate attributes into coordinate system objects. The job of the subclass may then reduce to adding these _Coordinate attributes to the file in the augmentDataset() method. The subclass may also need to create and add new Coordinate variables to the file, and/or to create CoordinateTransforms. Examples of existing CoordSystemBuilder subclasses are in the ucar.nc2.internal.dataset package.

The ucar.nc2.internal.dataset.CoordSystemBuilder class uses the _Coordinate attributes (“underscore Coordinate attributes”, described fully here to create CoordinateSystem objects. An attribute that starts with an underscore is a “system attribute”, which usually implies some special processing or behavior within the NetCDF library (both C and Java).

When you are subclassing ucar.nc2.internal.dataset.CoordSystemBuilder, you can ignore the setConventionUsed and addUserAdvice methods and let the superclass handle them. If not, you can just implement dummy methods.

The ToolsUI application has a CoordSys tab that is designed to help with the process of building coordinate systems. Open up your dataset in that tab, and 3 tables are presented: The data variables, the coordinate systems, and the coordinate axes. The Info button (top right) will show various information from the CoordSystemBuilder class that was used for the dataset.

Identifying which datasets your class should operate on

If your datasets use the global attribute convention, then you only need to pass in the value of that attribute into ucar.nc2.dataset.CoordSystemFactory.registerConvention(String conventionName, String className), and you do not need to implement the isMine() method.

Otherwise, your class must implement a static method isMine() that returns true when it is given a dataset that it knows how to handle.

For example:

public static  boolean isMine(NetcdfFile ncfile) {
  String stringValue =
  assert stringValue != null;
  return stringValue.equalsIgnoreCase("CRAFT/NEXRAD");

look to see if the global attribute sensor_name has the value CRAFT/NEXRAD. It is important that the isMine() method be efficient, ideally using only the dataset metadata (attributes, variable names, etc) rather than having to do any data reading.

Adding Attributes to the Dataset

For the simple case where you only need to add attributes to the file, you might implement the augmentDataset() method as follows.

Example implementation:

class yourSubclass {

  protected void augmentDataset(NetcdfDataset ncDataset) {
    this.conventionName = "ATDRadar";
    Variable time = ncDataset.findVariable("time");
    time.toBuilder().addAttribute(new Attribute("_CoordinateAxisType", "Time"));

You may find it easier to do the same thing using an NcML file, for example:

class yourSubclass {

  protected void augmentDataset(NetcdfDataset ncDataset, CancelTask cancelTask)
      throws IOException {
    NetcdfDataset.Builder<?> ncBuilder = ncDataset.toBuilder();
    this.conventionName = "ATDRadar";
    ucar.nc2.internal.ncml.NcmlReader.wrapNcml(ncBuilder, "file:/MyResource/ATDRadar.ncml",

The NcMLReader.wrapNcML() method wraps a NetcdfDataset in an NcML file, making whatever modifications are specified in the NcML file. You pass in the URL location of the NcML to use, typically a local file as above, but it may also be a remote access over http. Alternatively, you could add the /MyResource directory to your classpath, and call this variation:

ucar.nc2.internal.ncml.NcmlReader.wrapNcmlResource(ncDataset.toBuilder(), "ATDRadar.ncml",

The NcMLReader.wrapNcMLresource() looks for the NcML document by calling Class.getResource(). The example NcML file might look like:

<?xml version='1.0' encoding='UTF-8'?>
<netcdf xmlns="http://www.unidata.ucar.edu/namespaces/netcdf/ncml-2.2">
 <attribute name="Conventions" value="ATDRadar"/>
  <variable name="latitude">
  <attribute name="_CoordinateAxisType" value="Lat" />
  <variable name="longitude">
  <attribute name="_CoordinateAxisType" value="Lon" />
  <variable name="altitude">
  <attribute name="_CoordinateAxisType" value="Height" />
  <attribute name="_CoordinateZisPositive" value="up" />
  <variable name="time">
  <attribute name="_CoordinateAxisType" value="Time" />

The NcML adds the appropriate _CoordinateAxisType attribute to existing CoordinateAxes. Because the data variables all use coordinate variables, implicit CoordinateSystem objects are created and assigned. There is no need for CoordinateTransforms because all the coordinates are reference coordinates (lat, lon, height). Here is complete info on NcML.

If all you need to do is wrap the dataset in NcML, and the dataset already has a Convention attribute in it (before it is wrapped), then you can simply register the NcML directly, without having to write any code. For this, you use:

CoordSystemFactory.registerNcml(conventionNameAsString, ncmlLocationAsString);

Adding Coordinate Axes to the Dataset

When a CoordinateAxis is missing, you must add it. You can do this programmatically or through an NcML file, for example:

  <variable name="latitude" shape="row" type="double">
    <attribute name="long_name" value="latitide coordinate" />
    <attribute name="units" value="degrees_north" />
    <attribute name="_CoordinateAxisType" value="Lat" />
    <values start="90.0" incr="5.0" />

This file creates a new coordinate axis variable, and gives it evenly spaced values. You can also enumerate the values:

  <values>90.0 88.3 72.6 66.9</values>

When the values must be computed, then you need to do this programmatically. It’s convenient to wrap the dataset in NcML, even when you also have to do some programming. For one thing, you can change the NcML file without recompiling.

Example implementation of argumentDataset():

class yourSubclass {

  protected void augmentDataset(NetcdfDataset ncDataset, CancelTask cancelTask)
      throws IOException {
    this.conventionName = "Zebra";
    // The time coord variable is created in the NcML
        CoordSystemFactory.resourcesDir + "Zebra.ncml", cancelTask);

    Dimension timeDim = netcdfFile.findDimension("time");
    Variable base_time = netcdfFile.findVariable("base_time");
    Variable time_offset = netcdfFile.findVariable("time_offset");

    // The time coordinate is created in the NcML file, and we set its values here
    Variable time = netcdfFile.findVariable("time");

    Attribute att = base_time.findAttribute("units");
    String units = (att != null) ? att.getStringValue() : "seconds since 1970-01-01 00:00 UTC";

    // Time coordinate units are set equal to units on the base_time variable
    time.toBuilder().addAttribute(new Attribute("units", units));

    Array data;
    try {
      // Read in the (scalar) base_time
      double baseValue = base_time.readScalarDouble();
      // Read in the time_offset array
      data = time_offset.read();
      IndexIterator iter = data.getIndexIterator();
      while (iter.hasNext()) {
        // Add the baseValue to each value of the time_offset
        iter.setDoubleCurrent(iter.getDoubleNext() + baseValue);
        // For long-running calculations, check to see if the user has cancelled, and return ASAP
        if ((cancelTask != null) && cancelTask.isCancel())
    } catch (java.io.IOException ioe) {
      // Error message if there's an exception
      parseInfo.append("ZebraConvention failed to create time Coord Axis for "
          + netcdfFile.getLocation() + "\n" + ioe + "\n");
    // Set the data values of the time coordinate to the computed values
    Variable.Builder timeb = Variable.builder().setName("time");
    timeb.setCachedData(data, true);

    // When adding new variables to a dataset, you must call close() when all done

Identifying Coordinate Axis Types

Another simple case to handle is when you are using Coordinate variables for all data variables. Coordinate variables are 1D variables with the same name as their dimension, which encode the coordinate values for that dimension. In that case, you only need to identify the CoordinateAxes types, which you do by overriding the getAxisType() method.

Below is an example implementation to override getAxisType(). This will pass in all variables that have been identified as coordinate axes, and your job is to return their AxisType, if they have one:

protected AxisType getAxisType(NetcdfDataset ncDataset, VariableEnhanced v) {
  String unit = v.getUnitsString();
  if (unit == null)
    return null;
  if (unit.equalsIgnoreCase("degrees_east") || unit.equalsIgnoreCase("degrees_E")
      || unit.equalsIgnoreCase("degreesE") || unit.equalsIgnoreCase("degree_east")
      || unit.equalsIgnoreCase("degree_E") || unit.equalsIgnoreCase("degreeE"))
    return AxisType.Lon;

  if (unit.equalsIgnoreCase("degrees_north") || unit.equalsIgnoreCase("degrees_N")
      || unit.equalsIgnoreCase("degreesN") || unit.equalsIgnoreCase("degree_north")
      || unit.equalsIgnoreCase("degree_N") || unit.equalsIgnoreCase("degreeN"))
    return AxisType.Lat;

  if (SimpleUnit.isDateUnit(unit) || SimpleUnit.isTimeUnit(unit))
    return AxisType.Time;

  // look for other z coordinate
  if (SimpleUnit.isCompatible("m", unit))
    return AxisType.Height;
  if (SimpleUnit.isCompatible("mbar", unit))
    return AxisType.Pressure;
  if (unit.equalsIgnoreCase("level") || unit.equalsIgnoreCase("layer")
      || unit.equalsIgnoreCase("sigma_level"))
    return AxisType.GeoZ;

  Variable var = ncDataset.findVariable("fullName");
  String positive = var.findAttributeString("attributeName", "positive");
  if (positive != null) {
    if (SimpleUnit.isCompatible("m", unit))
      return AxisType.Height;
      return AxisType.GeoZ;
  return null;

Creating Coordinate Transformations

A more complex task is to create CoordinateTransforms, which map your coordinates to reference coordinates, such as lat/lon. A CoordinateTransform is typically represented by a CoordinateTransform variable, which may be a dummy variable (ie has no data in it), and whose attributes document the meaning and specify any needed parameters for it. You can create arbitrary transforms by creating ucar.nc2.dataset.CoordinateTransform objects, which your code will have access to when it opens a NetcdfDataset.

However, for your Transform to be used by the netCDF-Java library and standard applications built on top of it, the CoordinateTransform must have a reference to a ucar.unidata.geoloc.Projection or a ucar.unidata.geoloc.vertical.VerticalTransform object which knows how to do the actual mathematical transformation. The netCDF-Java library has a number of these, mostly following the CF-1.0 specification (Appendix F for projections, Appendix D for vertical transforms). You can also write your own implementation and add them at run time.

For this lesson, we will concentrate on what your CoordSystemBuilder needs to do to use an existing standard or user written Projection or VerticalTransform class.

You can create the CoordinateTransform objects yourself, by overriding the makeCoordinateTransforms() and assignCoordinateTransforms() methods in CoordSystemBuilder. Much easier is to use the existing machinery and create a CoordinateTransform variable which represents the parameters of the transform in a way recognized by a CoordTransformFactory class.

One way to do that is by overriding the augmentDataset() method:

For example:

protected void augmentDataset(NetcdfDataset ncDataset, CancelTask cancelTask)
    throws IOException {
  // Read the projection values stored as non-standard global attributes in your dataset
  double lat_origin =
      (double) ncDataset.findVariable("varName").findAttribute("LAT0").getNumericValue();
  double lon_origin =
      (double) ncDataset.findVariable("varName").findAttribute("LON0").getNumericValue();
  double scale =
      (double) ncDataset.findVariable("varName").findAttribute("SCALE").getNumericValue();
  if (Double.isNaN(scale))
    scale = 1.0;

  // A Coordinate Transform Variable is created, and the parameters are renamed according to the CF-1.0
  Variable.Builder v = Variable.builder().setName("ProjectionPS");

  v.addAttribute(new Attribute("grid_mapping_name", "polar_stereographic"));
  v.addAttribute(new Attribute("straight_vertical_longitude_from_pole", lon_origin));
  v.addAttribute(new Attribute("latitude_of_projection_origin", lat_origin));
  v.addAttribute(new Attribute("scale_factor_at_projection_origin", scale));

  // The CoordinateTransformType identifies this variable unambiguously as a CoordinateTransform
  v.addAttribute(new Attribute(ucar.nc2.constants._Coordinate.TransformType,
  // See NOTE below
  v.addAttribute(new Attribute(ucar.nc2.constants._Coordinate.AxisTypes, "GeoX GeoY"));
  // Fake data is added, in case someone accidentally tries to read it
  Array data = Array.factory(DataType.CHAR, new int[] {}, new char[] {' '});
  v.setCachedData(data, true);
  // The Coordinate Transform Variable is added to the dataset

NOTE: The _CoordinateAxisTypes attribute indicates that the transform is to be used for all CoordinateSystems that have a GeoX and GeoY coordinate axis. To be CF compliant, you would have to identify all data variables and add the attribute grid_mapping="ProjectionPS" to each.

This creates a CoordinateTransform variable in your dataset that looks like this:

 char Projection;
   :grid_mapping_name = "polar_stereographic";
   :straight_vertical_longitude_from_pole = "-150.0";
   :latitude_of_projection_origin = "90.0";
   :scale_factor_at_projection_origin = "0.996";
   :_CoordinateTransformType = "Projection";
   :_CoordinateAxisTypes = "GeoX GeoY";

A similar way to do this, which creates the same result, creates ProjectionImpl and ProjectionCT objects, and calls the makeCoordinateTransformVariable() utility method in CoordSystemBuilder to handle the details:

protected void augmentDataset(NetcdfDataset ncDataset, CancelTask cancelTask)
    throws IOException {
  // Read the projection values stored as non-standard global attributes in your dataset
  double lat_origin =
      (double) ncDataset.findVariable("varName").findAttribute("LAT0").getNumericValue();
  double lon_origin =
      (double) ncDataset.findVariable("varName").findAttribute("LON0").getNumericValue();
  double scale =
      (double) ncDataset.findVariable("varName").findAttribute("SCALE").getNumericValue();
  if (Double.isNaN(scale))
    scale = 1.0;

  // A Projection is created out of those parameters
  Projection proj =
      new ucar.unidata.geoloc.projection.Stereographic(lat_origin, lon_origin, scale);
  // A ProjectionCT wraps the Projection
  ProjectionCT projCT = (ProjectionCT) proj;
  // The makeCoordinateTransformVariable method creates the CoordinateTransform
  // The Projection knows what the standard names of its parameters are
  VariableDS v = makeCoordinateTransformVariable(ncDataset, projCT);
  // The _CoordinateAxisTypes attribute indicates that the transform is used for all Coordinate Systems that have a GeoX
  // and GeoY coordinate axis
  // The CoordinateTransform variable is added to the dataset
  NetcdfFormatWriter.Builder builder =
  builder.addVariable("time", v.getDataType(), v.getDimensionsString());

CoordSystemBuilder Reference

These are the steps taken by NetcdfDataset to add CoordinateSystems:

  1. Identify which subclass should be used
  2. Create a new object of that class
  3. Call augmentDataset( ncDataset, cancelTask)
  4. Call buildCoordinateSystems( ncDataset)

The buildCoordinateSystems() method is where CoordSystemBuilder constructs the CoordinateSystems and adds them to the dataset. In some special cases, the subclass may need to override some methods that are called by buildCoordinateSystems(). See ucar.nc2.internal.dataset.CoordSystemBuilder.buildCoordinateSystems() method implementation below.

// put status info into parseInfo that can be shown to someone trying to debug this process
parseInfo.format("Parsing with Convention = %s%n", conventionName);

// Bookkeeping info for each variable is kept in the VarProcess inner class

// identify which variables are coordinate axes
// identify which variables are used to describe coordinate systems
// identify which variables are used to describe coordinate transforms

// turn Variables into CoordinateAxis objects

// make Coordinate Systems for all Coordinate Systems Variables

// assign explicit CoordinateSystem objects to variables

// assign implicit CoordinateSystem objects to variables

// optionally assign implicit CoordinateSystem objects to variables that dont have one yet
if (useMaximalCoordSys) {

// make Coordinate Transforms

// assign Coordinate Transforms

To work at this level, you will need to study the source code of CoordSystemBuilder, and existing subclasses in the ucar.nc2.dataset.conv package. As a subclass, you will have access to the list of VarProcess objects, which wrap each variable in the dataset, and keep track of various information about them.