I.  Abstract

The netCDF Moving Features encoding extension is a summary of conventions that supports efficient exchange of simple moving features as binary files. This Best Practice is a complement to the Moving Features Encoding Part I: XML Core and an alternative to the Simple Comma Separated Values (CSV) extension. Compared to the CSV encoding, this netCDF encoding offers more compact storage and better performance at the cost of additional restrictions on the kinds of features that can be stored.

III.  Preface

This document does not define any new Moving Features elements, rather it provides a method to encode discrete sampling geometries as netCDF files. This Best Practice combines two existing OGC specifications:

• The Network Common Data Form (netCDF) standard [OGC 10-092r3], which provides the binary encoding of array-oriented scientific data; and

• A subset of the Climate and Forecast (CF) conventions [OGC 11-165r2], which provides standard names and layout for the arrays to encode in a netCDF file.

This encoding is intended to provide a faster and more compact alternative to the Moving Features CSV encoding using a format readable by existing software.

1.  Scope

This OGC^® Best Practice gives guidelines for encoding in netCDF files the geometries of features that move as rigid bodies. This is an alternative to the encoding defined by the OGC Moving Features Encoding Extension: Simple Comma Separated Values (CSV) standard. netCDF encoding can store moving features with the following characteristics:

• Each moving feature can be described with the Schema for Moving Features (ISO 19141:2008);

• The number of features simultaneously encoded with this format can be massive (many thousands of features);

• All features can be described using common space-time coordinates;

• Trajectories are described by a single point (not a line string) at each time step;

• Foliation order is sequential, i.e., trajectory elements that are parts of one track are ordered by time.

Deforming features such as floodwater and geometric complexes are out of scope.

This OGC document describes relevant attributes from the Climate and Forecast (CF) conventions in sufficient detail that client applications do not need to deal with the entire scope of CF conventions. Applications only need to understand a restricted subset of CF conventions in order to be able to interpret moving features in netCDF files.

2.  Conformance

Conformance with this specification shall be checked using all the relevant tests specified in Annex A (normative).

3.  Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

Akinori Asahara, Ryosuke Shibasaki, Nobuhiro Ishimaru, David Burggraf: OGC 14-083r2, OGC® Moving Features Encoding Part I: XML Core. Open Geospatial Consortium (2015). https://docs.ogc.org/is/14-083r2/14-083r2.html.

Akinori Asahara, Ryosuke Shibasaki, Nobuhiro Ishimaru, David Burggraf: OGC 14-084r2, OGC® Moving Features Encoding Extension: Simple Comma Separated Values (CSV). Open Geospatial Consortium (2015). https://docs.ogc.org/is/14-084r2/14-084r2.html.

Ben Domenico: OGC 10-090r3, OGC Network Common Data Form (NetCDF) Core Encoding Standard version 1.0. Open Geospatial Consortium (2011). https://portal.ogc.org/files/?artifact id=43732.

Ben Domenico: OGC 10-092r3, NetCDF Binary Encoding Extension Standard: NetCDF Classic and 64-bit Offset Format. Open Geospatial Consortium (2011). https://portal.ogc.org/files/?artifact id=43734.

Ben Domenico and Stefano Nativi: OGC 11-165r2, CF-netCDF3 Data Model Extension standard. Open Geospatial Consortium (2013). https://portal.ogc.org/files/?artifact id=51908.

ISO: ISO 19101-1:2014, Geographic information — Reference model — Part 1: Fundamentals. International Organization for Standardization, Geneva (2014). https://www.iso.org/standard/59164.html.

ISO: ISO 19141:2008, Geographic information — Schema for moving features. International Organization for Standardization, Geneva (2008). https://www.iso.org/standard/41445.html.

ISO: ISO 19107:2003, Geographic information — Spatial schema. International Organization for Standardization, Geneva (2003). https://www.iso.org/standard/26012.html.

ISO: ISO 19123:2005, Geographic information — Schema for coverage geometry and functions. International Organization for Standardization, Geneva (2005). https://www.iso.org/standard/40121.html.

Lawrence Livermore National Laboratory: NetCDF CF Metadata Conventions. http://cfconventions.org/

ESIP: Attribute Convention for Data Discovery (ACDD). http://wiki.esipfed.org/index.php/Attribute_Convention_for_Data_Discovery

NOAA: NCEI netCDF Templates. http://www.nodc.noaa.gov/data/formats/netcdf/

4.  Terms and definitions

This document uses the terms defined in OGC Policy Directive 49, which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this document and OGC documents do not use the equivalent phrases in the ISO/IEC Directives, Part 2.

This document also uses terms defined in the OGC Standard for Modular specifications (OGC 08-131r3), also known as the ‘ModSpec’. The definitions of terms such as standard, specification, requirement, and conformance test are provided in the ModSpec.

For the purposes of this document, the following additional terms and definitions apply.

This document uses the terms defined in Sub-clause 5.3 of OGC 06-121r8, which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard.

4.1. Moving feature

Representation, using a local origin and local ordinate vectors, of a geometric object at a given reference time.

[SOURCE: ISO 19141:2008]

4.2. Vector

Quantity having direction as well as magnitude.

[SOURCE: ISO 19123:2005]

4.3. Geometric object

Spatial object representing a geometric set.

[SOURCE: ISO 19107:2003]

4.4. Geometric primitive

Geometric object representing a single, connected, homogeneous element of space.

[SOURCE: ISO 19107:2003]

4.5. Period

1-dimensional geometric primitive representing extent in time.

[SOURCE: ISO 19141:2008]

4.6. Instant

0-dimensional geometric primitive representing a position in time.

[SOURCE: ISO 19141:2008]

4.7. Point

0-dimensional geometric primitive representing a position.

[SOURCE: ISO 19107:2003]

4.8. Trajectory

Path of a moving point described by a one parameter set of points.

[SOURCE: ISO 19141:2008]

4.9. Foliation

One parameter set of geometries such that each point in the prism of the set is in one and only one trajectory and in one and only one leaf.

[SOURCE: ISO 19141:2008]

4.10. One parameter set of geometries

Function $f$ from an interval $t\in \left[a,b\right.$] such that $f\left(t\right)$ is a geometry and for each point $P\in f\left(a\right)$ there is a one parameter set of points (called the trajectory of $P$) $P\left(t\right):\left[a,b\right]\to P\left(t\right)$ such that $P\left(t\right)\in f\left(t\right)$.

[SOURCE: ISO 19141:2008]

4.11. Prism (one parameter set of geometries)

Set of points in the union of the geometries (or the union of the trajectories) of a one parameter set of geometries.

[SOURCE: ISO 19141:2008]

4.12. Leaf (one parameter set of geometries)

Geometry at a particular value of the parameter.

[SOURCE: ISO 19141:2008]

4.13. Dataset

An identifiable collection of data.

4.14. Attribute

NetCDF attributes hold metadata. An attribute contains information about properties of a variable or an entire dataset.

4.15. Global attribute

Global attributes apply to a whole dataset and may be used to record properties of all the data in a file, such as processing history or conventions used.

[SOURCE: OGC 10-090r3]

4.16. Variable attribute

Variable attributes record the properties of one variable.

[SOURCE: OGC 10-090r3]

4.17. Variable

A netCDF variable has a name, type, shape, attributes, and values. In face, much of the netCDF specification consists of defining the specific characteristics of variables. Variables hold data values. In the netCDF model, a variable can hold a multidimensional array of values of the same type.

[SOURCE: OGC 10-090r3]

4.18. Shape

The shape of a variable is specified with a list of zero or more dimensions.

[SOURCE: OGC 10-090r3]

4.19. Dimension

NetCDF dimensions are used to specify variable shapes, common grids, and coordinate systems.

[SOURCE: OGC 10-090r3]

5.  Conventions

This section provides details and examples for conventions used in the document.

6.  Data model

The data model used by the binary encoding is equivalent to that used by the CSV encoding. Figure 1 shows an example for trajectories of three moving points $A$, $B$ and $C$. Each trajectory has the same start time and the same end time.

• At $t=0$ (start of all data), all points start moving.

• At $t=1$, the movement of $A$ is changed.

• At $t=2$ (end of all data), the movement of $A$, $B$ and $C$ are changed.

In the case, the trajectory of $A$ from $t=0$ to $t=1$ and from $t=1$ to $t=2$ is encoded, together with the trajectories of $B$ and $C$ from $t=0$ to $t=2$.

7.  Data structure and file format

The binary encoding of arrays and attributes is defined by OGC 10-092r3 – NetCDF Binary Encoding Extension Standard: netCDF Classic and 64-bit Offset Format. That netCDF standard defines only a “container” format – it does not define the meaning of attributes stored in the file (the “schema” in XML terminology).

The full content of OGC 10-092r3 standard is needed for Moving Feature Simple Binary; this Best Practice does not propose any addition or subset of OGC 10-092r3.

Requirement 1
http://www.opengis.net/spec/mf_binary/1.0/req/netcdf_valid
All Moving Features binary files shall be compliant with the netCDF binary encoding as specified by the OGC 10-092r3 standard.

Meanings of attributes in a netCDF file are defined by a separated standard, OGC 11-165r2 – CF-netCDF3 Data Model Extension. All remaining requirements and recommendations in this Moving Feature Simple Binary document apply to a subset of those CF conventions.

import java.util.Formatter;
import java.io.IOException;
import ucar.nc2.constants.FeatureType;
import ucar.nc2.ft.FeatureCollection;
import ucar.nc2.ft.FeatureDataset;
import ucar.nc2.ft.FeatureDatasetFactoryManager;
import ucar.nc2.ft.FeatureDatasetPoint;
import ucar.nc2.ft.PointFeature;
import ucar.nc2.ft.PointFeatureIterator;
import ucar.nc2.ft.TrajectoryFeature;
import ucar.nc2.ft.TrajectoryFeatureCollection;
import ucar.unidata.geoloc.EarthLocation;

public class TestMovingFeatureNetCDF {
    public static void main(String[] args) throws IOException {
        try (FeatureDatasetPoint fp = (FeatureDatasetPoint) FeatureDatasetFactoryManager
            .open(FeatureType.TRAJECTORY, “myFile.nc”, null, new Formatter(System.out)))
        {
            for (FeatureCollection fc : fp.getPointFeatureCollectionList()) {
                TrajectoryFeatureCollection tfc = (TrajectoryFeatureCollection) fc;
                while (tfc.hasNext()) {
                    TrajectoryFeature tf = tfc.next();
                    PointFeatureIterator it = tf.getPointFeatureIterator(-1);
                    while (it.hasNext()) {
                        PointFeature f = it.next();
                        EarthLocation loc = f. getLocation();
                        double time = f. getObservationTime();
                        Object value = f.getDataAll().getScalarObject(“myAttribute”);
                        System.out.println(loc + " at " + time + " = " + value);
                    }
                    it.finish();
                }
            }
        }
    }
}