H5M Convention
Version 0.1
Revision 15
Author: Gerco de Jager, MARIN MSG

Note before reading any further: This file format is still in development. Not all features and intended usages are supported yet by this standard. New features will be added gradually and lead to new versions of this standard.
Contents
1 Introduction
1.1 Intended usage
1.2 Supported features
2 Glossary
3 H5M Structure
3.1 Root : Group "/"
3.2 Signal Set : Group
3.3 Signal : Dataset
4 H5M Data Descriptions
4.1 Naming conventions
4.2 Attributes
4.3 Data types
4.3.1 Default value
4.3.2 Strings: favour unicode (UTF-8) over ascii
4.3.3 iso_fmt
4.3.4 unit_str
4.3.5 prop_str
4.3.6 obj_ref

Introduction

H5M is an acronym for HDF5 MARIN Datasets File.
The data which it contains can have any available source at MARIN: Basin measurements, simulator runs, CFD calculations, full scale measurements and post-processing cq analysis of this data.
This convention specifies how the content of a HDF5 file is organised for MARIN. For more information about HDF5 itself and for generic HDF5 viewers please visit https://support.hdfgroup.org/.
The glossary of terms is described in the next chapter.The data structure and their properties are specified in chapter 3.Details of the data types used and the followed naming convention are specified in chapter 4.

Intended usage

MARIN Internal use:

• Exchange and access results of the measurements in the basins to data analysts for further processing. This contains the raw data from MMS2 and supersedes the MMS format.
• Exchange the results of simulations and calculations.
• Exchange the results of analysis and other post-processing of raw data between data analysts and between departments.

MARIN External use:

• Exchange processed results of measurements or calculations or simulations or experiments or analysis with clients. (Portal, e-mail, ftp, …)

Supported features

The specified format supports the following features:

1. Identification of the software and system configuration used to write the data set.
2. Identification of the process step(s) that generated the data set. (i.e. experiment number, project number, etc)
3. Identification of the experiment variables (a.k.a. signals) and – if applicable - the sensors used.
4. Grouping of signals with shared origin or properties. This is called a signal set.
5. Link a signal set to another signal set to create a dependency of one to another.
6. Time branching; The available Signal Sets can form a graph from which a Signals can be composed by following a specific path.

Glossary

ExperimentIn the context of this convention, any action that results in one or more signals that need to be stored. E.g. MMS2 Measurement, ReFresco Calculation, XMF Simulation, SHARK data analysis.
H5MMARIN convention of structuring the content of a HDF5 container for Experiment Datasets.
Master Signal Independent value range. E.g. the timestamp values for a time series signal that correspond with each sample value.
Signal Measured, calculated or simulated variable; range of values.
Signal SetSet of signals grouped by one or more common properties. E.g. origin, experiment run, sample rate, sample count, timestamp.
Slave SignalDependent value range. E.g. the sample values at each corresponding timestamp for a time series signal or the RAO value at certain values on several frequency axis.

H5M Structure

The H5M content is structured as depicted in the diagram below.
Each tree node item will be explained in the next paragraphs in this chapter.The location of the item is defined by it's address in the HDF5 file, called path. Also all available attributes are specified per item.

Root : Group "/"

The root group contains:

• attributes that identify the used H5M format.
• zero or more HDF Groups for each set of signals.
• attributes that identify the provenance of the file.

Attributes:

Signal Set : Group

The signal set group contains a set of signals that are logically a single group. Such signals share common properties like number of samples andsample rate. Which properties are common has to be determined by the consumer of the data; either a human or an application.
Name: HDF Group name of the set of signals.
Path: "/<Signal set name>"
Attributes:

Signal : Dataset

The signal is a dataset containing the samples of the measurement or simulation or calculation or postprocessing step. A signal maps to an experiment variable.
Name: HDF Dataset name; must be unique in the set. It is the HDF-safe name of the signal. For reference the original potentially HDF-unsafe name is provided with the data in the 'signalSource' attribute.
Path: "/<set name>/<signal name>"
Additional properties of the signal are added as attributes. Below are the common attributes. In sub sections domain or source specific attributes can be found.
Attibutes:

Note: There is no explicit sample rate property. Sample rate is a specific attribute of equidistant time based signals. If sample rate needs to be visible it can be specified in the attribute 'description' of the signal set group.

H5M Data Descriptions

This chapter specifies some conventions with respect to the data and HDF attributes. Client code – a.k.a. applications – and h5m read/write implementations are allowed to fail if these conventions are not followed.

Naming conventions

Names of HDF5 attributes follow the camelCase naming convention, the first character of every word is capitalised except for the first word. This applies only to the attribute names and not the actual values. HDF5 Datasets and Groups get their name from the content (e.g. signal name, signal set name) and thus follow the convention of their own context.
Items in the hdf5 file are accessible via their path(s) through the tree and the path consists of the names of the tree items. (This implies that names must be unique at each level.)

Attributes

Any HDF tree node item can have attributes. Attributes can have optional values or be optional themselves. The list of attributes per HDF tree node item specifies these cases as described in the sections below.
Attributes can be mandatory or optional. This is specified in the column 'Exists?' as follows:
A: Always; Attribute is mandatory and must have a valid value.
O: Optional; Attribute is only present if a valid value is available.
NS: Not Specified; Attribute is always present and contains the string value 'not specified' if no valid value is available.Note: Although HDF supports a null dataspace for scalars and vectors it is not used because it lacks the ability of specifying why it has no value.
Attributes play a certain role. Currently the following roles are recognized in column 'Role':
i:Informational; Attribute is intended to be human interpretable.
a:Automation; One or more applications depend on the value of this attribute.
s:Structural; The value defines the relations in the data-model. Both human and computer interpretable.

Data types

Each section in this section defines a specific type notation that is used in the chapter 'H5M Structure'.

Default value

The default value for attributes is the UTF-8 string value 'not specified'. It is important to notice that the data type of any attribute changes to UTF-8 string if there is no value specified and this default is set.

Strings: favour unicode (UTF-8) over ascii

All strings (names, datasets and attributes) in this convention are UTF-8 unless specified otherwise. The use of UTF-8 instead of ascii is encouraged by the HDF Group.

iso_fmt

Values of datetime attributes like 'dateTimeOfCreation' are added to the hdf item as a string attribute with formatting in accordance with ISO 8601. These values can optionally include timezone information.The HDF string is fixed length utf-8.
Examples:

• "2017-09-19T08:26:30.5606524" – no offset specified, thus local and could be anywhere.
• "2017-09-19T06:26:30.5606524Z" – Zulu time, UTC offset 0: at Greenwich UK without daylight saving time correction.
• "2017-09-19T08:26:30.5606524+02:00" – including offset with respect to UTC.

Including thus the local date and time of creation and optionally including time zone offset with respect to UTC (as opposed to GMT which includes daylight saving time correction). This offers readability for the users and can be easily converted to a datetime in loader libraries in python or dotNet or otherwise.
To get the UTC time (i.e. UTC Zulu time) one has to substract the offset from the specified date-and-time. For example:

• "2017-09-19T08:26:30+02:00" is equal to "2017-09-19T06:26:30Z".
• "2017-09-19T08:26:30-04:00" is equal to "2017-09-19T12:26:30Z".

UTC differs from GMT in that it has no daylight saving time correction. To get the GMT time one has to add the daylight saving time correction. Using an API that supports iso 8601 format can do this automatically.

unit_str

The unit_str is a variable length HDF string . The content is human and computer readable. Therefore any unit is limited to a predefined list of acceptable units and formatted using a specific unit definition notation.
Examples of unit definition strings: "N.m", "kg.m^2/s^3", "kg.m.s^-2"
A list of available or possible values is available at MARIN and is not part of this convention (and probably never will be).

prop_str

The prop_str is a HDF string value which content is limited to a list of acceptable names for quantities, properties or experiment variables.
A list of predefined property / measurand names is not available yet.

obj_ref

The obj_ref is an HDF Object Reference value. It is used to define relations between individual entries in the file.
Note: Most common HDF viewers will only render this type as "object reference" and not specify to which item it refers. If applicable such an attribute is accompanied by an attribute that specifies the name(s) of the referred object(s).