Skip to content

EMC Model Files Standards and Conventions

Introduction

The EMC Standards and Conventions define the supported file formats, metadata requirements, and coordinate systems to ensure a consistent approach to creating, distributing, and using EMC model files. Adhering to these conventions improves the consistency, interoperability, and reliability of contributed models, enabling more effective collaboration and data sharing across the community.

This guide highlights the critical role of metadata, specifying the required attributes, their formats, and guidelines for coordinate system usage. Following these practices helps minimize distortion, improve accuracy, and maintain the overall integrity and usability of EMC data and tools.

Contributed Earth models are available for download from the EarthScope Data Repository. For interactive filtering and selection of models, visit the EMC Model Filtering & Access API.

File Format Standards

This section describes the required formats for EMC model files and the benefits of adopting the current standards.

Required Format

  • File format: All EMC models must use NetCDF-4 Classic as the required container format. Note: earlier versions of the EMC standard specified NetCDF-3 Classic).
  • Compression: Apply compression for large models to reduce file size and improve data transfer efficiency.
  • Metadata Compliance: Ensure all models are CF-compliant and fully compatible with EMC-Tools.

Format Conversion

  • Use the EMC-Tools utilities to convert Earth model files between GeoCSV and NetCDF-4 Classic.
  • The transition from NetCDF-3 Classic to NetCDF-4 Classic provides key benefits:
    • Enable internal compression and chunking to reduce storage and transfer costs.
    • Maintain backward compatibility with the NetCDF-3 data model to ensure broad software support.
    • Adopt a modern container format that improves long-term sustainability and usability of EMC model files.

Metadata Requirements

All EMC NetCDF files must include metadata following the CF Metadata Conventions. In addition, EMC introduces the following required attributes:

source (Variable-Level Attribute)

Each model data variable must include a source attribute describing the origin of the data:

  • If derived from observations: 
vp:source = data-derived
  • If derived from an empirical formula:
    Include the formula or reference. Example: 
vs:source = assumed to follow vp as vs = vp / sqrt(3)

data_layout (Global Attribute)

Specifies the layout of data in the file:

  • vertex — values specified at vertices
  • cell — values specified at the centers of grid cells

If data_layout is not defined, the default vertex is assumed.

Coordinate Systems (grid_ref)

The grid_ref attribute specifies the coordinate system used in the model file:

  • The default coordinate system is geographic: 
grid_ref = latitude_longitude
  • Models using a projected coordinate system (e.g., UTM) must include variables for both the projected coordinates and the geographic latitude/longitude coordinates.
  • The value of grid_ref should indicate the projection used, and metadata must clearly document the projection, including EPSG codes where applicable.

Additional Resources

The following resources provide additional standards, guidelines, and workflows for contributing and maintaining EMC models:

Comments or Questions?
For any questions or feedback about EMC Earth models or EMC-Tools,
please email: es-help@earthscope.org