The use of gridded data in geodetic applications is increasing. For some geodetic datum transformations the Natural Resources Canada NTv2 format is widely used although other national and ad hoc formats are also in use. There is no global standard for these and other geodetic data such as geoid grids, position displacement grids and numerous others. Producers often define a proprietary or some other convenient format. The adoption of a standard file format will facilitate the creation and use of gridded data sets. It would relieve grid producers of the necessity for producing file readers and will assist application developers to incorporate new grids with minimal effort. Users will benefit from quicker access to the data. GGXF is a proposed standard format for the exchange of gridded geodetic data.

GGXF is an Esri research and development project to define a standardized format for the exchange of gridded geodetic data. The project began in 2013 and has progressed in fits and starts since then. The requirements of GGXF are as follows:

• Multi-dimensional
• Multi-resolution
• Self-defining (metadata/header)
• Applicable to any datatype defined on a graticular grid
• Binary data storage structure
• Open-source GGXF reader/writers from commonly used existing formats

We quickly realized the formidible scope of this task. We also questioned the rationale of creating yet another standard format. After a fairly cursory review of existing geodetic data grid formats coupled with the fact that Esri already had capability to work with NetCDF/HDF5, we focused on leveraging the latter widely used, robust and mature format for GGXF. The fact that NetCDF was already an OGC standard bolstered our decision to proceed in this direction.

Having decided to adapt NetCDF/HDF5, we created a set of attribute and variable names relevant to GGXF based on the CF Conventions and Metadata guidelines. Currently, the set of attribute and variable names we use to describe the NetCDF file comprises a combination of NetCDF Attribute Convention for Dataset Discovery attributes to define those attributes of a GGXF file common to all grids in the file, along with our newly developed attribute and variable names to define specific attributes of any grid within the file. These GGXF standard attributes and variables define the metadata/header file needed to ensure the format is self-describing.

Using these parameters we can create a Common Data Form Language File (CDL) which is a human-readable notation for NetCDF objects and data. We tested this by creating a CDL file of a multi-resolution, nested NTv2 grid file and applied the ncgen function from the NetCDF library to convert the CDL file into binary NetCDF format. We successfully recovered the original CDL from the binary NetCDF file using the ncdump function. This process gave us confidence that NetCDF could possibly serve as a platform for GGXF.

A more complete background of the development of GGXF and an example of a GGXF format grid can be found in the files uploaded above. The table below describes the content of each of these files.

• Continue to vet GGXF throughout the community
• Develop a more comprehensive list of metadata/header standard attriute and variable names and their definitions
• Incorporate GGXF into the Esri Projection Engine (PE)
• Develop code samples to read/write GGXF files
• Develop code samples to convert between GGXF and some commonly used existing formats