Earth orientation data

The motion of the polar axis causes a slight correction to the absolute gravity value, see functions polar_motion_correction() and dg_polar(). Its calculation requires knowledge of the Earth orientation parameters (EOP) at the time of measurement. This data is published at https://www.iers.org/IERS/EN/DataProducts/EarthOrientationData/eop.html.

gravitools needs this data to propery work (polar motion correction). You can either download the data manually and store it at the recommended path or use a post-install script.

For manual download: get the CSV data file (https://datacenter.iers.org/data/csv/finals2000A.all.csv) and save it under <userspace>/.config/gravitools/finals2000A.all.csv (defined by EOP_PATH), so it can be found automatically.

If you use this default location, the function update_eop_data() facilitates updating it. If the file is older than 12 hours (by default), it will download a new copy and replace the old file. This is useful for cronjobs, for instance.

You can now call read_eop_data() to read and parse this data file.

    from gravitools.corrections import read_eop_data
    eop = read_eop_data()
    print(eop.loc["2024":].head())

                                  x         y   type
utc
2024-01-01 00:00:00+00:00  0.136911  0.202214  final
2024-01-02 00:00:00+00:00  0.134904  0.202515  final
2024-01-03 00:00:00+00:00  0.133129  0.203146  final
2024-01-04 00:00:00+00:00  0.131589  0.203979  final
2024-01-05 00:00:00+00:00  0.130043  0.205166  final

You can also supply an alternative path, if the file is not at the default location.

  eop = read_eop_data("path/to/finals2000A.all.csv")

Since this data will likely be used multiple times, it is automatically cached by read_eop_data() as a global variable eop_data of the gravitools.corrections module. You can also write this variable directly, if you wish to load the data from an alternative source.

The function get_eop_data() provides a wrapper for accessing this global variable. If eop_data has been set, it will return the cached data. Otherwise, it will call read_eop_data() first to read it from the default location.

dg_polar() calculates the polar motion correction at a given location coordinate and list of time indices. If you do not explicitly supply EOP data to it, it will acquire this data using get_eop_data().

The published EOP data contains final results for past dates and predictions for future dates, as indicated by the "type" column. Predictions are generally accurate enough for the purpose of absolute gravimetry. Nonetheless, dg_polar() will throw a warning message, if you use them.

In summary, if you supply the EOP data file at the default location, you can simply call dg_polar() and it will automatically acquire the EOP data. If it is located at a different location, call read_eop_data('your/path/finals2000A.all.csv') first. If you use a different source, either provide the eop argument to dg_polar(), or set corrections.eop_data first. If you run an automated processing, consider including a call to update_eop_data().