Download, clean and reformat weather data from USA National
Climatic Data Center (NCDC) Global Surface Summary of the Day (GSOD) weather
stations,
<https://data.noaa.gov/dataset/global-surface-summary-of-the-day-gsod>.
The function, get_GSOD(), retrieves data from the GSOD ftp site and
reformats it from United States Customary System (USCS) units to metric
units, also for convenience elevation is converted from decimetres to
metres. Stations are individually checked for number of missing days, as
defined by the user, to assure data quality. Stations with too many missing
observations are omitted from final file. Also omitted are stations with a
latitude of < -90 or > 90 or longitude of < - 180 or > 180. Output is
returned as a comma-separated values (CSV) file written to disk in a
location selected by the user, which summarises each year by station and
includes new variables: actual and saturation vapor pressure and relative
humidity calculated from the original GSOD data. The resulting files can be
as large as 500mb depending on the user's stringency for missing data and
geographic area of interest. Be sure to have sufficient RAM and disk space
as well as a reasonably fast internet connection to use this package to
perform this operation. However, for much smaller and more manageable data
sets, an individual country of interest may be selected as well as only
stations falling between -60/60 degrees latitude for agroclimatology work.
The resulting files include station data (e.g., station name, country,
latitude, longitude, elevation) for use in a geographic information system
(GIS). The function was largely based on T. Hengl's 'getGSOD.R' script,
available from
<http://spatial-analyst.net/book/system/files/getGSOD.R> with enhancements
to be more cross-platform, faster and more flexible. Currently flags are
maintained for all of the columns except for MIN/MAX. In the original
MIN/MAX columns of the GSOD data, "blank indicates max temp was taken from
the explicit max temp report and not from the 'hourly' data. * indicates max
temp was derived from the hourly data (i.e., highest hourly or
synoptic-reported temperature).", GSOD readme.txt
<http://www1.ncdc.noaa.gov/pub/data/gsod/readme.txt>. For CSV files
resulting from GSODR the flags are stripped.