Download, clean, reformat and create new variables from the
USA National Climatic Data Center (NCDC) Global Surface Summary of the Day
(GSOD) weather stations data,
<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
International System of Units (SI), 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.
Stations with missing latitude or longitude or values for both of 0 are
omitted. 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 vapor
pressure, saturation vapor pressure and relative humidity are 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 or
individual stations if the station ID is known. 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. See the GSOD readme.txt
<http://www1.ncdc.noaa.gov/pub/data/gsod/readme.txt>.