def _proj4(self):
_proj4 = ("+proj=eqc +units=meters +a={} +b={} "
"+lon_0={}".format(Constants.WRF_EARTH_RADIUS,
Constants.WRF_EARTH_RADIUS,
self.stand_lon))
return _proj4
# Notes (may not be correct since this projection confuses me):
# Each projection system handles this differently.
# 1) In WRF, if following the WPS instructions, POLE_LON is mainly used to
# determine north or south hemisphere. In other words, it determines if
# the globe is tipped toward or away from you.
# 2) In WRF, POLE_LAT is always positive, but should be negative in the
# proj4 based systems when using the southern hemisphere projections.
# 3) In cartopy, pole_longitude is used to describe the dateline, which
# is 180 degrees away from the normal central (standard) longitude
# (e.g. center of the projection), according to the cartopy developer.
# 4) In basemap, lon_0 should be set to the central (standard) longitude.
# 5) In either cartopy, basemap or pyngl, I'm not sure that projections with
# a pole_lon not equal to 0 or 180 can be plotted. Hopefully people
# follow the WPS instructions, otherwise I need to see a sample file.
# 6) For items in 3 - 4, the "longitude" (lon_0 or pole_longitude) is
# determined by WRF's
# STAND_LON values, with the following calculations based on hemisphere:
# BASEMAP: NH: -STAND_LON; SH: 180.0 - STAND_LON
# CARTOPY: NH: -STAND_LON - 180.; SH: -STAND_LON
# 9) For PYNGL/NCL, you only need to set the center lat and center lon,
# Center lat is the offset of the pole from +/- 90 degrees. Center
# lon is -STAND_LON in NH and 180.0 - STAND_LON in SH.
# 10) It also appears that NetCDF CF has no clear documentation on what
# each parameter means. Going to assume it is the same as basemap, since
# basemap appears to mirror the WMO way of doing things (tilt earth, then
# spin globe).
# 11) Basemap and cartopy produce projections that differ in their extent
# calculations by either using negative values or 0-360 (basemap). For
# this reason, the proj4 string for this class will use cartopy's values
# to keep things in the -180 to 180, -90 to 90 range.
# 12) This projection makes me sad.
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