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Fix stereographic scaling issues #929

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Merged
QuLogic merged 2 commits into from
Feb 2, 2018
Merged

Fix stereographic scaling issues #929

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dab79a6
Fix issues with scaling and stereographic projections
Jul 6, 2014
da433a5
Address comments from review
dopplershift Sep 1, 2017
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33 changes: 27 additions & 6 deletions lib/cartopy/crs.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1310,12 +1310,27 @@ def y_limits(self):
class Stereographic(Projection):
def __init__(self, central_latitude=0.0, central_longitude=0.0,
false_easting=0.0, false_northing=0.0,
true_scale_latitude=None, globe=None):
true_scale_latitude=None,
scale_factor=None, globe=None):
proj4_params = [('proj', 'stere'), ('lat_0', central_latitude),
('lon_0', central_longitude),
('x_0', false_easting), ('y_0', false_northing)]
if true_scale_latitude:

if true_scale_latitude is not None:
if central_latitude not in (-90., 90.):
warnings.warn('"true_scale_latitude" parameter is only used '
'for polar stereographic projections. Consider '
'the use of "scale_factor" instead.')
proj4_params.append(('lat_ts', true_scale_latitude))

if scale_factor is not None:
if true_scale_latitude is not None:
raise ValueError('It does not make sense to provide both '
'"scale_factor" and "true_scale_latitude". '
'Ignoring "scale_factor".')
else:
proj4_params.append(('k_0', scale_factor))

super(Stereographic, self).__init__(proj4_params, globe=globe)

# TODO: Let the globe return the semimajor axis always.
Expand Down Expand Up @@ -1358,17 +1373,23 @@ def y_limits(self):


class NorthPolarStereo(Stereographic):
def __init__(self, central_longitude=0.0, globe=None):
def __init__(self, central_longitude=0.0, true_scale_latitude=None,
globe=None):
super(NorthPolarStereo, self).__init__(
central_latitude=90,
central_longitude=central_longitude, globe=globe)
central_longitude=central_longitude,
true_scale_latitude=true_scale_latitude, # None is +90
globe=globe)


class SouthPolarStereo(Stereographic):
def __init__(self, central_longitude=0.0, globe=None):
def __init__(self, central_longitude=0.0, true_scale_latitude=None,
globe=None):
super(SouthPolarStereo, self).__init__(
central_latitude=-90,
central_longitude=central_longitude, globe=globe)
central_longitude=central_longitude,
true_scale_latitude=true_scale_latitude, # None is -90
globe=globe)


class Orthographic(Projection):
Expand Down
42 changes: 35 additions & 7 deletions lib/cartopy/tests/crs/test_stereographic.py
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
# (C) British Crown Copyright 2013 - 2017, Met Office
# (C) British Crown Copyright 2013 - 2018, Met Office
#
# This file is part of cartopy.
#
Expand Down Expand Up @@ -52,14 +52,42 @@ def test_eccentric_globe(self):
[-3932.82587779, 3932.82587779], decimal=4)

def test_true_scale(self):
# The "true_scale_latitude" parameter to Stereographic appears
# meaningless. This test just ensures that the correct proj4
# string is being created. (#339)
stereo = ccrs.Stereographic(true_scale_latitude=10)
expected = ('+ellps=WGS84 +proj=stere +lat_0=0.0 +lon_0=0.0 '
'+x_0=0.0 +y_0=0.0 +lat_ts=10 +no_defs')
# The "true_scale_latitude" parameter only makes sense for
# polar stereographic projections (#339 and #455).
# For now only the proj4 string creation is tested
# See test_scale_factor for test on projection.
globe = ccrs.Globe(ellipse='sphere')
stereo = ccrs.NorthPolarStereo(true_scale_latitude=30, globe=globe)
expected = ('+ellps=sphere +proj=stere +lat_0=90 +lon_0=0.0 '
'+x_0=0.0 +y_0=0.0 +lat_ts=30 +no_defs')
assert stereo.proj4_init == expected

def test_scale_factor(self):
# See #455
# Use spherical Earth in North Polar Stereographic to check
# equivalence between true_scale and scale_factor.
# In these conditions a scale factor of 0.75 corresponds exactly to
# a standard parallel of 30N.
globe = ccrs.Globe(ellipse='sphere')
stereo = ccrs.Stereographic(central_latitude=90., scale_factor=0.75,
globe=globe)
expected = ('+ellps=sphere +proj=stere +lat_0=90.0 +lon_0=0.0 '
'+x_0=0.0 +y_0=0.0 +k_0=0.75 +no_defs')
assert stereo.proj4_init == expected

# Now test projections
lon, lat = 10, 10
projected_scale_factor = stereo.transform_point(lon, lat,
ccrs.Geodetic())

# should be equivalent to North Polar Stereo with
# true_scale_latitude = 30
nstereo = ccrs.NorthPolarStereo(globe=globe, true_scale_latitude=30)
projected_true_scale = nstereo.transform_point(lon, lat,
ccrs.Geodetic())

assert projected_true_scale == projected_scale_factor

def test_eastings(self):
stereo = ccrs.Stereographic()
stereo_offset = ccrs.Stereographic(false_easting=1234,
Expand Down