--- /dev/null
+[pyproj][] is a Python wrapper around [PROJ.4][]. Here's a quick
+walkthrough.
+
+Initialize a [geodetic][] converter:
+
+ >>> from pyproj import Geod
+ >>> g = Geod(ellps='clrk66')
+
+where `ellps='clrk66'` selects [Clarke's 1866][clrk66] [reference
+ellipsoid][rell]. `help(Geod.__new__)` gives a list of possible
+ellipsoids.
+
+Calculate the distance between two points, as well as the local
+heading, try
+
+ >>> lat1,lng1 = (40.7143528, -74.0059731) # New York, NY
+ >>> lat2,lng2 = (49.261226, -123.1139268) # Vancouver, Canada
+ >>> az12,az21,dist = g.inv(lng1,lat1,lng2,lat2)
+ >>> az12,az21,dist
+ (-59.10918706123901, 84.99453463527395, 3914198.2912370963)
+
+which gives forward and back [azimuths][] as well as the geodesic
+distance in meters. Not that longitude comes *before* latitude in the
+these pyproj argument lists.
+
+Calculate the terminus of a geodesic from an initial point, azimuth,
+and distance with:
+
+ >>> lng3,lat3,az3 = g.fwd(lng1,lat1,az12, dist)
+ >>> lat3,lng3,az3
+ (49.26122600306212, -123.11392684861474, 84.99453467574762)
+
+Plan your trip with:
+
+ >>> pts = g.npts(lng1,lat1,lng2,lat2,npts=5)
+ >>> pts.insert(0, (lng1, lat1))
+ >>> pts.append((lng2, lat2))
+ >>> import numpy
+ >>> npts = numpy.array(pts)
+ >>> npts
+ array([[ -74.0059731 , 40.7143528 ],
+ [ -80.93566289, 43.52686057],
+ [ -88.48167748, 45.87969433],
+ [ -96.61187851, 47.6930911 ],
+ [-105.22271807, 48.89347605],
+ [-114.13503215, 49.42510006],
+ [-123.1139268 , 49.261226 ]])
+
+To plot the above New York to Vancouver route on a flat map, we need a
+`Proj` instance:
+
+ >>> from pyproj import Proj
+ >>> awips221 = Proj(proj='lcc', R=6371200, lat_1=50, lat_2=50,
+ ... lon_0=-107, ellps='clrk66')
+ >>> awips218 = Proj(proj='lcc', R=6371200, lat_1=25, lat_2=25,
+ ... lon_0=-95, ellps='clrk66') #x_0=-llcrnrx,y_0=-llcrnry)
+
+ #llcrnrlon,llcrnrlat are lon and lat (in degrees) of lower
+ # left hand corner of projection region.
+
+where `proj='lcc` selects the [Lambert conformal conic][lcc]
+projection for the x/y points, and `ellps='clrk66'` selects the
+reference ellipsoid for the lat/lng coordinates. The other
+coordinates are LCC parameters that select the [AWIPS 221][awips221]
+and [AWIPS 226][awips226] projections respectively (`lat_1`
+corresponds to `Latin1`, `lat_2` corresponds to `Latin2`, and `lon_0`
+corresponds to `Lov`).
+
+Convert our lat/lng pairs into grid points:
+
+ >>> awips221(lng1, lat1)
+ (2725283.842678774, 5823260.730665273)
+ >>> x221,y221 = awips221(npts[:,0], npts[:,1])
+ >>> # xy221 = numpy.concatenate((a1, a2, ...), axis=0) # numpy-2.0
+ >>> xy221 = numpy.ndarray(shape=npts.shape, dtype=npts.dtype)
+ >>> xy221[:,0] = x221
+ >>> xy221[:,1] = y221
+ >>> xy221
+ array([[ 2725283.84267877, 5823260.73066527],
+ [ 2071820.3526011 , 5892518.49630526],
+ [ 1422529.71760395, 5967565.49899035],
+ [ 775650.03731228, 6046475.43928965],
+ [ 129946.46495299, 6127609.80532071],
+ [ -515306.57275941, 6209785.69230076],
+ [-1160447.80254759, 6292455.41884832]])
+
+Finally, you can convert points from one projection to another.
+
+ >>> from pyproj import transform
+ >>> x218,y218 = transform(awips221, awips218, x221, y221)
+ >>> xy218 = numpy.ndarray(shape=npts.shape, dtype=npts.dtype)
+ >>> xy218[:,0] = x218
+ >>> xy218[:,1] = y218
+ >>> xy218
+ array([[ 1834251.59591561, 4780900.70184736],
+ [ 1197541.13209718, 5028862.9881648 ],
+ [ 542391.04388716, 5258740.71523961],
+ [ -131577.34942316, 5464828.45934687],
+ [ -822685.42269932, 5641393.59760613],
+ [-1527077.85176048, 5783597.16169582],
+ [-2239159.34620498, 5888495.91009021]])
+
+Another useful coordinate system is the [Universal Transverse
+Mercator][UTM] projection which slices the world into [zones][].
+
+ >>> p = Proj(proj='utm', zone=10, ellps='clrk66')
+
+Putting everything together, here's a route map based on digital
+lat/lng pairs stored in a text file:
+
+ >>> from numpy import array
+ >>> from pylab import plot, show
+ >>> from pyproj import Geod, Proj
+ >>> latlng = array([[float(x) for x in ln.split()]
+ ... for ln in open('coords', 'r')
+ ... if not ln.startswith('#')])
+ >>> g = Geod(ellps='WGS84')
+ >>> az12s,az21s,dists = g.inv(latlng[:-1,1], latlng[:-1,0],
+ ... latlng[1:,1], latlng[1:,0])
+ >>> print('total distance: %g m' % dists.sum())
+ total distance: 2078.93 m
+ >>> mlng = latlng[:,1].mean()
+ >>> zone = int(round((mlng + 180) / 6.))
+ >>> p = Proj(proj='utm', zone=zone, ellps='WGS84')
+ >>> xs,ys = p(latlng[:,1], latlng[:,0])
+ >>> lines = plot(xs, ys, 'r.-')
+ >>> show()
+
+Note that you can easily get lat/lng pairs using [geopy][]:
+
+ >>> import geopy
+ >>> g = geopy.geocoders.Google()
+ >>> place1,(lat1,lng1) = g.geocode("New York, NY")
+ >>> place2,(lat2,lng2) = g.geocode("Vancouver, Canada")
+ >>> place1,(lat1,lng1)
+ (u'New York, NY, USA', (40.7143528, -74.0059731))
+ >>> place2,(lat2,lng2)
+ (u'Vancouver, BC, Canada', (49.261226, -123.1139268))
+
+If you're looking for a more compact [[C++]] package for geographic
+conversions, [GeographicLib][] looks promising.
+
+[pyproj]: http://code.google.com/p/pyproj/
+[PROJ.4]: http://trac.osgeo.org/proj/
+[geodetic]: http://en.wikipedia.org/wiki/Geodesy
+[clrk66]: http://en.wikipedia.org/wiki/Alexander_Ross_Clarke
+[rell]: http://en.wikipedia.org/wiki/Reference_ellipsoid
+[azimuths]: http://en.wikipedia.org/wiki/Azimuth
+[LCC]: http://en.wikipedia.org/wiki/Lambert_conformal_conic_projection
+[awips221]: http://www.nco.ncep.noaa.gov/pmb/docs/on388/tableb.html#GRID221
+[awips226]: http://www.nco.ncep.noaa.gov/pmb/docs/on388/tableb.html#GRID218
+[UTM]: http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system
+[zone]: http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system#UTM_zone
+[geopy]: http://code.google.com/p/geopy/
+[GeographicLib]: http://geographiclib.sourceforge.net/html/
+
+[[!tag tags/fun]]
+[[!tag tags/linux]]
+[[!tag tags/python]]
+[[!tag tags/tools]]
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