Re: basemap: Mask the ocean [SEC=UNCLASSIFIED]
Jeff Whitaker <
jswhit@...>
2009-11-02 15:52:48 GMT
Stephane Raynaud wrote:
> Ross,
>
>
> one way is to mask (or remove) ocean points using the _geoslib module
> provided with basemap.
> When you create a Basemap instance, you can retrieve all its polygons
> land (continents and islands) with "mymap.coastpolygons".
> Thay are stored as numpy arrays, and you can convert them to
> _geoslib.Polygon objects :
>
> poly = _geoslib.Polygon(N.asarray(coastalpoly).T)
>
> Then you loop over all Polygons and all (x,y) points and test :
>
> good_point = _geoslib.Point((x,y)).within(poly)
>
> Thanks to this method, you can choose you optimal resolution.
> You can even compute the intersection of you hexagons with coastal
> polygons using .intersection() and .area (instead of simply checking
> if the center is inside) and then reject points depending the fraction
> of the cell covered by land (or ocean).
Following Stephane's excellent suggestion, here's a prototype Basemap
method that checks to see if a point is on land or over water. Ross -
if you find it useful I'll include it in the next release. Note that it
will be slow for lots of points or large map regions.
-Jeff
>
> On Mon, Nov 2, 2009 at 8:07 AM, <Ross.Wilson@...
> <mailto:Ross.Wilson@...>> wrote:
>
> Listers,
>
> I'm using basemap to plot randomly sampled values (x,y,z) through
> hexbin. This produces a very nice result. Some sample code is:
> ----------
> import numpy as np
> from numpy.random import seed
> import matplotlib.pyplot as plt
> from mpl_toolkits.basemap import Basemap
> from matplotlib.mlab import griddata
>
> ll_lat = -38.39477 # extent of area of interest
> ll_lon = 144.54767
> ur_lat = -37.51642
> ur_lon = 145.67144
>
> num_points = 100 # sample points
>
> # create random sampling over the area of interest
> seed(0)
> data = np.ones((3, num_points))
> data[0,:] *= ll_lon + np.random.random((num_points))*(ur_lon-ll_lon)
> data[1,:] *= ll_lat + np.random.random((num_points))*(ur_lat-ll_lat)
> data[2,:] *= np.random.random((num_points))*10000
>
> # plot the data
> fig = plt.figure()
> ax = fig.add_subplot(111)
> m = Basemap(projection='cyl', llcrnrlat=ll_lat, urcrnrlat=ur_lat,
> llcrnrlon=ll_lon, urcrnrlon=ur_lon, resolution='f',
> suppress_ticks=False, area_thresh=0.5)
> plt.hexbin(data[0,:], data[1,:], data[2,:], zorder=3)
> m.fillcontinents(color=(0.8,0.8,0.8,0), zorder=1)
> m.drawcoastlines(linewidth=0.25, color='k', zorder=2)
> plt.show()
> ----------
>
> This contrived example shows a sparse set of hexagons on both land
> and ocean. I would like the hexagons over the ocean to be hidden.
> I can make the ones on land disappear by changing the 'zorder'
> parameter of .hexbin() to 0. However I have found no way of doing
> the inverse and hiding hexagons over the ocean.
>
> Using drawlsmask() is too crude at a 5-minute resolution.
>
> Any ideas?
>
> Thanks,
> Ross
>
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> --
> Stephane Raynaud
> ------------------------------------------------------------------------
>
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