plotmap_masked.py

# make plot of etopo bathymetry/topography data on
# lambert conformal conic map projection, drawing coastlines, state and
# country boundaries, and parallels/meridians.

# demonstrates use of masked arrays to mask out certain regions
# (in this case the oceans)

frommpl_toolkits.basemapimportBasemap, shiftgrid
importnumpy.maasma
importnumpyasnp
importmatplotlib.pyplotasplt
importmatplotlib.colorsascolors

# read in topo data (on a regular lat/lon grid)
# longitudes go from 20 to 380.
topoin=np.loadtxt('etopo20data.gz')
lonsin=np.loadtxt('etopo20lons.gz')
latsin=np.loadtxt('etopo20lats.gz')
# shift data so lons go from -180 to 180 instead of 20 to 380.
topoin,lonsin=shiftgrid(180.,topoin,lonsin,start=False)

# setup of basemap ('lcc' = lambert conformal conic).
# use major and minor sphere radii from WGS84 ellipsoid.
m=Basemap(llcrnrlon=-145.5,llcrnrlat=1.,urcrnrlon=-2.566,urcrnrlat=46.352,\
rsphere=(6378137.00,6356752.3142),\
resolution='l',area_thresh=1000.,projection='lcc',\
lat_1=50.,lon_0=-107.)
# transform to nx x ny regularly spaced native projection grid
nx=int((m.xmax-m.xmin)/40000.)+1; ny=int((m.ymax-m.ymin)/40000.)+1
topodat,x,y=m.transform_scalar(topoin,lonsin,latsin,nx,ny,returnxy=True)
# create the figure.
fig=plt.figure(figsize=(8,8))
# add an axes.
ax=fig.add_axes([0.1,0.1,0.8,0.8])
# associate this axes with the Basemap instance.
m.ax=ax
# make topodat a masked array, masking values lower than sea level.
topodat=np.where(topodat<0.,1.e10,topodat)
topodatm=ma.masked_values(topodat, 1.e10)
palette=plt.cm.YlOrRd
palette.set_bad('aqua', 1.0)
# plot image over map with imshow.
im=m.imshow(topodatm,palette,norm=colors.normalize(vmin=0.0,vmax=3000.0,clip=False))
m.colorbar(im,pad='12%') # draw colorbar
# plot blue dot on boulder, colorado and label it as such.
xpt,ypt=m(-104.237,40.125) 
m.plot([xpt],[ypt],'bo') 
ax.text(xpt+100000,ypt+100000,'Boulder')
# draw coastlines and political boundaries.
m.drawcoastlines()
m.drawcountries()
m.drawstates()
# draw parallels and meridians.
# label on left, right and bottom of map.
parallels=np.arange(0.,80,20.)
m.drawparallels(parallels,labels=[1,1,0,1])
meridians=np.arange(10.,360.,30.)
m.drawmeridians(meridians,labels=[1,1,0,1])
# set title.
ax.set_title('Masked ETOPO Topography - via imshow')
plt.show()