Question

I would like to create a density plot using basemap. The data I have is ungridded and repeating or very close to each other. I’ve tried gridding the data and then plotting the number of bins using pcolor but I keep getting a buffer size error even though all the datasets have equal length. My original idea was to use the basemap script below but I can only get scatter to work, though this does not give me a density plot.

m = Basemap(resolution='f',projection='merc',                                        
            lon_0=160,                                                               
            llcrnrlat=-30.0,                                                         
            urcrnrlat=30.0,                                                          
            llcrnrlon=100.,                                                          
            urcrnrlon=270.0,                                                         
            lat_ts=0.0) 

m.drawmapboundary(fill_color='white')                                                
m.fillcontinents(color='#F5DEB3',lake_color='#85A6D9')                               
m.drawcoastlines(color='black', linewidth=.4)                                        
m.drawcountries(color='#6D5F47', linewidth=.4)                                       
m.drawmeridians(np.arange(0, 360, 20),                                               
                labels=[0,0,0,1],                                                    
                color='black',                                                       
                dashes=[1,0],                                                        
                labelstyle='+/-',                                                    
                linewidth=0.2)         
m.drawparallels(np.arange(-90, 90, 10),                                              
                labels=[1,0,0,0],                                                    
                color='black',                                                       
                dashes=[1,0],                                                        
                labelstyle='+/-',                                                    
                linewidth=0.2) 
lats = 
array([ 14.375,  14.125,  14.125,   9.375,  13.625,  14.375,   5.625,
    13.875,  14.625,   5.875,   8.875,   5.625,   8.875,  13.375,
    11.125,   8.375,  12.375,   6.125,   5.375,   8.375,   7.375,
     7.875,  14.375,  14.875,   9.875,  11.125,  14.875,   7.875,
     9.125,  11.625,   5.125,  10.875,   5.125,  12.125,  12.625,
     8.625,   5.125,   8.375,  11.625,  11.375,  12.875,  14.375,
     8.875,   8.375,   6.375,   8.625,   5.875,   8.125,   9.375,
     5.875,   8.125,   8.875,   5.375,   8.875,   5.625,  11.875,
     9.875,   9.875,  10.875,  11.375,   9.875,   9.375,  13.125,
    14.125,   8.125,  14.875,   9.875,   9.625,  10.625,  12.125,
     9.375,   5.625,  13.625,   6.375,  10.125,  14.875,   8.875,
     5.125,   6.125,   8.625,   6.875,   9.375,   9.125,   9.375,
    13.625,   6.125,   6.125,   6.875,  11.375,  13.375,  10.375,
     6.875,   7.625,   7.625,  13.875,   5.125,   6.125,  14.125,
     7.375,   5.375])
lons = array([ 122.125,  125.875,  122.375,  132.125,  124.375,  122.625,
    206.125,  122.625,  120.375,  187.625,  243.625,  161.625,
    220.375,  121.875,  125.625,  130.125,  219.625,  223.875,
    223.375,  190.125,  132.875,  185.875,  122.625,  120.875,
    259.375,  125.875,  204.875,  131.875,  130.875,  125.375,
    234.375,  241.375,  188.125,  124.375,  124.375,  140.375,
    205.625,  130.875,  256.875,  239.375,  124.125,  123.125,
    131.625,  126.375,  241.125,  130.875,  233.625,  184.375,
    205.125,  169.375,  150.375,  183.375,  168.875,  239.625,
    180.125,  241.375,  131.375,  244.875,  244.375,  125.625,
    131.375,  150.125,  203.625,  122.125,  243.125,  159.125,
    125.625,  243.375,  125.875,  127.375,  130.125,  146.625,
    203.125,  185.125,  204.625,  120.625,  130.125,  233.875,
    131.875,  258.625,  130.125,  173.375,  258.125,  129.125,
    137.875,  215.625,  214.125,  234.625,  125.375,  136.625,
    231.125,  145.625,  128.625,  232.875,  125.125,  145.125,
    239.875,  138.625,  241.375,  169.625])
data = 
array([ 84.8125 ,  81.6875 ,  79.75   ,  78.5625 ,  70.8125 ,  70.75   ,
    69.0625 ,  69.     ,  67.3125 ,  63.65625,  63.375  ,  62.5625 ,
    62.46875,  61.09375,  60.4375 ,  59.21875,  58.84375,  58.8125 ,
    58.75   ,  58.4375 ,  57.8125 ,  57.375  ,  57.1875 ,  57.15625,
    57.15625,  57.09375,  56.96875,  56.875  ,  56.3125 ,  56.28125,
    56.1875 ,  55.59375,  55.5    ,  55.46875,  55.4375 ,  55.375  ,
    55.3125 ,  55.21875,  55.21875,  55.15625,  54.90625,  54.75   ,
    54.6875 ,  54.65625,  54.625  ,  54.59375,  54.4375 ,  54.34375,
    54.21875,  54.0625 ,  53.9375 ,  53.75   ,  53.65625,  53.21875,
    53.15625,  53.0625 ,  52.78125,  52.75   ,  52.4375 ,  52.25   ,
    52.0625 ,  51.71875,  51.71875,  51.59375,  51.4375 ,  51.3125 ,
    51.28125,  51.28125,  51.25   ,  51.03125,  51.     ,  50.96875,
    50.9375 ,  50.875  ,  50.8125 ,  50.75   ,  50.71875,  50.6875 ,
    50.5    ,  50.4375 ,  50.4375 ,  50.28125,  50.21875,  50.125  ,
    50.     ,  49.5    ,  49.5    ,  49.40625,  49.21875,  49.21875,
    49.0625 ,  48.96875,  48.53125,  48.46875,  48.375  ,  48.28125,
    48.21875,  48.0625 ,  47.875  ,  47.84375])

Any ideas on how to get a density plot of this data? Appreciate the help.
/M

Answer 1

If your interested in doing a frequency count for each lat lon in a gridbox, you can use the numpy function histogram2d.

The following code might be what you are looking for:

nx, ny = 10, 3

# compute appropriate bins to histogram the data into
lon_bins = numpy.linspace(lons.min(), lons.max(), nx+1)
lat_bins = numpy.linspace(lats.min(), lats.max(), ny+1)

# Histogram the lats and lons to produce an array of frequencies in each box.
# Because histogram2d does not follow the cartesian convention 
# (as documented in the numpy.histogram2d docs)
# we need to provide lats and lons rather than lons and lats
density, _, _ = numpy.histogram2d(lats, lons, [lat_bins, lon_bins])

# Turn the lon/lat bins into 2 dimensional arrays ready 
# for conversion into projected coordinates
lon_bins_2d, lat_bins_2d = numpy.meshgrid(lon_bins, lat_bins)

# convert the xs and ys to map coordinates
xs, ys = m(lon_bins_2d, lat_bins_2d)

plt.pcolormesh(xs, ys, density)
plt.colorbar(orientation='horizontal')

# overlay the scatter points to see that the density 
# is working as expected
plt.scatter(*m(lons, lats))

plt.show()

EDIT: I have added more inline comments to help with comprehension/readability.