plot gebco data in python basemap

Question

I have downloaded some gebco bathymetry data as a netCDF file. I would like to plot it with python-basemap. I have tried,

import netCDF4
from mpl_toolkits.basemap import Basemap


# Load data
dataset = netCDF4.Dataset('/home/david/Desktop/GEBCO/gebco_08_-30_45_5_65.nc')

# Extract variables
x = dataset.variables['x_range']
y = dataset.variables['y_range']
spacing = dataset.variables['spacing']

# Data limits
nx = (x[-1]-x[0])/spacing[0]   # num pts in x-dir
ny = (y[-1]-y[0])/spacing[1]   # num pts in y-dir

# Reshape data
zz = dataset.variables['z']
Z = zz[:].reshape(ny, nx)



# setup basemap.
m = Basemap(llcrnrlon=-30,llcrnrlat=45.0,urcrnrlon=5.0,urcrnrlat=65.0,
            resolution='i',projection='stere',lon_0=-15.0,lat_0=55.0)


# Set up grid
lons, lats = m.makegrid(nx, ny)
x, y = m(lons, lats)

m.contourf(x, y, flipud(Z))
m.fillcontinents(color='grey')
m.drawparallels(np.arange(10,70,10), labels=[1,0,0,0])
m.drawmeridians(np.arange(-80, 5, 10), labels=[0,0,0,1])

this gives the figure below, clearly not correct. The problem stems from how the areas are defined. For basemap area is defined by lower left corner lat,lon and upper right corner lat, lon. But the gebco data takes a maximum and minimum lon/lat defined along a center line. Anyone have any experience with gebco data or see a solution??

thanks D

Answer 1

so just for the record, here's the answer that works, using the comments above:

import netCDF4
from mpl_toolkits.basemap import Basemap

# Load data
dataset = netCDF4.Dataset('/usgs/data1/rsignell/bathy/gebco_08_-30_-45_5_65.nc')

# Extract variables
x = dataset.variables['x_range']
y = dataset.variables['y_range']
spacing = dataset.variables['spacing']

# Compute Lat/Lon
nx = (x[-1]-x[0])/spacing[0]   # num pts in x-dir
ny = (y[-1]-y[0])/spacing[1]   # num pts in y-dir

lon = np.linspace(x[0],x[-1],nx)
lat = np.linspace(y[0],y[-1],ny)

# Reshape data
zz = dataset.variables['z']
Z = zz[:].reshape(ny, nx)

# setup basemap.
m = Basemap(llcrnrlon=-30,llcrnrlat=45.0,urcrnrlon=5.0,urcrnrlat=65.0,
            resolution='i',projection='stere',lon_0=-15.0,lat_0=55.0)

x,y = m(*np.meshgrid(lon,lat))

m.contourf(x, y, flipud(Z));
m.fillcontinents(color='grey');
m.drawparallels(np.arange(10,70,10), labels=[1,0,0,0]);
m.drawmeridians(np.arange(-80, 5, 10), labels=[0,0,0,1]);

which produces this plot.