Re: [Matplotlib-users] help plotting vector field in basemap

[Eric F. <ef...@ha...>]

On 2013/04/08 4:13 AM, epi wrote:
> Hi All,
>
> i'm new to basemap, truing to learn how to plot vector from a netcdf file
>
> the variables in my file are :
>
> - time
> - Significant_height_of_combined_wind_waves_and_swell_surface
> - u-component_of_wind_surface
> - v-component_of_wind_surface
>
> this is my code, the pcolormesh is fine
>
> ####
>
> import netCDF4
> import datetime as dt
>
> import numpy as np
> import numpy.ma as ma
> from datetime import date, datetime, timedelta
>
> from mpl_toolkits.basemap import Basemap
>
>
>
> url='http://geoport.whoi.edu/thredds/dodsC/fmrc/NCEP/ww3/cfsr/4m/best'
> #url = 'http://geoport.whoi.edu/thredds/dodsC/fmrc/NCEP/ww3/cfsr/10m/best'
> nc = netCDF4.Dataset(url)
> #nc.variables.keys()
>
>
> time_var = nc.variables[str('time')]
> wave_var = nc.variables['Significant_height_of_combined_wind_waves_and_swell_surface']
> date = datetime(1991,11,1,12)
> istart = netCDF4.date2index(date,time_var,select='nearest')
> lat = nc.variables['lat'][:]
> lon = nc.variables['lon'][:]
> uin = nc.variables['u-component_of_wind_surface'][istart,:,:]
> vin = nc.variables['v-component_of_wind_surface'][istart,:,:]
> var = wave_var[istart,:,:]
> wave = ma.masked_where(np.isnan(var),var)
>
>
> m = Basemap(llcrnrlon=-71.5,llcrnrlat=39.5,urcrnrlon=-63.0,urcrnrlat=46.0,\
>              lat_0=20.,lon_0=-60.,lat_ts=20.)
>
>
> lons, lats = np.meshgrid(lon,lat)
> x, y = m(lons, lats)
>
>
> m.pcolormesh(lon[:], lat[:], wave, vmin=0, vmax=3);
> m.quiver(x, y, uin, vin);

Try something like this:

m.quiver(x[::5,::5], y[::5,::5], uin[::5,::5], vin[::5,::5], scale=200);

You can use the scale and the scale_units kwargs to control the arrow 
lengths.  Quiver plots don't work visually when there are too many 
arrows, so given the scale of your plot, you need to subsample the wind 
vectors as illustrated.

Eric


>
>
> ####
>
> .. but the vector plot in overlay doesn't render what i'm looking for .. obviously my fault in the code
> thank you for your precious help!
>
>
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