Re: [Matplotlib-users] plot arrows for wind direction (degrees)

[questions a. <que...@gm...>]

Another quick question,
I noticed that the wind direction appeared back-to-front so I have added a
negative  to my equation as I am in the southern hemisphere. It now looks
correct but I wanted to check if this is the right way to do this? I saw
something called 'flip' for barb but nothing for quiver.
Thanks, see code below

               map = Basemap(projection='merc',llcrnrlat=-40,urcrnrlat=-33,

llcrnrlon=139.0,urcrnrlon=151.0,lat_ts=0,resolution='i')

                x,y=map(*N.meshgrid(LON,LAT))
                u=-1*N.sin(WDIR*N.pi/180)# added negative to account for
southern hemisphere
                v=-1*N.cos(WDIR*N.pi/180)
                map.quiver(x,y,u, v)



On Wed, Oct 5, 2011 at 3:20 PM, questions anon <que...@gm...>wrote:

> thank you! of course!
>
>
> On Wed, Oct 5, 2011 at 3:08 PM, Eric Firing <ef...@ha...> wrote:
>
>> On 10/04/2011 05:40 PM, questions anon wrote:
>> > Excellent, thank you. That works for both quiver and barb.
>> > In regards to the shape that has to do with the netcdf file, wind
>> > direction variable also has lat and lon. I ended up needing to slice so
>> > I could skip a few points otherwise there was too many arrows on the
>> map.
>> > Below is the code that worked in case someone else needs help.
>>
>> Are you sure it worked correctly?  I don't think so.  See below.
>>
>> > Thanks again
>> >
>> > from netCDF4 import Dataset
>> > import numpy as N
>> > import pylab as plt
>> > from numpy import ma as MA
>> > from mpl_toolkits.basemap import Basemap
>> > import os
>> >
>> > ncfile=Dataset('E:/temp_winddir/IDZ00026_VIC_ADFD_Wind_Dir_SFC.nc',
>> > 'r+', 'NETCDF4')
>> > WDIR=ncfile.variables['Wind_Dir_SFC'][20,0::5,0::5]
>>
>> Don't you need to convert from degrees to radians?  sin and cos (used
>> below) expect radians, not degrees.  You can use N.deg2rad(WDIR) to
>> convert.
>>
>> > LAT=ncfile.variables['latitude'][0::5]
>> > LON=ncfile.variables['longitude'][0::5]
>> > TIME=ncfile.variables['time'][20]
>> > ncfile.close()
>> >
>> > map = Basemap(projection='merc',llcrnrlat=-40,urcrnrlat=-33,
>> >                llcrnrlon=139.0,urcrnrlon=151.0,lat_ts=0,resolution='i')
>> > x,y=map(*N.meshgrid(LON,LAT))
>> > u=1*N.sin(WDIR)
>> > v=1*N.cos(WDIR)
>> > map.quiver(x,y,u, v)
>>
>> Assuming you do convert to radians the above will work for your mercator
>> projection but it will not align the vectors correctly with respect to
>> the parallels and meridians for other projections, so you should use
>> map.rotate_vector as in the basemap quiver demo.
>>
>> Eric
>>
>>
>> >
>> > #  otherwise use the barb tool - map.barbs(x,y,u, v)
>> >
>> > plt.title('Wind Direction')
>> > plt.show()
>>
>>
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