Plotting a map : arrows disappear

Question

Good afternoon, I'm trying to plot surface current's data on a map thanks to a csv file. Here's my code :

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.mlab as mlab
from pylab import *


# read CSV as a numpy array
data = mlab.csv2rec('datasets/mix.csv')

# print CSV file headers
print data.dtype.names

# load columns as vectors
data_x = data['longitude']
data_y = data['latitude']
data_u = data['x']
data_v = data['y']

U = cos(data_u)
V = sin(data_v)


# plot raw data
Q = quiver(data_x, data_y, U, V, color='black', units='width')
qk = quiverkey(Q, 0.5, 0.92, 2, '.', labelpos='W',  
               fontproperties={'weight': 'bold'})
title('Current Surface')

plt.show()

With a small part of that csv file (300 lines), my result contains arrows :

But when i want to model all my csv file, there are no arrows anymore, but points (which results in the map below): Previous figure zoomed in to show there are no arrows:

Have you got any idea about this behaviour? Regards.

Answer 1

In the last image you have arrows, but they are so short that you cannot see the "tails". The problem is that in your second plot, you have too dense datapoints: The quiver command automatically scales the arrwos such that they do not overlap. If you then zoom in (I guess you zoomed in the window, not by selecting a smaller region in the script?), this scaling is not recalculated.

I would suggest to plot only every 10th or so datapoint (e.g. U[::10]), this should help. ALternatively, play around with the "scale" keyword argument:

http://matplotlib.org/api/pyplot_api.html