(Python) Unable to See Matplotlib Plot

Question

I'm using the following code to try to generate a contour plot using matplotlib.

Script Code:

#!/usr/bin/python

from mpl_toolkits.basemap import Basemap
import matplotlib.pyplot as plt
import matplotlib.cm as cm
import matplotlib.colors as colors
import numpy as np
from matplotlib.mlab import griddata

a = np.loadtxt('/home/weather/site_data')

def show_map(self, a):

    # 'a' is of the format [(lats, lons, data), (lats, lons, data)... (lats, lons, data)]
    lats = [ x[0] for x in a ]
    lons = [ x[1] for x in a ]
    data = [ x[2] for x in a ]

    lat_min = min(lats)
    lat_max = max(lats)
    lon_min = min(lons)
    lon_max = max(lons)
    data_min = min(data)
    data_max = max(data)

    spatial_resolution = 0.5
    fig = plt.figure()

    x = np.array(lons)
    y = np.array(lats)
    z = np.array(data)

    xinum = (lon_max - lon_min) / spatial_resolution
    yinum = (lat_max - lat_min) / spatial_resolution
    xi = np.linspace(lon_min, lon_max + spatial_resolution, xinum)        # same as [lon_min:spatial_resolution:lon_max] in matlab
    yi = np.linspace(lat_min, lat_max + spatial_resolution, yinum)        # same as [lat_min:spatial_resolution:lat_max] in matlab
    xi, yi = np.meshgrid(xi, yi)

    zi = griddata(x, y, z, xi, yi)

    m = Basemap(
        projection = 'merc',
        llcrnrlat=lat_min, urcrnrlat=lat_max,
        llcrnrlon=lon_min, urcrnrlon=lon_max,
        rsphere=6371200., resolution='l', area_thresh=10000
    )

    m.drawcoastlines()
    m.drawstates()
    m.drawcountries()

    lat, lon = m.makegrid(zi.shape[1], zi.shape[0])
    x,y = m(lat, lon)
    m.contourf(x, y, zi)

plt.show()

Data:

[(45.43, -75.65, 75.9), (44.30, -73.63, 85.2), (45.76, -65.76, 68.2)]

I'm trying to figure out if I am inputting the data incorrectly or if there is another underlying issue at hand.

I keep getting the following error code:

ValueError: could not convert string to float: [(45.43,

I tried fixing it myself and thought that I figured it out by removing all parentheses and commas but, then the script didn't even plot an image.

Read the error message. Your data seems to be a list/array of strings, not a list/array of floats. Maybe it is read as a string by np.loadtxt(). Check type of data and data[0]! — sascha
– sascha, Commented Jul 27, 2015 at 15:56
Can you interate through your data and print the respective types? Something along the lines of for i in data: print type(i), i. The error message says that you have a ValueError (because of incorrect type) after all, most likely your problem is fixed if you cast it into the correct type. — Bas Jansen
– Bas Jansen, Commented Jul 27, 2015 at 15:58

Pencil Von Radiergummi · Accepted Answer · 2015-07-27 18:17:34Z

1

I altered your input data file to look like this, which I believe is the format that np.loadtxt() is looking for

45.43 -75.65 75.9
44.30 -73.63 85.2 
45.76 -65.76 68.2

As for why you aren't seeing an image, I removed the self from your function arguments, and called

show_map(a)

a line before plt.show(). You never actually call the function show_map() in your code block, which is why it wasn't executing. I obtain a picture that looks like this:

which may or may not be what you're looking for (I've never used basemap before). Hope this helps.

edited Jul 27, 2015 at 18:17

answered Jul 27, 2015 at 16:28

Pencil Von Radiergummi

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2 Comments

Aaron Perry Over a year ago

So you changed "def show_map(self, a)" -> "def show_map(a)" and then the last two lines in the script read, "show_map(a) plt.show()"?

Pencil Von Radiergummi Over a year ago

Correct, in addition to changing site_data to the format in my answer.

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