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tacaswell opened this issue Aug 26, 2014 · 1 comment
Open

call shiftdata on both lat and lon #163

tacaswell opened this issue Aug 26, 2014 · 1 comment

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@tacaswell
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This is migrated from matplotlib/matplotlib#3404

There is a bug with point shifting when using latlon=True in Basemap.plot(), as discussed on http://stackoverflow.com/questions/25471723/plotting-on-a-basemap-unexpected-result/25472915: for instance, see the following code

from mpl_toolkits.basemap import Basemap
import numpy as np
import matplotlib.pyplot as plt

m = Basemap(projection='merc',llcrnrlat=-80,urcrnrlat=80,\
            llcrnrlon=-180,urcrnrlon=180,lat_ts=20,resolution='l')
m.drawparallels(np.arange(-90.,91.,30.))
m.drawmeridians(np.arange(-180.,181.,60.))

lon=[
 -44.897539694478894,
 -79.56264363246461,
 -108.31264586027467,
 -129.5832433799378,
 -149.11755440233293,
 173.04624586158417,
 57.26114485166647,
 26.06650557322952,
 6.8910540489469785,
 -15.059586144625898]

lat=[
 -23.30021206811055,
 -22.174848810053106,
 -6.169632450760373,
 18.199421172044598,
 45.95724594253466,
 72.89364342463014,
 69.39230460744983,
 41.88542137864501,
 14.50656439517308,
 -8.974170076274387]

m.plot(lon, lat, 'ro', markersize=14, mec='none', latlon=True)
m.plot(lon[:-1], lat[:-1], 'bo', markersize=10, mec='none', latlon=True)
m.plot(lon[1:], lat[1:], 'go', markersize=6, mec='none', latlon=True)

Output:
2pwkx

The red points should all coincide with the blue and green ones, but they don't.
@tacaswell
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From the user DrV at SO:

Update: I think I found the bug. The plot method of Basemap first shifts the data so that it starts from the edge of the plot. For example:

In [27]: m.shiftdata(lon)
Out[27]: 
array([ 173.04624586,   57.26114485,   26.06650557,    6.89105405,
        -15.05958614,  -44.89753969,  -79.56264363, -108.31264586,
       -129.58324338, -149.1175544 ])

This is quite ok, as now lines will be contiguous. Unfortunately, the latitude data is not shifted, and the results are exactly what is shown above.

If you shift both latitudes and longitudes by:

lons, lats = m.shiftdata(lon, lat)

and use the shifted version to plot the data, everything is fine.

To my eye it seems that there is some hassle with the decorator _transform1d used around plot and scatter methods. Monkey-patching this (init.py, lines 3239 and 3277 in today's git) by changing the decorator into _transform should help, but it may break other functionality in plot and scatter.

I think the best solution is to sort the data before plotting by using shiftdata as above. (And of course to file a bug report.)

@tacaswell tacaswell mentioned this issue Aug 26, 2014
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@tacaswell