Re: [Matplotlib-users] basemap imshow() extents

[Jeff W. <js...@fa...>]

On 8/20/12 8:21 PM, Scott Henderson wrote:
> On Mon 20 Aug 2012 06:29:01 PM EDT, Jeff Whitaker wrote:
>> On 8/20/12 4:41 PM, Scott Henderson wrote:
>>> I'm having trouble with transform_scalar() and imshow() with basemap.
>>> Essential I have data from satellite tracks that are either smaller
>>> or larger than the map extent, so I don't want to use
>>> Basemap.imshow() which sets the 'extent' keyword automatically.
>>>
>>> I tried following the following suggestion, but I can't get things to
>>> line up..
>>> http://comments.gmane.org/gmane.comp.python.matplotlib.general/25085
>>>
>>> I suspect I've done something wrong in the code below?:
>>
>> Scott:  If you use the pcolormesh or contourf Basemap methods (instead
>> of imshow), you can specify the map projection coordinates of your
>> data and it will be plotted correctly (whether or not it covers the
>> whole map projection region).
>>
>> -Jeff
>>>
>>>
>>> # -------------------------------
>>> from mpl_toolkits.basemap import Basemap
>>> import matplotlib.pyplot as plt
>>>
>>> LL = (-68, -26)
>>> UR = (-66, -21)
>>> bmap = Basemap(projection='merc',
>>>                llcrnrlat=LL[1],
>>>                urcrnrlat=UR[1],
>>>                llcrnrlon=LL[0],
>>>                urcrnrlon=UR[0],
>>>                resolution='i',
>>>                suppress_ticks=False,
>>>                ax=ax)
>>> bmap.drawcountries(linewidth=1)
>>>
>>> # Overlay image with extents that don't match map boundaries
>>> nLon, nLat = image.shape
>>> LL = (-69.915, -31.161)
>>> UR = (-65.075, -17.334)
>>> dy = -0.006666664
>>> lats = UR[1] + dy * np.arange(nLat)
>>> extent = (LL[0], UR[0], LL[1], UR[1])
>>> nx = nLon; ny = nLat
>>> image_xy = bmap.transform_scalar(image, lons, lats[::-1], nx, ny)
>>> x, y = bmap(extent[:2],extent[2:])
>>> extent_xy = (x[0], x[1], y[0], y[1])
>>> im = plt.imshow(image_xy, extent=extent_xy)
>>>
>>>
>>>
>>> -- 
>>> ---------------
>>> Scott T. Henderson
>>> http://www.geo.cornell.edu/eas/gstudent/sth54/contact.html
>>>
>>>
>>> ------------------------------------------------------------------------------ 
>>>
>>> Live Security Virtual Conference
>>> Exclusive live event will cover all the ways today's security and
>>> threat landscape has changed and how IT managers can respond. 
>>> Discussions
>>> will include endpoint security, mobile security and the latest in 
>>> malware
>>> threats.http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/
>>>
>>>
>>> _______________________________________________
>>> Matplotlib-users mailing list
>>> Mat...@li...
>>> https://lists.sourceforge.net/lists/listinfo/matplotlib-users
>>
>>
>> -- 
>> Jeffrey S. Whitaker         Phone  : (303)497-6313
>> Meteorologist               FAX    : (303)497-6449
>> NOAA/OAR/PSD  R/PSD1        Email  :Jef...@no...
>> 325 Broadway                Office : Skaggs Research Cntr 1D-113
>> Boulder, CO, USA 80303-3328 Web    :http://tinyurl.com/5telg
>
> Thanks Jeff,
>
> pcolormesh seems to get the georeferencing correct. Why is it that 
> imshow doesn't work? The trouble with pcolormesh is that it's much 
> slower, the file sizes get bigger and when zooming into detailed 
> regions the pixels edges show up even when I set edgecolors='None'. 
> I've attached a few screenshots for comparison.

Scott: That's weird - I don't know why the edges show.  The only thing 
you're getting with imshow is interpolation.  To use imshow, you need to 
have a grid that exactly fits the map projection region.  You could do 
this with transform_scalar using the keyword masked=True, which will 
assign missing values to grid points outside the range of the input 
grid.  When you plot with imshow, these areas should just show up as 
transparent.

-Jeff
>
> # New version:
> # -------------------
> data_xy, x, y = bmap.transform_scalar(data[::-1], lons, lats[::-1], 
> nx, ny, returnxy=True)
> data_xym =  np.ma.array(data_xy, mask=np.isnan(data_xy))
> im = bmap.pcolormesh(x,y,data_xym,
>                     shading='flat', #'gouraud'
>                     edgecolors='None',
>                     alpha=alpha,
>                     vmin=cmin,
>                     vmax=cmax)
> plt.colorbar(im)
>
>
> # Different figures attached
>
>
>
> -- 
> ---------------
> Scott T. Henderson
> http://www.geo.cornell.edu/eas/gstudent/sth54/contact.html
>