[Matplotlib-users] Fwd: basemap and imshow at high latitudes

[Phil E. <pel...@gm...>]

It would be easier if there were some code to reproduce the problem - it
wouldn't have to be the actual data you are using.

I wasn't able to reproduce the problem with the following code:

import mpl_toolkits.basemap as bm
import matplotlib.pyplot as plt

# Geotiff from http://earthobservatory.nasa.gov/IOTD/view.php?id=49687
# FROM gdalinfo
#Corner Coordinates:
#Upper Left  ( -11.1339000,  55.8714000) ( 11d 8' 2.04"W, 55d52'17.04"N)
#Lower Left  ( -11.1339000,  50.9251367) ( 11d 8' 2.04"W, 50d55'30.49"N)
#Upper Right (  -4.8622296,  55.8714000) (  4d51'44.03"W, 55d52'17.04"N)
#Lower Right (  -4.8622296,  50.9251367) (  4d51'44.03"W, 50d55'30.49"N)
#Center      (  -7.9980648,  53.3982684) (  7d59'53.03"W, 53d23'53.77"N)
im = plt.imread('ireland_amo_2010284_geo.tif')
# extent = [left, right, bottom, top]
extent = [-11.1339000, -4.8622296, 50.9251367, 55.8714000]

m = bm.Basemap(projection='cyl',
               llcrnrlon=extent[0], urcrnrlon=extent[1],
               llcrnrlat=extent[2], urcrnrlat=extent[3],
               resolution='i')
plt.imshow(im, extent=extent)
m.drawcoastlines()
plt.show()

[image: Inline images 1]

Your data appears to be a long way off. Are you certain of the bounds that
you are setting? Are you using plt.imshow or doing some other operation to
draw (e.g. contour)?


Cheers,





On 12 March 2013 15:00, Hearne, Mike <mh...@us...> wrote:

> The relevant code is:
> It's kind of hard to post code that's reproducible (lots of
> dependencies on code and data), but I'll attach an image at high
> latitudes that demonstrates the problem.
>
> The image is a shaded relief of topography, and the coastlines are
> drawn in black.
>
> The topography is cut from a large ESRI format BIL (Band Interleaved
> by Line) file, and it's bounds define the boundaries of the map.  The
> image data is NOT projected (it's in lat/lon).
>
> --Mike
>
> On Tue, Mar 12, 2013 at 3:46 AM, Phil Elson <pel...@gm...> wrote:
> > Hi Mike,
> >
> > Do you have any code or an image to show the problem?
> >
> > Cheers,
> >
> >
> > On 11 March 2013 20:44, Hearne, Mike <mh...@us...> wrote:
> >>
> >> I have an issue with basemap.imshow() at higher latitudes - namely the
> >> image (high-res topography, in this case) becomes distorted with
> >> respect to the coastlines the higher I go.    I assume it has to do
> >> with the image pixels becoming more non-square the higher I go in
> >> latitude.
> >>
> >> I found this discussion:
> >> http://matplotlib.1069221.n5.nabble.com/Basemap-and-imshow-td14115.html
> >>
> >> where Jeff indicates that the user is using a non-rectangular map
> >> projection.  I'm thinking that is perhaps my problem (I'm using
> >> Transverse Mercator), but I'm not sure which projections Basemap
> >> supports that *are* rectangular.
> >>
> >> Or perhaps it's something else entirely.
> >>
> >> Any hints?
> >>
> >> Thanks,
> >>
> >> Mike Hearne
> >>
> >>
> >>
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