Dear Phil,
Thank you. This solves my problem. So the title of my mail is wrong, the
behaviour is reasonable but I am using wrong coordinates.
And also thanks to Jeff.
Cheers,
Chao
On Wed, Apr 16, 2014 at 10:14 AM, Phil Elson <pelson....@gmail.com> wrote:
> Hi Chao,
>
> The warning you are getting:
>
> WARNING: x coordinate not monotonically increasing - contour plot
> may not be what you expect. If it looks odd, your can either
> adjust the map projection region to be consistent with your data, or
> (if your data is on a global lat/lon grid) use the shiftgrid
> function to adjust the data to be consistent with the map projection
> region (see examples/contour_demo.py).
>
> Is important here. It looks like the x coordinate is not in appropriate
> longitudes. Printing the first 5 and last 5 longitudes gives us our first
> clue:
>
> First 5: [-180. -178.99720764 -177.99443054 -176.99163818
> -175.98886108]
> Last 5 : [ 175.98886108 176.9916687 177.9944458 178.9972229
> 180.00003052]
>
> Notice that the last longitude wraps around beyond 180. So if we were to
> clip these numbers to -180 and +180 we will see that the warning disappears
> and the contour is correct. This can be achieved with:
>
> lon = np.clip(lon, -180, 180)
>
> Alternatively, we can just construct the latitudes and longitudes
> ourselves directly with:
>
> lon, lat = np.meshgrid(np.linspace(-180, 180, 360), np.linspace(-90, 90,
> 180))
>
> Incidentally, I tried these numbers with cartopy which has been designed
> to handle dateline wrapping automatically, and the contour worked with the
> unmodified longitudes (http://nbviewer.ipython.org/gist/pelson/10830039).
>
> ---------------------------
>
> @JeffWhitaker - This looks like a bug with float tolerances in the
> makegrid function. It currently does:
>
> def makegrid(self,nx,ny,returnxy=False):
> dx = (self.urcrnrx-self.llcrnrx)/(nx-1)
> dy = (self.urcrnry-self.llcrnry)/(ny-1)
>
> But might be better if it did:
>
> def makegrid(self,nx,ny,returnxy=False):
> x = np.linspace(self.llcrnrx, self.urcrnrx, nx)
> y = np.linspace(self.llcrnry, self.urcrnry, ny)
>
> To avoid the multiplicative floating point drift that is currently being
> seen.
>
> HTH,
>
> Phil
>
--
please visit:
http://www.globalcarbonatlas.org/
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Chao YUE
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