Re: [Matplotlib-users] Fwd: Strange behaviour on plotting data on Ronbinson projection using Basemap

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

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