Re: [Matplotlib-users] Combining pcolormesh and contour

[Ryan M. <rm...@gm...>]

On Tue, Feb 28, 2012 at 9:10 AM, Benjamin Root <ben...@ou...> wrote:
> On Tuesday, February 28, 2012, Andreas H. wrote:
>>
>> Good morning,
>>
>> I'm creating the attached plot using pcolormesh(). What I would like to
>> do now is draw contour lines at +/- 2.5%, which follow the grid edges.
>>
>> The problem is that when I use contour(), the lines drawn do not follow
>> the grid edges but seem to be interpolated somehow.
>>
>> Do you have an idea how to draw the contour lines following the grid
>> edges?
>>
>> Your insight is very much appreciated :)
>>
>> Cheers,
>> Andreas.
>
>
> This is because of a subtle difference in how pcolor-like functions and
> contour-like functions work.  I always forget which is which, but one
> assumes that the z value lies on the vertices of the grid while the other
> assumes that it lies in the middle of each grid point.  This is why you see
> them slightly offset from each other.

By definitition, with pcolormesh you're giving it the edges of the
grid cells. In fact, for an NxN grid of data, you should have N+1xN+1
grids of x,y values. Contour is given the actual grid and values.

Ryan

-- 
Ryan May
Graduate Research Assistant
School of Meteorology
University of Oklahoma