mbauer wrote:
> Matplotlib users,
>
> I've been using pcolor and pcolormesh to plot results from the NCEP
> Reanalysis. I've noticed that the plotted values are slightly offset.
> Googling around I see that matlab has this quality, which I assume
> matplotlib inherited.
>
> # If your georeferenced image is in lat/long coordinates (i.e.
> each data row is along a line of
> constant latitude, each column a line of equal longitude), ...
> you MUST remember to offset your
> coordinates by one-half of the pixel spacing. This is because
> of the different behaviors of
> p_color and image when given the same data.
> 1. image will center the drawn (i,j) pixel on the (i,j)th entry of
> the X/Y matrices.
> 2. p_color with shading flat will draw a panel between the (i,j),
> (i+1,j),(i+1,j+1),(i,j+1)
> coordinates of the X/Y matrices with a color corresponding to
> the data value at (i,j). Thus
> everything will appear shifted by one half a pixel spacing.
>
> and
> % Since the grid is rectangluar in lat/long (i.e. not
> % really a projection at all, althouhg it is included in
> % m_map under the name 'equidistant cyldindrical'), we
> % don't want to use the 'image' technique. Instead...
> % Create a grid, offsetting by half a grid point to account
> % for the flat pcolor
> [Plg,Plt]=meshgrid(Plon-0.25,Plat+0.25);
>
> The data I'm using uses polar grids centered on +-90.0 which give a
> latitude array as such
> [-90. -87.5 -85. -82.5 -80. -77.5 -75. -72.5 -70. -67.5 -65. -62.5
> -60. -57.5 -55. -52.5 -50. -47.5 -45. -42.5 -40. -37.5 -35.
> -32.5
> -30. -27.5 -25. -22.5 -20. -17.5 -15. -12.5 -10. -7.5 -5.
> -2.5
> 0. 2.5 5. 7.5 10. 12.5 15. 17.5 20. 22.5 25.
> 27.5
> 30. 32.5 35. 37.5 40. 42.5 45. 47.5 50. 52.5 55.
> 57.5
> 60. 62.5 65. 67.5 70. 72.5 75. 77.5 80. 82.5 85.
> 87.5
> 90. ]
>
> Is there a simple way to "shift" this data so my global plots look
> correct? So far my results result in an "empty" line along the south
> pole or I end up with an extra latitude which pcolor doesn't like.
>
> Thanks,
>
> Mike
>
>
Mike: From the pcolor docstring:
X and Y, if given, specify the (x,y) coordinates of the colored
quadrilaterals; the quadrilateral for C[i,j] has corners at
(X[i,j],Y[i,j]), (X[i,j+1],Y[i,j+1]), (X[i+1,j],Y[i+1,j]),
(X[i+1,j+1],Y[i+1,j+1]). Ideally the dimensions of X and Y
should be one greater than those of C; if the dimensions are the
same, then the last row and column of C will be ignored.
So it may be easier to modify your data (by averaging adjacent values to
they reflect the mid-point of each grid box) than to modify the vertices.
-Jeff
--
Jeffrey S. Whitaker Phone : (303)497-6313
Meteorologist FAX : (303)497-6449
NOAA/OAR/PSD R/PSD1 Email : [EMAIL PROTECTED]
325 Broadway Office : Skaggs Research Cntr 1D-124
Boulder, CO, USA 80303-3328 Web : http://tinyurl.com/5telg
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