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
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