Thread: [Matplotlib-users] Basemap projection issue

Hi

I'm running into a problem using the mollweide projection, with the
following simplified code, my actual code doesn't use random data for
values but this is a clearer example to the problem I'm experiencing:

lon = numpy.arange(0, 361, 1)
lat = numpy.arange(-90, 91, 1)
x, y = numpy.meshgrid(lon, lat)
values = numpy.random.random((181, 361))

map = basemap.Basemap(projection='moll', lon_0=0, resolution='c')
map.drawcoastlines()
map.drawcountries()
map.drawmapboundary()
map.drawmeridians(numpy.arange(0,360,30))
map.drawparallels(numpy.arange(-90,90,30))

X, Y = map(x, y)

map.contourf(X, Y, values)

I get the following error:

 WARNING: x coordinate not montonically 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).

And the resulting map is incorrect due to the horizontal trends, see
http://tier2.ihepa.ufl.edu/~ram/files/moll.jpg


but if I use the orthographic projection ie:

map = basemap.Basemap(projection='ortho', lat_0=-20, lon_0=-30, resolution='c')

Then there are no errors, and the map is displayed as expected, see
http://tier2.ihepa.ufl.edu/~ram/files/ortho.jpg

I am unsure as to why I am receiving this error for the mollweide
projection, as my coordinate grid is covering the entire surface.  I
am clearly doing something wring here but I don't know what - does
anyone know what is causing this issue?

Cheers

Adam

Adam Mercer wrote:
> Hi
>
> I'm running into a problem using the mollweide projection, with the
> following simplified code, my actual code doesn't use random data for
> values but this is a clearer example to the problem I'm experiencing:
>
> lon = numpy.arange(0, 361, 1)
> lat = numpy.arange(-90, 91, 1)
> x, y = numpy.meshgrid(lon, lat)
> values = numpy.random.random((181, 361))
>
> map = basemap.Basemap(projection='moll', lon_0=0, resolution='c')
> map.drawcoastlines()
> map.drawcountries()
> map.drawmapboundary()
> map.drawmeridians(numpy.arange(0,360,30))
> map.drawparallels(numpy.arange(-90,90,30))
>
> X, Y = map(x, y)
>
> map.contourf(X, Y, values)
>
> I get the following error:
>
>  WARNING: x coordinate not montonically 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).
>
> And the resulting map is incorrect due to the horizontal trends, see
> http://tier2.ihepa.ufl.edu/~ram/files/moll.jpg
>
>
> but if I use the orthographic projection ie:
>
> map = basemap.Basemap(projection='ortho', lat_0=-20, lon_0=-30, resolution='c')
>
> Then there are no errors, and the map is displayed as expected, see
> http://tier2.ihepa.ufl.edu/~ram/files/ortho.jpg
>
> I am unsure as to why I am receiving this error for the mollweide
> projection, as my coordinate grid is covering the entire surface.  I
> am clearly doing something wring here but I don't know what - does
> anyone know what is causing this issue?
>
> Cheers
>
> Adam
>   

Adam:  I assume your data is on a latitude-longitude grid?  You've asked 
for a mollweide projection centered on the Greenwich meridian.  Your 
data is not centered on Greenwich - but the error message is trying to 
say that you can shift your grid (with the shiftgrid function) so that 
is has the same orientation as the map projection region.  This only 
comes into play with global projections that 'wrap-around' at the edges, 
like the mollweide and mercator projections.  The orthographic 
projection does not 'wrap around' - hence you don't get the error message.


-Jeff

-- 
Jeffrey S. Whitaker         Phone  : (303)497-6313
Meteorologist               FAX    : (303)497-6449
NOAA/OAR/PSD  R/PSD1        Email  : Jef...@no...
325 Broadway                Office : Skaggs Research Cntr 1D-124
Boulder, CO, USA 80303-3328 Web    : http://tinyurl.com/5telg

On 11/10/2007, Jeff Whitaker <js...@fa...> wrote:

> Adam:  I assume your data is on a latitude-longitude grid?  You've asked
> for a mollweide projection centered on the Greenwich meridian.  Your
> data is not centered on Greenwich - but the error message is trying to
> say that you can shift your grid (with the shiftgrid function) so that
> is has the same orientation as the map projection region.  This only
> comes into play with global projections that 'wrap-around' at the edges,
> like the mollweide and mercator projections.  The orthographic
> projection does not 'wrap around' - hence you don't get the error message.

Thanks, that makes sense now.

Cheers

Adam

On 11/10/2007, Jeff Whitaker <js...@fa...> wrote:

> Adam:  I assume your data is on a latitude-longitude grid?  You've asked
> for a mollweide projection centered on the Greenwich meridian.  Your
> data is not centered on Greenwich - but the error message is trying to
> say that you can shift your grid (with the shiftgrid function) so that
> is has the same orientation as the map projection region.  This only
> comes into play with global projections that 'wrap-around' at the edges,
> like the mollweide and mercator projections.  The orthographic
> projection does not 'wrap around' - hence you don't get the error message.

Looking at the shiftmap function it looks like I should shift the
co-ordinate grid so that my longitude runs from -180 to 180 instead of
0 to 360, therefore the following shiftgrid call should do this

values, lon = basemap.shiftgrid(180, values, lon)

However I still get the same error and the corruption in the resulting
plot.  I feel like I'm missing something obvious here but can't find
it.

Cheers

Adam

Adam Mercer wrote:
> On 11/10/2007, Jeff Whitaker <js...@fa...> wrote:
>
>   
>> Adam:  I assume your data is on a latitude-longitude grid?  You've asked
>> for a mollweide projection centered on the Greenwich meridian.  Your
>> data is not centered on Greenwich - but the error message is trying to
>> say that you can shift your grid (with the shiftgrid function) so that
>> is has the same orientation as the map projection region.  This only
>> comes into play with global projections that 'wrap-around' at the edges,
>> like the mollweide and mercator projections.  The orthographic
>> projection does not 'wrap around' - hence you don't get the error message.
>>     
>
> Looking at the shiftmap function it looks like I should shift the
> co-ordinate grid so that my longitude runs from -180 to 180 instead of
> 0 to 360, therefore the following shiftgrid call should do this
>
> values, lon = basemap.shiftgrid(180, values, lon)
>
> However I still get the same error and the corruption in the resulting
> plot.  I feel like I'm missing something obvious here but can't find
> it.
>
> Cheers
>
> Adam
>   
Adam:

 From the basemap docs;

shiftgrid(lon0, datain, lonsin, start=True)
    shift global lat/lon grid east or west.
    assumes wraparound (or cyclic point) is included.
     
    lon0:  starting longitude for shifted grid
           (ending longitude if start=False). lon0 must be on
           input grid (within the range of lonsin).
    datain:  original data.
    lonsin:  original longitudes.
    start[True]: if True, lon0 represents the starting longitude
    of the new grid. if False, lon0 is the ending longitude.
     
    returns dataout,lonsout (data and longitudes on shifted grid).


You did

values, lon = basemap.shiftgrid(180, values, lon)

but I think you want

values, lon = basemap.shiftgrid(180, values, lon, start=False)

HTH,

-Jeff

-- 
Jeffrey S. Whitaker         Phone  : (303)497-6313
Meteorologist               FAX    : (303)497-6449
NOAA/OAR/PSD  R/PSD1        Email  : Jef...@no...
325 Broadway                Office : Skaggs Research Cntr 1D-124
Boulder, CO, USA 80303-3328 Web    : http://tinyurl.com/5telg

On 11/10/2007, Jeff Whitaker <js...@fa...> wrote:

> but I think you want
>
> values, lon = basemap.shiftgrid(180, values, lon, start=False)

Thats it! Thanks a lot!

Cheers

Adam