Evan Mason wrote:
> Thanks for the replies. The map you produced, Jeff, looks as it
> should. However, I am trying to make an ocean model grid, and so I
> require two 2d arrays of lon and lat, at my desired grid spacing.
> This is why I try the steps:
>
> dl = 20000.
> nx = int((M.xmax - M.xmin) / dl) + 1
> ny = int((M.ymax - M.ymin) / dl) + 1
> lonr, latr = M.makegrid(nx, ny) <- it seems to be here that it loses
> 'memory' of omerc projection that I specified, and maybe there is a
> bug here?
Evan: Why do you say it 'loses' memory of the projection? The values
look fine to me - they are just equally spaced points in map projection
coordinates that cover the map projection region. Take a look at
M = Basemap(projection = 'omerc', \
resolution = 'l', \
llcrnrlon = -43.7, \
llcrnrlat = 14.7, \
urcrnrlon = -4.0, \
urcrnrlat = 41.9, \
lat_2 = 11.0, \
lat_1 = 45.5, \
lon_2 = -27.8, \
lon_1 = -19.9)
dl = 200000.
nx = int((M.xmax - M.xmin) / dl) + 1
ny = int((M.ymax - M.ymin) / dl) + 1
lonr, latr,x,y= M.makegrid(nx, ny, returnxy=True)
M.drawcoastlines()
M.scatter(x.flatten(), y.flatten(),5,marker='o')
M.drawparallels(arange(10,51,10))
M.drawmeridians(arange(-50,1,10))
show()
>
> If you have matlab, the following lines do what I am looking for:
>
> incx = 0.00310/2;
> incy = 0.00306/2;
> Xstr = -0.275;
> Xend = 0.275;
> Ystr = 0.17;
> Yend = 0.8;
> X = [Xstr:incx:Xend];
> Y = [Ystr:incy:Yend];
> [XX,YY] = meshgrid(X,Y);
> [Lonr,Latr] = m_xy2ll(XX,YY);
> m_proj('Oblique Mercator','lon',[-23.75 -28.25],'lat',[45.5
> 11],'direction','vertical');
> plot(Lonr, Latr, 'c.')
Sorry, I don't have matlab - but it looks at first glance like it ought
to be doing the same thing.
-Jeff
>
>
>
> -Evan
>
>
>
>
>
> On Feb 13, 2008 5:14 AM, Jeff Whitaker <[EMAIL PROTECTED]
> <mailto:[EMAIL PROTECTED]>> wrote:
>
> Evan Mason wrote:
> > Hi, I am having some problems using the oblique mercator
> projection in
> > basemap. I want to define a rectangular orthogonal grid, rotated
> > clockwise by about 13 degrees. I want to define grid cells of size,
> > say, about 20x20 km. The script I have so far is below. The
> problem
> > is that at some point (the makegrid step) I lose the rotation,
> as seen
> > in the plot.
> >
> > I'd appreciate any help with this, thanks, Evan
> >
> >
> > from matplotlib.toolkits.basemap import Basemap
> >
> > M = Basemap(projection = 'omerc', \
> > resolution = None, \
> > llcrnrlon = -43.7, \
> > llcrnrlat = 14.7, \
> > urcrnrlon = -4.0, \
> > urcrnrlat = 41.9, \
> > lat_2 = 11.0, \
> > lat_1 = 45.5, \
> > lon_2 = -27.8, \
> > lon_1 = -19.9)
> >
> > dl = 20000.
> > nx = int((M.xmax - M.xmin) / dl) + 1
> > ny = int((M.ymax - M.ymin) / dl) + 1
> >
> > lonr, latr = M.makegrid(nx, ny)
> >
> > plot(lonr, latr, 'c.')
> > show()
>
> Evan: I have to admit, I'm not too familiar with the Oblique Mercator
> projection. What exactly should it look like?
>
> If I plot
>
> M = Basemap(projection = 'omerc', \
> resolution = 'l', \
> llcrnrlon = -43.7, \
> llcrnrlat = 14.7, \
> urcrnrlon = -4.0, \
> urcrnrlat = 41.9, \
> lat_2 = 11.0, \
> lat_1 = 45.5, \
> lon_2 = -27.8, \
> lon_1 = -19.9)
> M.drawcoastlines()
> M.drawparallels(arange(10,51,10))
> M.drawmeridians(arange(-50,1,10))
> M.show()
>
> I see a reasonable looking map, but then I don't really know exactly
> what to expect.
>
> It seems that there are two ways to specify oblique mercator in proj4
>
> 1) by specifying 2 points (lon1,lat1), (lon2,lat2) along the
> central line
> 2) by specifying a central point and an azimuth that passes
> through the
> central point.
>
> Basemap uses (1), but every example on the web I've seen uses (2). It
> could be there are bugs in (1), and (2) would produce more reasonable
> results in your case. If you can give me an example of what your map
> *should* look like, it would help a lot.
>
> -Jeff
>
>
>
>
> --
> Jeffrey S. Whitaker Phone : (303)497-6313
> NOAA/OAR/CDC R/PSD1 FAX : (303)497-6449
> 325 Broadway Boulder, CO, USA 80305-3328
>
>
--
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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