Re: [Matplotlib-users] Projection Direction Distortion

[Mathew Topper]

Hi Jody,

Thanks for getting in touch. My reply in line also:

On 26/04/13 18:34, Jody Klymak wrote:
Not clear how a direction is given as a lon lat. Do you mean you have 
a set of vectors, each defined as lon/lat pairs? 


My usage is to examine boundary conditions for a spectral wave model. 
Thus, I have data points at locations given in**geographic coordinates 
with bearings to describe the wave directions.


It depends on what you want - if you want the arrow to point where the 
wind would go after X minutes, then you want the distortion. If you 
want the viewer to be able to pick off the geographic heading by eye, 
then simply convert your lon lat pairs to heading/length pairs and 
plot them in the axis frame. See 
http://matplotlib.org/basemap/users/mapcoords.html for how to convert 
from basemap to the underlying axis frame.


To clarify, the wave model I am using demands wave directions in the 
projection of the grid which is being used, and I am trying to figure 
out if I need to change the inputs to the wave model, using basemap as 
an indicator, potentially a pre-processor.


In fact, I'm having a bit of a intellectual crisis trying to understand 
what an instantaneous wave direction is and whether such a thing should 
be effected by the projection it is in!


In an ideal world your projection would not be over such a large area 
that any of this matters - if your vector is off by 1 degree, who will 
be able to tell in a plot?


In the plots and also in the model, I think it will make very little 
difference to the outputs. However, I am very interested in 
quantification of uncertainty and therefore I want to demonstrate that 
it could (should the directions need adjustment) make a difference to 
the results, particularly if you were a bit slack with your choice of 
grid projection.


Thanks again for your help and I'm sorry if this is a little off 
topic, but I know basemap users are a smart bunch to consult with. :)


All the best,

Mat

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Re: [Matplotlib-users] Basemap plotting data on projection

[ChaoYue]

Hi all,

Indeed it's a bit strange, I can reproduce the problem. But when using
'cyl' projection I don't have this issue.
attahced two figures showed the difference.

the tested data is also attached.

complete code is below:

from mpl_toolkits.basemap import Basemap, cm, maskoceans
import matplotlib.pyplot as plt


# set up the Mercator projection
m = Basemap(projection='merc',llcrnrlat=9,urcrnrlat=54.5,\
llcrnrlon=74,urcrnrlon=142,lat_ts=20,resolution='l')

m.drawcountries()
m.drawcoastlines()
m.drawmapboundary(fill_color='aqua')
m.drawrivers()

pdata = np.genfromtxt('pdata.txt')
pdata = np.ma.masked_greater(pdata,1E20)
lonm,latm=m.makegrid(pdata.shape[1],pdata.shape[0])

latm=np.flipud(latm)
lonpro,latpro=m(lonm,latm)
cs = m.contourf(lonpro,latpro,pdata)
m.colorbar(cs)

cheers,

Chao

On Mon, Apr 29, 2013 at 12:53 PM, Ann [via matplotlib] 
ml-node+s1069221n40973...@n5.nabble.com wrote:

 Dear All,


 I am encountering the following problem:
 I have some data which I want to plot on a map. Unfortunately, I do not
 manage to plot the data according to the shape of the projection! What it
 appears to do is to simply plot the data on top of the projection without
 taking the actual position (latitude and longitude) into account. However,
 this is crucial for my analysis.
 Could anybody spot the mistake in my code?
 Any suggestion would be of great help!!
 THANKS



 from mpl_toolkits.basemap import Basemap, cm, maskoceans
 import matplotlib.pyplot as plt


 # set up the Mercator projection
 m = Basemap(projection='merc',llcrnrlat=9,urcrnrlat=54.5,\
 llcrnrlon=74,urcrnrlon=142,lat_ts=20,resolution='l')

 m.drawcountries()
 m.drawcoastlines()
 m.drawmapboundary(fill_color='aqua')
 m.drawrivers()


 parallels = N.arange(0.,81,10.)
 labels = [left,right,top,bottom]
 m.drawparallels(parallels,labels=[1,0,0,0])
 meridians = N.arange(10.,351.,20.)
 m.drawmeridians(meridians,labels=[0,0,0,1])


 ny=resmat.shape[0] # resmat is the matrix containing the data which I want
 o plot.
 nx=resmat.shape[1]

 #get lat/lons of ny by nx evenly space grid
 lons, lats =m.makegrid(nx, ny)


 # compute map projection coordinates
 x, y= m(lons, lats)

 maskdata = maskoceans(lons, lats, resmat, inlands= True, resolution = 'l')


 cs = m.contourf(x,y,maskdata,150,cmap=cm.GMT_no_green_r)

 plt.show()


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pdata.txt (413K) 
http://matplotlib.1069221.n5.nabble.com/attachment/40975/0/pdata.txt
cyl.jpg (540K) 
http://matplotlib.1069221.n5.nabble.com/attachment/40975/1/cyl.jpg
merc.jpg (435K) 
http://matplotlib.1069221.n5.nabble.com/attachment/40975/2/merc.jpg




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