2-dimensional interpolation and gridding

2-dimensional interpolation and gridding is a common problem for the representation of measurements on a map. Usually measurements are taken at irregular sample points and not in a regular grid. There are various approaches for the problem and the best solution depends on the data.

In the following we will look at oceanographic parameters that have been measured with the Argo system.

Example data from Argo system

Argo observation system

Data are provided at, i.e. ftp://ftp.ifremer.fr//ifremer/argo/

First we have to download the data. Here is a /download script that can be used to retrieve one month of Argo data.

We extract only the surface temperature to reduce the large amount of data. This /extract script generates an overview of the data and writes the surface temperature in a file lat_lon_T.tab which contains latitude, longitude and surface temperature. We can use this file in the following.

LehreWiki: OpenSource2010/Lesson12 (last edited 2011-01-17 13:34:14 by anonymous)

-  ⇤ ← Revision 3 as of 2011-01-17 11:21:36 → 
  Size: 3394
  Editor: anonymous
  Comment:
+   ← Revision 6 as of 2011-01-17 11:28:53 → ⇥
  Size: 1318
  Editor: anonymous
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
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-== Example: ARGO floats ==
+== Example data from Argo system ==
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-http://en.wikipedia.org/wiki/Argo_%28oceanography%29
+[[http://en.wikipedia.org/wiki/Argo_%28oceanography%29|Argo observation system]]
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-First we have to download the data. Here is a [[/download script]] that can be used to retrieve one month of data in a temporary directory called {{{tmp_dir}}}:
+First we have to download the data. Here is a [[/download script]] that can be used to retrieve one month of Argo data.
 Line 21:
-{{{#!python
#!/usr/bin/env python
import os,os.path
from ftplib import FTP
+We extract only the surface temperature to reduce the large amount of data. This [[/extract script]] generates an overview of the data and writes the surface temperature in a file [[attachment:lat_lon_T.tab]]  which contains latitude, longitude and surface temperature.  We can use this file in the following.
-Line 26:
+Line 23:
-tmp_dir='/scratch/clisap/seaice/TMP/u242023/ARGO/'

ftp_adr='ftp.ifremer.fr'
ftp_dir='/ifremer/argo/geo/atlantic_ocean/2011/01/'

ftp = FTP(ftp_adr)   # connect to host, default port
ftp.login()          # user anonymous, passwd anonymous@
ftp.cwd(ftp_dir)

file_list=ftp.nlst()

for f in file_list:
    urlfile='ftp://'+ftp_adr+ftp_dir+f
    localfile=tmp_dir+f
   
    if not(os.path.exists(localfile)):
        print 'Getting file '+f
         #We use curl instead of ftp.retrbinary for download
        os.system("curl -s -k -o "+localfile+"  "+urlfile)
    else:
        print 'file '+f+' exists'

ftp.close()
}}}

The next script generates an overview of the data and writes the surface temperature in a file called {{{lat_lon_T.tab}}} which contains latitude, longitude and surface temperature. [[attachment:lat_lon_T.tab]]
-Line 55:
+Line 26:
-{{{#!python

import scipy.io as io
import glob
from pylab import *
from mpl_toolkits.basemap import Basemap

tmp_dir='/scratch/clisap/seaice/TMP/u242023/ARGO/'


file_liste=glob.glob(tmp_dir+'*.nc')
D={}# Empty dictionary to store selected profiles
for f in file_liste:# Loop over all data


    # Open netcdf data file
    fid=io.netcdf_file(f,'r')

    # Read content into variables
    lat=fid.variables['LATITUDE'][:].copy()
    lon=fid.variables['LONGITUDE'][:].copy()
    T=fid.variables['TEMP'][:].copy()
    P=fid.variables['PRES'][:].copy()
    
    T[T>=99999]=nan # Set 99999.0 to "Not a Number"
    P[P>=99999]=nan

    (nr_profs,Z)=T.shape # Get dimension

    for i in range(nr_profs):
        D[(lon[i],lat[i])]=(T[i,:],P[i,:])

    # Close data file
    fid.close()

fid=open('lat_lon_T.tab','w')
for k in D.keys():# write position (lat,lon), surface temperature to file
    fid.write(str(k[0])+'\t'+str(k[1])+'\t'+str(D[k][0][0])+'\n')
fid.close()
stop


# Draw map of positions
m = Basemap(projection='ortho',lon_0=-45,lat_0=0,resolution='l')
m.bluemarble()
for lon,lat in D.keys():
    x,y=m(lon,lat) # Coordinate transfer
    m.plot(x,y,'r.')
    
savefig('argo_position.png',dpi=150)

# Plot profiles
figure()
for k in D.keys():
#    print k
    plot(D[k][0][:],D[k][1][:])
axis([-2,30,2000,0])
xlabel('T')
ylabel('P')
show()
savefig('Argo_plot.png',dpi=75)
}}}