Introduction

The -lon option prints the value of the grid point closest to the specified longitude latitude. The latitude-longitude of the grid point are also printed. If you use the verbose mode, the grid coordinates (i,j) and the number of the element are also printed. The -lon option can be repeated to save processing time.

-sh-2.05b$ wgrib2 eta.t00z.awphys18.grb2 -d 1 -s -lon 249 39 -lon 255 33
1:0:d=2003090300:MSLET:mean sea level:18 hour fcst:lon=249.035,lat=38.9912,val=101685:
lon=254.964,lat=32.9671,val=101668

-sh-3.00$ wgrib2 rtma.t12z.2dvaranl_ndfd.grb2.c2 -d 1 -v -lon -120 30
1:0:lon=240.008805,lat=29.988418,i=220037,ix=72,iy=206,val=0

Usage

-lon LONGITUDE LATITUDE
      LONGITUDE = 0 .. 360
      LATITUDE = -90 .. 90

      If the verbosity is 0, the print out the longitude and
        latitude of the nearest grid point as well as the grid value.

      If the verbosity is 1 or higher, the prints out the longitude and
        latitude of the nearest grid point, the index (i) to the data,
        the grid coordinates (ix,iy) as well as the grid point value.

        i = 1..number of grid points
        ix = 1..nx
        iy = 1..ny

Example

$ wgrib2 test.grb2 -s -lon -90 20
1:0:d=2005090200:HGT:1000 mb:60 hour fcst:lon=270,lat=20,val=121.3
2:133907:d=2005090200:HGT:975 mb:60 hour fcst:lon=270,lat=20,val=344.4
3:263511:d=2005090200:HGT:950 mb:60 hour fcst:lon=270,lat=20,val=573
4:389058:d=2005090200:HGT:925 mb:60 hour fcst:lon=270,lat=20,val=806.5
...

Old vs New

The original code for the -lon option used the internal geolocation package. This package could compute the lat/lon of the grid points but nothing else. To find the grid point closest to a specified lat/lon, the distance to every grid point had to be calculated. Later, a short cut was added for lat-lon grids. Finally the geolocation packages gctpc and Proj4 have inverse functions which allow you to find the closest grid point to specified point for the the supported grids. For unsupported grids like the Gaussian grid and staggered grids, the original brute force code is used. You can turn off the new code by the -gctpc 0 option. The old code did not know about grid domains and would find the closest point even if the point were outside of the grid domain. The gctpc-based closest will return a lat=lon=999 to signal an intial point outside of the domain.

Want Speed?

You want extract the values for a 1000 different points. So you call wgrib2 1000 times and complain that wgrib2 is slow. Well decoding a jpeg2000 compressed file 1000 times does take time. It's better to add a 1000 -lon options to the command line and only decode the file once.

The number of -lon options on a command line is limited by a compile-time option. Try running wgrib2 -config and look for the line "maximum number of arguments on command line:". The current value is 5000 which allows you 5000 words on the command line. Each -lon option takes 3 words, so that gives you about 1600 -lon options you can run on one line. Of course, limitations such as the maximum line length or maximum number of continuations may stop you first.

Text, Binary and CSV Output

The -lon option writes the grid value to the inventory. What happens if you want the output written to a file. You could write the output of -lon to a file by using the -last option.

$ wgrib2 gep19.aec -lon 10 20  -last junk -nl_out junk -for 1:3
1:0:lon=10.000000,lat=20.000000,val=12391.6
2:70707:lon=10.000000,lat=20.000000,val=219.5
3:96843:lon=10.000000,lat=20.000000,val=85
$ cat junk
lon=10.000000,lat=20.000000,val=12391.6
lon=10.000000,lat=20.000000,val=219.5
lon=10.000000,lat=20.000000,val=85

You can also use the -lola option which can write a 1x1 grid to binary, text or a grib file.

$ wgrib2 gep19.aec -no_header -lola "10:1:1" "20:1:1" out.txt text -for 1:3
1:0:d=2009060500:HGT:200 mb:180 hour fcst:ENS=+19
2:70707:d=2009060500:TMP:200 mb:180 hour fcst:ENS=+19
3:96843:d=2009060500:RH:200 mb:180 hour fcst:ENS=+19
$ cat out.txt
12391.6
219.5
85

$ wgrib2 gep19.aec -no_header -lola "10:1:1" "20:1:1" out.grb grib -for 1:3
1:0:d=2009060500:HGT:200 mb:180 hour fcst:ENS=+19
2:70707:d=2009060500:TMP:200 mb:180 hour fcst:ENS=+19
3:96843:d=2009060500:RH:200 mb:180 hour fcst:ENS=+19
$ wgrib2 out.grb -csv out.csv
1:0:d=2009060500:HGT:200 mb:180 hour fcst:ENS=+19
2:182:d=2009060500:TMP:200 mb:180 hour fcst:ENS=+19
3:364:d=2009060500:RH:200 mb:180 hour fcst:ENS=+19
$ cat out.csv
"2009-06-05 00:00:00","2009-06-12 12:00:00","HGT","200 mb",10,20,12391.6
"2009-06-05 00:00:00","2009-06-12 12:00:00","TMP","200 mb",10,20,219.5
"2009-06-05 00:00:00","2009-06-12 12:00:00","RH","200 mb",10,20,85

See also: -last, -lola, -config