Extract Required Bytes From A File In Python

I have a binary file here: ftp://n5eil01u.ecs.nsidc.org/SAN/GLAS/GLA06.034/2003.02.21/GLA06_634_1102_001_0079_3_01_0001.DAT I have to extract the following data from that file: Byt

Solution 1:

Using a hard coded offset is a rather fragile solution. But assuming you know what you are doing:

Byte Offset: 176 
Data type: 4-byte (long) integer
Total bytes: 160

AKAICT, that leads to 160/4 = 40 values to read (could you confirm that?)

In addition, the type should be one of the numpy defined type. Here np.int32 might be the right one:

data= np.fromfile(fi,dtype=np.int32,count=40)

On my computer, this produces the following result:

[1919251297  997485633 1634494218 1936678771 1634885475  825124212
  808333629  808464432  942813232 1818692155 1868526433 1918854003
 1600484449 1702125924  842871086  758329392  841822768 1728723760
 1601397100 1600353135 1702125938 1835627615 1026633317  809119792
  808466992 1668483643 1668509535 1952543327 1026633317  960048688
  960051513  909654073  926037812 1668483643 1668509535 1952543327
 1633967973  825124212  808464957  842018099]

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If this is not what expected, maybe you have a problem of endianness.

Numpy as support for custom defined types to solve that problem:

For example:

• np.dtype('<i4') is 4 bytes (signed) integer little endian
• np.dtype('>i4') is 4 bytes (signed) integer big endian

In you case, to force reading data as little endian, you might write:

dt = np.dtype('<i4')

with open(fname,'rb') as fi:
    fi.seek (176,0)
    data= np.fromfile(fi,dtype=dt,count=40)
    print data