Güralp Compressed Format
8 Güralp Compressed Format
Güralp Systems sensors and data modules use Güralp Compressed Format (GCF) to
share seismic data. The format can be used for data storage or transmission over a
serial link or TCP/UDP network.
This section describes the specification of GCF format.
8.1 GCF blocks
A GCF file or stream consists of a sequence of
blocks
, which can be up to 1024 bytes
long. Each block consists of a 16-byte header followed by either
•
a series of data records, containing initial and final sample values and a
sequence of first differences between intervening sample values, or
•
status information in ASCII text format.
The format of the block's body is determined by information in the header.
8.1.1 The GCF header
The header is 16 bytes long, split into four 4-byte fields:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
System ID
Stream ID
Date code
Data format
System ID :
If the top bit of this field is unset, the bottom 31 bits of this field specify a
label of up to 6 characters identifying the originating system, encoded as a base 36
number. The base 36 digit in each position corresponds to a single character (0 – “
0
”
… 9 – “
9
” , 10 – “
A
” … 35 – “
Z
”), with the least significant digit placed at the right-hand
end of the string.
If the most significant base 36 digits are zero, they are omitted from the string; hence
the encoding 1 corresponds to the string
1
, not
000001
.
For example, the string
HPA1
would be encoded as the number (17 × 36 × 36 × 36) + (25
× 36 × 36) + (10 × 36) + 1 = 825913.
The field is a signed 31-bit positive integer, allowing System IDs up to ZIK0ZJ.
If the top bit of this field is set, the bottom 26 bits of this field specify a label of up to 5
characters using the same encoding. In this case, the field is a signed 26-bit positive
integer, allowing System IDs up to ZZZZZ. Bits 26 – 30 of the System ID field are
reserved in this case.
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Issue U - December, 2021